Comprehensive WBBSE Class 10 Physical Science Notes Chapter 8.4 Inorganic Chemistry in the Laboratory and in Industry can help students make connections between concepts.

Inorganic Chemistry in the Laboratory and in Industry Class 10 WBBSE Notes

Discovery: In 1774 Priestly prepared the ammonia gas by heating a mixture of ammonium chloride and calcium hydroxide.

Occurrence: Traces of ammonia occur in the atmosphere. Ammonia is a product of the decomposition of organic matter containing nitrogen. The stable manure e.g. urea [CO(NH₂)₂], derived from the urine of animals. The urea is converted by the action of bacteria into ammonium carbonate which slowly decomposes, yielding ammonia and hence its smell is a stable and its presence in traces in air.

• Molecular formula : NH₃
• Molecular weight : 17

Preparation of ammonia:
Principle Normally ammonia gas is prepared in the laboratory by heating a mixture of ammonium chloride (1 part) and slaked lime (3 parts). Instead of using slaked lime quick lime also be used.
Chemicals required: Ammonium chloride (NH₄Cl) and quick lime (CaO) or dry slaked lime [Ca(OH)₂]

Chemical reactions:
2NH₄Cl + Ca (OH)₂ = 2NH₃↑ + CaCl₂ + 2H₂O
2NH₄Cl + CaO = 2NH₃↑ + CaCl₂ + H₂O

Collection: Ammonia is lighter than air, it may be collected by the down­ward displacement of air. Ammonia is not collected through the downward displacement of water because, it is highly soluble in water. Precautions: The ingradients, the test-tube, the delivery pipes and the gas- jar should be absolutely dry. All the connections in the apparatus should be leak-proof.

Drying of ammonia : As ammonia is a basic substance, it cannot be direct by acidic drying agents like cone. H₂SO₄ or P₂O₅.
2NH₃ + H₂SO₄ = (NH₄)₂SO₄; 6NH₃ + P₂O₅+ 3H₂O = 2 (NH₄)₃PO₄ 

It forms an additive compound with calcium chloride. So fused CaCl₂ cannot also be used to dry ammonia.
CaCl₂ + 8NH₃ = CaCl₂-8HO (Additive compound)
It is best dried with the basic drying agent, quicklime (CaO).

2. Other methods of preparation of Ammonia

(a) By hydrolysis of a metallic nitride by heating with an alkali solution.
Mg₃N₂ + 6H₂O= 3Mg (OH)₂ + 2NH₃↑
A1N + NaOH + H₂O = NaAlO₂ + NH₃↑

(b) By heating a solution of a nitrate or nitrite with zinc and strong caustic soda solution.
3NaNO₃ + 8Al + 5NaOH + 2H₂O = 8NaAlO₂ + 3NH₃↑
NaNO₂ + 3Zn + 5NaOH = 3N₂ZnO₂ + H₂O + NH₃↑

(c) By heating ammonium salts at high temperature.
(NH₄)₂SO₄ = NH₃↑ + NH₄HSO₄
2(NH₄)₃PO₄ = 6NH₃↑ + P₂O₅ + 3H₂O

(d) By the reduction of the oxides of nitrogen (except nitrous oxide) e.g. by passing a mixture of nitric oxide and hydrogen over heated spongy platinum.
2NO + 5H₂ = 2NH₂+ 2H₂O

(e) Calcium cyanamide is hydrolysed by highly heated steam to form ammonia.
CaCN₂ + 3H₂O = CaCO₃ + 2NH₃↑

(f) Harber’s process : In Haber’s synthetic process the mixture of one volume of nitrogen and three volume of hydrogen is heated at 550°C temperature and 200 atm pressure under the influence of iron as catalyst and MO as promoter to form ammonia.

Physical properties of ammonia :

• Smell Ammonia is a colourless gas with very pungent smell that affects eyes, nose and throat.
• Density Ammonia has a density (vapour density = 8 5) less than air (vapour density of air = 144). The gas is readily liquefied by pressure alone (6 atm at 0°C).
• It is highly soluble in water. 1 volume of water at 0°C dissolves 1150 volumes of ammonia and 739 at 20°C.

Liquor ammonia : A saturated aqueous solution of ammonia (sp. gr. = 0.88) contains only 35% NH₃ by weight is known as liquor ammonia. Liquor ammonia is dangerous for eyes.
NH₃ + H₂O = NH₄OH

The solution of ammonia is alkaline and turns red litmus to blue. Its basic character is due to the formation of OH^–.
NH₃+ H₂O \(\rightleftharpoons\) NH₄⁺+ OH^–
A bottle of liquor ammonia should be carefully opened after cooling in ice, as there is always a high pressure inside.

Chemical properties of NH₃

(i) Reaction with oxygen:

(a) It does not support combustion, nor does it burn in air, but in oxygen it burns with a greenish-yellow flame, forming nitrogen and steam. 4NH₃ + 3O₂ = 2N₂ + 6H₂O

(b) In presence of heated platinum gauze catalyst at 500°C – 700°C, ammonia is oxidised to nitric oxide by air or oxygen. 4NH₃ + 5O₂ = 4NO + 6H₂O

(ii) Reaction with acid : As ammonia is a base, it readily reacts with an acid to form salt.

NH₃ + HCl = NH,Cl
NH₃ + HNO₃ = NH₄NO₃
2NH₃ + H₂SO₄ = (NH₄)₂ SO₄

The reaction of NH₃ (gas) with HCl (gas) produces NH₄Cl as a white solid.
This reaction shows the formation of a solid substance from the reaction of two gaseous substances.
NH₃(gas) + HCl (gas) = NH₄Cl (solid)

(iii) Reaction with alkali metals: Ammonia reacts with alkali metals at red-heat (360°C), forming amides which are violently decomposed by water, yielding ammonia.
2Na + 2NH₃ = 2NaNH₂ + H₂↑
2K + 2NH₃ = 2KNH₂ + H₂↑
NaNH₂ + H₂O = NaOH + NH₃↑

(iv) Reaction with chlorine (non-metal): Ammonia reacts with chlorine in two ways.

(a) In excess ammonia Excess ammonia is oxidised by chlorine forming nitro­gen and is reduced itself to hydrochloric acid. Hydrochloric acid thus formed combines with ammonia producing ammonium chloride. This reaction also proves that ammonia contains nitrogen.

(b) In excess chlorine : When excess chlorine reacts ammonia forming nasent nitrogen which again combines with chlorine producing nitrogen trichloride, an oily yellow explosive compound.

(v) Reducing property: Ammonia reduces some metal oxides at high tempera­ture lo the F corresponding metals and itself gets oxidised to nitrogen gas. Ammonia gas is passed over heated black cupric oxide which is reduced to red metallic copper and ammonia is oxidised to nitrogen.

(vi) Formation of additive compounds :
Anhydrous CaCl₂,  ZnCl₂ etc. absorb ammonia to form additive compounds.
CaCl₂ + 8NH₃ = CaCl₂ .8NH₃
ZnCl₂ + 8NH₃ = ZnCl₂ .8NH₃

(vii) Reaction with carbon dioxide :
(a) Urea is formed with the reaction of ammonia and carbon dioxide at 200°C and 150 atm pressure.

(b) Ammonia is converted into ammonium sulphate with the direct use of sul­phuric acid.
2NH₃ + CO₂ + H₂O + CaSO₄ = (NH₄)₂SO₄ + CaCO₃

(vii) Reactions with salts: Salts like ferric chloride, aluminium chloride react with ammonium hydroxide forming brown precipitate of ferric hydroxide and white gelatinus precipitate of aluminium hydroxide respectively.

(ix) Formation of complex salts: In some cases the precipitated hydroxide dissolves in excess of ammonia forming a cationic complex e.g. (a) copper sulphate gives a pale blue precipitate of basic copper sulphate with ammo­nia which dissolves in excess of the precipitant forming a deep blue solution containing the complex salt, tetra-tetra-ammine cupric sulphate [Cu(NH₃)₄] SO₄.

(b) Silver nitrate solution gives a white precipitate which quickly passess into brown oxide, soluble in excess of ammonia.

(c) Ammonia produces brown colouration or precipitate in Nessler’s reagent (an alkaline solution of potassium mercuric iodide, k₂Hgl₄

Precaution to be taken to combat the effect of NH₃ leaked from industires and ammonia tanks :
Ammonia has a strong pungent smell and is highly soluble in water. It is harmful for eyes. It gives troubles in eyes when exposed in small quantities but in excess it could damage eyes permanently.

Possible measure :
(a) As NH₃ is highly soluble in cold water, it is neces­sary to spray cold water so that dispersed ammonia gas would be dissolved and minimizes its bad effect.

(b) If individual is affected then eyes should be washed readily with cold water time and again. If ammonia is leaked from ammonia tanks then the portion from which it is leaked should be kept immersed in cold water.

Identification  of Ammonia : Ammonia can be identified by the following tests:

• Ammonia is a colourless gas with a strong pungent smell.
• White fumes are obtained when ammonia gas brought in contact with a glass rod moistened, with hydrochloric acid.
• Ammonia (or its aqueous solution) forms a brown colour precipitate with Nessler’s reagent solution (an alkaline solution of K₂Hgl₂.
• Ammonia turns moist red litmus paper into blue.
• A strip of filter paper, soaked in mercurous salt solution, when exposed to ammonia gas, turns black.

Using Ammonia:
(a) Ammonia is used for the industrial preparation of nitric acid (by Ostwald process), sodium carbonate (by Solvay process).

(b) Large quantities of ammonia are used in the manufacture of fertilizers, such as-urea [CO(NH₂)₂], ammonium sulphate [(NH₄)₂SO] etc.

(c) It is also used in pharmaceutical industries and also in the preparation of smelling salt.

(d) Ammonia is used as solvent.

(e) It is used as laboratory reagent.

(f) Liquid ammonia is used as a refrigerant in ice making.

(g) Ammonia is used as cleaning agent for removing grease.

Structure of  Ammonia molecule :
Ammonia molecule has a pyramidal sharp vith nitrogen atom at the apex. The nitrogen atom of ammonia is sp³ hybridized and < H-N-H = 106°45C

(b) Hydrogen Sulphide (H₂S)

Discovery: In 1774, Shele discovered the gas sulphurated hydrogen.

Occurrence : Hydrogen sulphide is found in volcanic gases and in many hot spring waters. Its presence in the atmosphere is in very small amount. Hydrogen sulphide is also present in rotten egg as also in leather.

Molecular formula : H₂S

Molecular weight : 34

Preparation of hydrogen sulphide :

1. Laboratory preparation:

Principle: At ordinary temperature ferrous sulphide taken in a woulfe’s bottle reacts with dilute sulphuric acid (1 volume acid and 6 volumes water), hydrogen sulphide is prepared.

Chemical required . Ferrous sulphide (FeS) and dilute sulphuric acid (H₂SO₄).
Chemical reaction : FeS + H₂SO₄ = FeSO₄ + H₂ST↑
Collection : Because the gas is heavier than air, it is collected over the upward displacement of air. Hydrogen sulphide is dissolved in water, so it is not colleced over downward displacement of water. It can also be collected in downward displacement of hot water because the gas is insoluble in hot water.

Purification : The hydrogen sulphide gas can be purified by absorbing it into a suspension of magnesium oxide in water and then regenerating it by heating magnesium bisulphide [Mg(HS)₂] thus formed at 60°C.

Pure H₂S gas is obtained by heating antimony sulphide, Sb₂S₃ with pure cone hydrochloric acid.
Sb₂S₃ + 6HCl = 2SbCl₃ + 3H₂S

Drying of hydrogen sulphide :
Hydrogen sulphide may be dried by passing it through phosphorus pen oxide (P₂O₅)

(i) Anhydrous CaCl₂ is not used for drying H₂S gas.
CaCl₂ + H₂S = CaS + 2HCl

(ii) Conc. H₂SO₄ is not used for drying H₂S gas.
H₂S + H₂SO₄ = S↓ + SO₂↑ + H₂O

(iii) Quick lime also cannot be used for drying H₂
CaO + H₂S = CaS + H₂O

(iv) Hg is not used for drying H₂ But pure H₂S does not react with Hg.

N.B. Dilute sulphuric acid is used in the preparation of H₂S because it is non-volatile and dilute H₂SO₄ does not behave as an oxidising agent but cone. H₂SO₄ oxidises H₂S when sulphur is precipited.
H₂S + H₂SO₄ = S’ + SO₂↑ + 2H₂O

Nitric acid cannot be used for preparing H₂S from a sulphide for H₂S thus produced is oxidised by HNO₃ into sulphur. Dilute HCl is not used for the preparation of H₂S because HCl is a vol­atile acid.

It is mixed with hydrogen sulphide gas in such a way, separation becomes difficult.

Procautions should be taken not to inhale the gas and not to allow its prolonged contact with skin, during handling H₂S.

2. Other methods of preparation of H₂S
From metallic sulphide : Different metallic sulphides react with dilute HCl or H₂SO₄ to form H₂S.
Na₂S + 2HCl = 2NaCl + H₂S↑
CaS + H₂SO₄ = CaSO₄ + H₂S↑
Reaction of insoluble sulphide with dilute acid reacts with insoluble .sulphide As₂S₃ to form H₂S.
As₂S₃ + 12H (Zn + dil H₂SO₄) = 12 AsH₃ + 3H₂S↑

Synthetic process : Hydrogen gas and sulphur vapour are mixed with each other at 450°C in the presence of Ni-dust to produce hydrogen sulphide.

Physical properties of hydrogen sulphide :

(i) Smell : Hydrogen sulphide is a colourless gas with bad smell like rotten eggs.
(ii) Density : The density of H₂S (vapour density = 17) is higher than that of air (vapour density of air = 14 4)
(iii) Solubility in water: Hydrogen sulphide is appreciably soluble in cold water, (4-37 volumes at 0°C, 3-40 volumes at 10°C and 2-6 volumes at 20°C) but practically insoluble in hot water. The solution of H₂S in water is acidic and it turns blue litmus red.
(iv) It is poisonous when inhaled in small amounts, it causes headache and is fatal in large amounts.
(v) It liquefies at 212 2K and freezes at -190 K to a transparent solid.

Chemical properties of H₂S
(i) Combustibility : It does not support combustion, but it burns a blue flame in excess of air or oxygen, giving water and sulphur dioxide; but sulphur is deposited, if the supply of oxygen is limited.
2H₂S + 3O₂ = 2SO₂ + 2H₂O
2H₂S + O₂ = 2Sl + 2H₂O

(ii) Acidic properties: Hydrogen sulphide is a weak dibasic acid. Its aqueous solution is acidic to litmus. With sodium hydroxide (NaOH), it forms sodium sulphide (NagS) as the normal salt and sodium hydrogen sulplide (NaHS) as the acid salt.

Reducing property :

(a) Reaction with Cl₂ : When hydrogen sulphide is passed over chlorine water, chlorine is reduced to HC1, on the other hand H₂S is oxidised to sulphur.
\(\stackrel{0}{\mathrm{C}} l_2+\mathrm{H}_2 \stackrel{-2}{\mathrm{~S}}=2 \mathrm{H} \stackrel{-1}{\mathrm{C}} 1+\stackrel{0}{\mathrm{~S}} \downarrow\)

(b) Reaction with SO₂ : Hydrogen sulphide reduces sulphur dioxide to sulphur and itself oxidised to sulphur.
\(\stackrel{+4}{\mathrm{~S}} \mathrm{O}_2+2 \mathrm{H}_2 \stackrel{-2}{\mathrm{~S}}=3 \stackrel{0}{\mathrm{~S}}+2 \mathrm{H}_2 \mathrm{O}\)

(c) Reaction with HNO₃ : H₂S reduces cone. HNO_s to brown nitrogen dioxide (NO₂) and is itself oxidised to sulphur.
\(2 \stackrel{+5}{\mathrm{H}^{\mathrm{N}}} \mathrm{O}_3+\mathrm{H}_2 \stackrel{-2}{\mathrm{~S}}=2 \stackrel{+4}{\mathrm{NO}_2}+\stackrel{0}{\mathrm{~S}}+2 \mathrm{H}_2 \mathrm{O}\)

(d) Reaction with cone. H₂SO₄ H₂S reduces cone. H₂SO₄ to SO₂ and is itself oxidised to sulphur.
\(\mathrm{H}_2 \stackrel{+6}{\mathrm{~S}} \mathrm{O}_4+\mathrm{H}_2 \stackrel{-2}{\mathrm{~S}}=\stackrel{+4}{\mathrm{~S}} \mathrm{O}_2+\stackrel{0}{\mathrm{~S}}+2 \mathrm{H}_2 \mathrm{O}\)

(e) When H₂S is passed through a yellow solution of ferric chloride (FeCl₃), the salt is reduced to colourless ferrous chloride (FeCl₂) and H₂S itself on oxidation gives a precipitate of sulphur.
\(2 \stackrel{+3}{\mathrm{~F}} \mathrm{eCl}_3+\mathrm{H}_2 \stackrel{-2}{\mathrm{~S}}=2 \stackrel{+2}{\mathrm{FeCl}_2}+2 \cdot \mathrm{\textrm {HCl }}+\stackrel{0}{\mathrm{~S}} \downarrow\)

(f) H₂S redues pink solution of potassium permanganate acidified with dilute sulphuric acid to a colourless solution.

(g) H₂S also reduces potassium dichromate solution acidified with dilute sulphuric acid-the orange-red colour of the solution turns green.

(h) H₂S reduces H₂O₂ to water and H₂S is oxidised to sulphur.

(iv) Reactions with metallic salts: Hydrogen sulphide reacts with different metallic salts giving rise different coloured metallic sulphides. The basic redicals in the salts are separated from different in colours and solubilities.

(a) The following precipitates ate obtained in acid medium
CuSO₄ + H₂S = H₂SO₄ + CuSl ↓ (Black)
Pb(NO₃)₂ + H₂S = 2HNO₃ + PbSl ↓(Black)
CdCl₂ + H₂S = 2HCl + CdS ↓ (Yellow)
SnCl₂ + H₂S = 2HCl + SnS ↓ (Brown)

(b) Certain sulphides are precipitated only in alkaline solution. In ammonical solution zinc salts give a white sulphide, iron salts give black sulphide.
ZnSO₄ + (NH₄)₂S = (NH₄)₂SO₄ + ZnS ↓ (White)
FeSO₂ + (NHS = (NH₄)₂SO₄ + FeS ↓ (Black)

Identification of Hydrogen sulphide:

Hydrogen sulphide can be identified  by the following tests.

(i) It is a colourless gas with foul smell like rotten egg.
(ii) A bright silver coin, when held in the gas, turns black.
2Ag+ H₂S = Ag₂S4- (black) + H₂
(iii) Lead acetate paper turns black when it is held in H₂S gas.
Pb(CH₃COO)₂ + H₂S = PbS↓ (black) + 2CH₃COOH
(iv) The gas is passed through a solution of sodium hydroxide solution and then sodium nitroprusside solution is added to it. The solution turns purple colour.

Absorbent: An acidic gas H₂S is absorbed by the caustic alkalis, NaOH and KOH; lead nitrate solution also absorbs the gas.
Pb (NO₃)₂ + H₂S = 2HNO₃ + PbSl (black)

Uses of H₂S :

• H₂S is used as a reagent in the separation of metal ions in group analysis.
• It is sometimes used as a reducing agent.
• It is used for preparing metallic sulphides. Some sulphides are used as pigments.

Structure of hydrogen  Sulphate : It has bent structure like water and H-S-H bond angle is 92-2°. In H₂S molecule S atom is sp³ hybridized.

C. Nitrogen (N₂)

Nitrogen was discovered by Danil Rutherford in 1772 Laviosier showed that nitrogen is an elementary gas present in air and it is not a supporter of combustion and respiration.

Atomic Number : 7; Symbol: N ; Molecular formula : H₂; Atomic weight : 14008

Electronic configuration : \(\text { Is } 2 s^2 2 p_x^1 2 p_y^1 2 p_z^{11}\)
Postion in priodic Table : Period 2, Group VA ;
Oxidation number : -3 to + 5.

Comparation of Nitrogen :

A concentrated aqueous solution of ammonium nitrite is heated to produce nitrogen gas. But this reaction is very fast so there is a chance of explosion. For this reason, the concentrated aqueous solution of sodium nitrite and ammonium are mixed molar ratio then if this mixture is heated nitrogen gas is produced.

Chemical  Requird : (i) Sodium nitrite (NaNO₂)
(ii) Ammonium Chloride (NH₄Cl)

Chemical Reaction  :
(i) NaNO₂ + NH₄Cl = NH₄NH₂ + NaCl

Collection : Though nitrogen gas is slightly soluble in water, it is collected by the download displacement of water.

Drying : The gas is dried by passing through a U-tube containing cone. H₂SO₄.

2. Other methods of preparation of nitrogen :
(a) From ammonium dichromate : Ammonium dichromate on gentle heating decomposes violently, evolving nitrogen.

(b) From ammonia : N₂ is also formed by slowly passing chlorine into concen­trated ammonia, when ammonium chloride (NH₄Cl) and nitrogen (N₂) are formed.
8NH₃ + 3Cl₂ = 6NH₄Cl + N₂↑

(c) From urea Nitrogen is obtained by reaction with alkaline hypobromite solution.
CO (NH₂)₂ + 3NaOBr = CO₂↑+ 3NaBr + 2H₂O + N₂↑
From azide compounds: Spectroscopically pure nitrogen is obtained by heating barium azide at 300°C. The metal remains.

(d) From nitric acid : Moderately dilute nitric acid reacts with copper evolving nitric oxide, which when passed over the heated metal yields nitrogen.
Ba(N₃)₂ = Ba + 3N₂↑

(e) From nitric acid : Moderately dilute nitric acid reacts with copper evolving nitric oxide, which when passed over the heated metal yields nitrogen.
3Cu + 8HNO₃ = 3Cu (NO₃)₂ + 2NO + 4H₂O
2NO + 2Cu = 2CuO + N₂

(f) From air : By passing over red-hot copper filings which fix the oxygen as oxide of copper : 2Cu + O₂ = 2CuO, and nitrogen passes out. It contains about 1 percent argon.

Physical properties of Nitrogen :

• Nitrogen is a colourless gas without any smell or taste.
• Vapour density of nitrogen (14) is slightly less than that of air (14 4).
• It is slightly soluble in water (23 5 ml of N₂ is dissolved in 1 lit water at STP).

Chemical properties : It is an inert element at ordinary temperature because of very large of dissociation of the molecules, but it enters into combination with many elements at higher temperature.

(i) Reactions with non-metals:

(a) Reaction with hydrogen : At 550°C and 200 atmopsheric pressure in presence of iron catalyst reacts with hydrogen to produce ammonia. This is an industrial process of NH₃ (Haber’s process).
N₂ + 3H₂ \(\rightleftharpoons\) 2NH₃ + 22.4 Kcal

(b) Reaction with oxygen : Nitrogen combines with oxygen forming nitric oxide under the influence of electric arc at a temperature of 3000°C.
N₂ + O₂ \(\rightleftharpoons\) 2NO – 43.3 Kcal

(c) Covalent nitrides are obtained by reacting nitrogen with Boron and Silican at high temperature.
2B + N₂ = 2BN ; 6Si +4N₂= 2Si₃N₄

Nitrogen is absorbed by heated metals like Ca, Mg and Al to form nitrides which by Hydrolysis gives ammonia.
3Ca + N₂ = Ca₃N₂ ; Ca₃N₂ + 6H₂O= 3Ca (OH)₂ + 2NH₃↑
3Mg + N₂ = Mg₃N₂ ; Mg₃N₂ + 6H₂O = 3Mg(OH)₂ + 2NH₃↑
2Al + N₂= 2AlN ; AlN + 3H₂O = Al (OH)₃ + NH₃↑

(iii) Reaction with compounds : calcium carbide is heated in a current of nitrogen at temperature of 1100° C, calcium cyanamide and carbon are formed which is commercially known as nitrolim.

Significance of the presence of nitrogen in air : About 78% by volume of nitrogen is present in the air but neither plant nor animal tissues can directly absorb nitrogen from air except a few leguminous plants, such as pea, bean clover etc. There are two ways of fixation of nitrogen as

(i) By electric discharge
(ii) Bio-chemical reaction through bacteria.

(i) Electric discharge due to thundering : During electric discharge in the atmosphere nitrogen and oxygen present in air combine to form nitric oxide. This nitric oxide is then oxidised by atmospheric oxygen to produce nitrogen dioxide. Later this oxide upon mixing with water vapour or rain water forms nitric acid which falls upon our earth. Nitric acid then reacts with the bases present in the soil forming nitrate salts.

(ii) Fixation of nitrogen due to bacteria : Some micro organism and blue green algae convert nitrogen present in air to ammonia and nitric salts by chemical process.

Uses of nitrogen

• Atmospheric nitrogen is fixed in large quantities as ammonia, nitric acid and nitrolim.
• Liquid nitrogen is a refirgerant.
• Nitrogen gives an inert atmosphere in certain metallurgical operations.
• In making gas thermometers and for filling electric bulbs.
• Nitrogen is used in the preparation of different explosives.

d. Hydrogen Chloride (Hydrochloric Acid, HCl), Nitric Acid (HNO_s), Sulphuric Acid (H₂SO₄)

Hydrogen Chloride œ Hyrochioric Acid (HCl)

• Molecular weight; 365; Formula : HCl (Murlatic Acid)
• Prepared from sea-salt first; Prestly (1772)
• Devy established that it is a compound of hydrogen and chlorine.

Laboratory method of preparation of hydrogen chloride : Hydrogen chloride is obtained by heating a mixture of common salt or sodium chloride and concentrated sulphuric acid.

Reaction occurs in two steps :
(a) When the mixture is heated at 150°C – 200°C then sodium bisulphate and hydrogen chloride gas are obtained.

(b) When it is heated at 500°C then sodium sulphate and hydrogen chloride gas are produced.

• In the laboratory the reaction is performed at lower temperature because :
• At high temperature the flask used may be cracked.
• Sodium sulphate at higher temperature forms a hand crust and sticks to the glass. Its removal is very difficult.
• Collection : As dry hydrogen chloride gas is heavier than air, it is collected by downward displacement of air.
• Hydrogen chloride is highly soluble in water, so it is not collected over displacement of water.

Drying agent To remove water vapour, hydrogen chloride is passed over concentrated sulphuric acid.
P₂O₅ is not used for drying HCl gas because

(ii) CaO, NaOH, KOH are not used for drying HCl gas because :
CaO + 2HCl = CaCl₂ + H₂O
NaOH + HCl = NaCl + H₂O
KOH + HCl = KCl + H₂O

Nitric acid la not used to prepare HCl gas :

(a) Nitric acid is not used because during the preparation of HCl gas, nitric acid oxidises the produced HCl gas into Cl₂ gas.
HNO₃ + 3HCl = HOCl + 2Cl + 2H₂O

(v) Reaction with silver nitrate : An aqueous solution of hydrochloric acid (or any soluble metallic chloride) gives a curdy white precipitate of silver chloride with silver nitrate solution—the precipitate is soluble in ammonia but insoluble in nitric acid.
HCl + AgNO₃ = AgCl + HNO₃
AgCl + 2NH₃ = [Ag(NH₃)₂]Cl

(vi) Reaction with Nitric acid : The mixture of three volume of cone. HCl acid and one volume of cone. HNO₃ acid is called aqua-regia. Noble metals like gold, platinum etc. are dissolved in aquaregia.
3HCl + HNO₃ = NOCl + 2 [Cl] + 2H₂O
Au + 3[Cl] = AuCl₃ ; AuCl₃ + HCl = HAuCl₄ (soluble)
Pt + 4[Cl] = PtCl₄ ; PtCl₄ + 2HCl = H₂PtCl₆ (soluble)

Identification of hydrogen chloride and hydrochloric acid :

• HCl gas is identified by its strong choking smell.
• It fumes in moist air. Dense white fumes are produced when a glass-rod, moistened with strong ammonia solution, is held in the gas.
• HCl gas turns a moist blue litmus paper red. Hydrochloric acid also turns blue litmus paper red.
• It forms a white curdy precipitate of AgCl with colourless. AgNO₃ solution. The curdy precipitate is soluble in ammonium hydroxide solution.

Uses of hydrochloric acid :

• It is used in dyeing and calico-printing.
• It is used in the manufacture of glucose, glue and many useful metal-chlorides. HCl is used as reagents in chemical laboratories.
• It is used in preparing aqua-regia.
• It is used for washing (pickling) iron sheets before galvanization and tinning. It is used for the preparation of chlorine and chlorides.

Nitric Acid (HNO₃)

Molecular weight: 63 ; Formula : HNO₃ (aqua forties)
Prepared by distilling KNO₃ (nitre) with concentrated sulphuric acid : Glauber (1650)

Preparation :
Laboratory method of preparation : Nitric acid is prepared in the lab­oratory by heating a mixture of sodium nitrate (or potassium nitrate) and concentrated sulphuric acid (in 3 : 2 mole proportion)

Reaction occurs in two steps :
When the reaction is kept at 200°C -300°C temperature, then sodium bisul­phate or potassium bisulphate and nitric acid are produced.
NaNO₃ + H₂SO₄ = NaHSO₄ + HNO₃ KNO₃ + H₂SO₄ = KHSO₄ + HNO₃

At 800°C, sodium sulphate and nitric acid are formed in the reaction between sodium nitrate or potassium nitrate and concentrated sulphuric acid.
2NaNO₃ + H₂SO₄ = Na₂SO₄ + 2HNO₃
2KNO₃ + H₂SO₄ = K₂SO₄ + 2HNO₃

In the laboratory the reaction is performed at lower temperature because :
(a) At high temperature of about 800°C, nitric acid is decomposed to nitrogen dioxide (NO₂) and oxygen (O₂).
4HNO₃ = 4NO₂ + O₂ + 2H₂O

(b) Both HNO₃ and HCl are volatile in nature. So, they will be collected jointly in a receiver where HNO₃ may oxidise HCl into Cl₂. This difficulty is removed by using cone. H₂SO₄ (b.p. 338°C) non-volatile acid.

Pure HCl preparation: Pure hydrogen chloride is prepared by the action of water upon silicon tetrachloride:
SiCl₄ + 4H₂O = Si(OH)₄ + 4HCl↑

Hydrochloric acid is not prepared by dissolving hydrogen chloride gas in water directly because : HCl gas is highly soluble in water and the rate of dissolving of HCl gas in water is much higher than the rate of formation of HCl gas as a result there is a vacuum in the flask. To fill up the vacuum water in the beaker enters into the flask and creates explosion when it contacts with H₂SO₄.

Properties :

Physical properties :

• Colour ; HCl gas and hydrochloric acid are both colourless.
• Odour : HCl gas has a strong choking odour and the odour of hydrochloric acid is less choking.
• Solubility : It is highly soluble in water. At 0°C, 450 c.c. of HCl are dis­solved in 1c. of water.
• Boiling Point : HCl gas is easily converted to colourless liquid by applying pressure at low temperature. The liquid HCl is of boiling point -84’5°C. Liquid hydrochloride is transformed crystals at -111-4°C.
• Density : The density of cone, hydrochloric acid is 119 g/ml. The vapour density of HCI gas is 18.25.

Chemical properties :

(i) Reaction with alkali Hydrochloric acid reacts with alkalis forming salt and water.
NaOH + HCl = NaCl + H₂O
Ca(OH)₂ + 2HCl = CaCl₂ + 2H₂O

(ii) Reactions with metals : Metals lying above hydrogen in the electrochemical series react with dilute HCI forming hydrogen gas and chlorides of metals.
Mg + 2HCl = MgCl₂ + H₂↑
Fe + 2HCl = MgCl₂ + H₂↑

Noble metals, such as — gold, platinum, etc. are not reacts with the acid. Copper slowly dissolves in hot and concentrated acid and silver is slowly reacts in the presence of air only.
2Cu + 4HCl + O₂ = 2CuCl₂ + 2H₂O
4Ag + 4HCl + O₂ = 4AgCl + 2H₂O

(iii) The aqueous acid dissolves metallic oxides, hydroxides and carbonates. Hy­drogen chloride reacts with ammonia in presence of trace of moisture, forming dense fumes of ammonium chloride.
CuO + 2HCl = CuCl₂ + H₂O
NaOH + HCl = NaCl + H₂O
NH₃(g) + HCl(g) = NH₄Cl(s)
CaCO₃ + 2HCl = CaCl₂ + H₂O + CO₂

(iv) Reaction with MnO₂ : HCl is readily oxidised to chlorine by manganese dioxide or potassium permanganate.

• At high temperature glass retort may be cracked.
• At high temperature sodium sulphate (Na₂O₄) or potassium sulphate (KgSO) produced forms a hard emst and sticks to tine glass. It cannot be removed easily.

Concentrated HCl is not used in the preparation of HNO_a because:

• It is because of the fact that HCl is more volatile than HNO₃.
• During heating with HCl will be collected in the receiver, as a result HNO₃ will not be produced.
  3HCl + HNO₃ = NOCl 4- 2[Cl] + 2H₂O

Fumming nitric acid : When NO₂ is dissolved in concentrated HNO₃, it is then called fumming nitric acid. NO₂ is evolved as brown fumes from the acid. Hence, it is called fumming nitric acid. It is a strong oxidising agent. Fumming nitric acid is prepared when cone, nitric acid is distilled with As₂O₃ (arsenious oxide) or starch.

Large scale production : (Ostwald process, 1914) :
The following steps are followed :

Properties :

Physical properties:

• Colour : Pure nitric acid is a colourless liquid.
• Odour : Nitric acid has a choking smell.
• Solubility : It is highly soluble in water.
• Density : Cone. HNO₃ has a density of 142 g/ml
• Boiling point : Pure nitric acid boils at 86°C and freezes to a white solid at -42°C. The specific gravity of pure acid is T52.
• It fumes in air if it is kept opened.
•  Concentrated nitric acid is corrosive to skin.

Chemical properties:

(i) Acidic character : Aqueous solution of nitric acid is ionised to great extent, hence it is strong acid. It is a monobasic acid.
\(\mathrm{HNO}_3 \rightleftharpoons \mathrm{H}^{+}+\mathrm{NO}_3^{-}\)
HNO₃ turns blue litmus to red.

(ii) Reactions with alkali : It reacts with alkalis forming salt and water.
NaOH + HNO₃ = NaNO_s + H₂O
Ca(OH)₂ + 2HNO₃ = Ca(NO₃)₂ + 2H₂O

(iii) Reaction with metals : Nitric acid reacts all metals with the exception of gold and platinum forming different products. The actual product formed depends upon the following factors :

• nature of the metal.
• concentration of the acid.
• temperature

(a) Reaction with magnesium : Strong nitric acid on reaction with magnesium forms magnesium nitrate and nitric oxide.
3Mg + 8HNO₃ = 3Mg(NO₃)₂ + 2NO + 4H₂O
Very dilute nitric acid on treatment with magnesium and manganese liberates hydrogen.
Mg + 2HNO₃ = Mg (NO₃)₂ + H₂

(b) Reaction with iron : Hot and cone. HNO₃ makes metallic iron passive. Passive iron does not exhibit its normal chemical properties.

(c) Reaction with copper : Hot concentrated nitric acid reacts with copper metal forming copper nitrate and nitrogen dioxide.
Cu + 4HNO₃ = Cu(NO₃)₂ + 2NO₂ + 2H₂O

(iv) Reaction with AgNO_s solution and BaCl₂ solution : Nitric acid does not react with AgNO₃ solution and BaCl₂

(v) Decomposition : When nitric acid is strongly heated, it forms nitrogen dioxide and oxygen
4HNO₃ = 2H₂O + 4NO₂ + O₂

(vi) Oxidising nature: Since nitric acid has a strong tendency to give nascent oxygen it therefore, acts as a poweful oxidising agent both in the concentrated and dilute solutions. Concentrated nitric acid is generally related to nitrogen dioxide while the dilute acid is reduced to nitric oxide.
2HNO₃ (cone.) = 2NO₂ + H₂O + [O]
2HNO₃ (dilute) = 2NO + H₂O + 3 [O]

(a) Concentrated nitric acid oxidises copper turnings to copper nitrate and itself is reduced to brown coloured nitrogen dioxide gas.

(b) Concentrated nitric acid oxidises charcoal i.e. carbon to carbondioxide and itself reduced to nitrogen dioxide.

(vii) Aquaregia : A mixture to concentrated nitric acid (1 volume) and hydrochloric acid (3 volume) is called aquaregia; it dissolves gold and platinum.


(viii) A mixture of concentrated nitric acid and sulphuric acid is used in the nitration of aromatic compounds.

Identification of Nitric acid :

• Nitric acid is a colourless liquid with a choking smell.
• On heating with copper turnings, nitric acid produces broun fumes to NO₂ and copper turnings dissolve to form a blue solution of copper nitrate.
• Ring test (or a nitrate): Equal volumes of freshly prepared ferrous sulphate solution and dilute HNO₃ (dilute solution of a nitrate) are mixed together in a test tube and cooled. Cone. H₂SO₄ is now added carefully into the inner side of the test tube so as to form a heavy bottom layer; a brown ring is formed at the junction of the two liquids.

6FeSO₄ + 2HNO₃ + 3H₂SO₄ = 3Fe₂(SO₄)₃ + 4H₂O +NO
FeSO₄ + NO = FeSO₄.NO
(nitroso ferrous sulphate, brown in colour)

Uses of nitric acid :

• In the manufacture of explosives like T.N.T. (trinitrotoluene), nitroglycerine, picric acid etc.
• As a laboratory reagent.
• It is used in the manufacture of fertilizers, such as — calcium ammonium nitrate.
• In the manufacture of sulphuric acid by chamber process.
• In the manufacture of dyes, artifical silk and perfumes.
• It is used in the purification of gold and silver.
• It is used in making celluloid, rayon and other nitro cellulose products.

Pollution of air, water from Goldsmith’s workshop : Pure gold is of 24 carats. But in ornaments it is 22 carats or less. Gold ornaments are prepared by mixing requisite amount of copper with gold. When these gold ornaments are redesigned to any other form it requires breaking. Goldsmiths use nitric acid to purify gold ornaments.’ Fumes of nitric acid and brown NO₂ are formed in the process. These gases pollute air in the locality.
Cu + 4HNO₃ = Cu(NO₃)₂ + 2NO₂ + 2H₂O

Air pollution : Excess amount of NO₂ is coming from Goldsmith’s workshop is inhaled. There is an oozing of blood from lungs. Photochemical oxidant produced from NO and NO₂ have harmful effect on plant and animals.

Water pollution : Copper nitrate produced in Goldsmith’s workshop is poisonous. This compound is mixed in ponds through drains. As a result of this, fishes and plants like algae in water of this ponds may be destroyed to certain extent.

Remedy from the pollution : The works in a Goldsmith factory should Be carried out in a fume cupboard with effective device for the gaseous and particulate pollutants to escape high in the atmosphere.

Acid rain: The acidic gaseous oxides, such as – SO₂, NO₂ and CO₂ in the atmosphere in the formation of acid rain :
S + O₂ = SO₂ ; 2SO₂ + O₂ = 2SO₃; SO₃ + H₂O= H₂SO₄
NO + O₃ = NO₂ + O₂ ; NO₂ + O₃ = NO₃ + O₂
NO₃ + NO₂ = N₂O₅ ; N₂O₅ + H₂O= 2HNO₃

Acid rain affect

• Plantation and agriculture by washing out the soil nutrients.
• It also damages the building materials of houses, historic monuments and sculptures and may even destroy aquatic lives like fish.

Sulphuric Acid (H₂SO₄)

• Moleculars weight : 98; Formula : H₂O₄ (oil of vitriol)
• It is called : king of chemicals

Preparation:
Contact process :
Chemicals required : Sulphur or iron pyrities or spent sulphide, excess air, platinised asbestos or V₂O₅ (vanadium pentoxide) and water.

Principle :
(i) Sulphur or iron pyrites are burnt in aim to form SO₂.
S + O₂ = SO₂
4FeS₂ + 11O₂ = 2Fe₂O₃ + 8SO₂

(ii) The process is named as contact process because conversion of SO₂ to SO₃ carried out in presence of porous catalyst having large contact surface.

According to Le-chatlier’s principle, the conditions for the maximum yield of SO₃ are :

• Excess of O₂ (SO₂ : O₂ ratio is 2 : 3)
• High pressure (2 atm to avoid corrosion of plant)
• Low temperature (450°C)
• Presence of catalyst (finely divided platinum or V₂O_s)

(iii) SO₃ formed is absorbed into concentrated H₂SO₄ when oleum or fuming sulphuric acid gets formed.
SO₃ + H₂SO₄ = H₂S₂O₇ (oleum)

(iv) Oleum formed is diluted with water to get sulphuric acid or any desired concentration.
H₂S₂O₇ + H₂O = 2H₂SO₄

Sulphuric acid is not prepared directly by adding water to SO₃ because:
The gas forms a dense sulphuric fog or mist with water on account of the following highly exothermic reaction. H₂O + SO₃ = H₂SO₄ + 89.2 KJ

Fuming sulphuric acid or oleum : Fuming sulphuric acid or oleum is obtained when sulphur trioxide is passed over 98% sulphuric acid.
H₂SO₄ + SO₃ = H₂S₂O₇ (Pyro sulphuric acid)
Sulphuric acid can further be obtained if requisite amount of water is added to oleum.

Properties :

• Physical properties :
• Colour : It is a colourless oily liquid.
• Odour : It is an odourless liquid.
• Solubility : It is soluble in water in any proportions.
• Density : Cone. H₂SO₄ is a heavy liquid with a density 1-84 g/ml.
• Boiling point : Cone. H₂SO₄ boils at 338°C under atmospheric pressure.
• It produces severe burns on the skin.

Chemical properties :

(i) Dissociation : It is quite stable but on strong heating, it dissociates into SO₃ and H₂
\(\mathrm{H}_2 \mathrm{SO}_4 \rightleftharpoons \mathrm{H}_2 \mathrm{O}+\mathrm{SO}_3\)

(ii) Acidic character : In aqueous solutions, sulphuric acid turns blue litmus red indicating its acidic character. It inoises in two steps as :

Sulphuric acid is dibasic acid and reacts with metals, metaloxides and car­bonates etc. which are the characteristic reactions of an acid.

Reactions with alkalis :
NaOH + H₂SO₄ = NaHSO₄ (acid salt) + H₂O
2NaOH + H₂SO₄ = Na₂SO₄ (normal salt) + 2H₂O

Reaction with metals : Metals lying above hydrogen in the electrochemical series react with dilute H₂SO₄ yielding hydrogen gas.
Mg + H₂SO₄ = MgSO₄ + H ↑
Fe + H₂SO₄ = FeSO₄ + H₂↑
Hot concentrated sulphuric acid reacts with metal copper forming sulphur dioxide.
Cu + 2H₂SO₄ = CuSO₄ + SO₂ + 2H₂O

(iii) Reactions with solutions of AgNO_s and BaCl₂ : It does not react with AgNO₃ sulphuric acid gives a heavy white precipitation with BaCl₂ solution. This precipitation is in soluble in any mineral acids.
H₂SO₄ + BaCl₂ = BaSO₄ (white precipitate) ↓ + 2HCl

(iv) Oxidising action Hot concentrated sulphuric acid is an oxidising agent since it decomposes to give atomic oxygen. H₂SO₄ = H₂O + SO₂ + [O]

(a) Concentrated sulphuric acid oxidises copper metal to copper sulphate and itself reduced to sulphur dioxide.

(b) Hot and concentrated sulphuric acid oxidises carbon to carbon dioxide and itself reduced to sulphur dioxide.

(v) Dehydration action of cone. H₂SO₄ H₂SO₄ is a strong dehydrating agent and desiccating agent due to its great affinity for water.

(a) It dehydrates the white crystals of cane sugar to black mass of carbon.
C₁₂H₂₂O₁₁(sugar) + [H₂SO₄] = 12C + [11H₂O + H₂SO₄]

(b) Concentrated sulphuric acid absorbs water molecule from formic acid yielding carbon monoxide.
HCOOH + [H₂SO₄] = CO + [H₂O + H₂SO₄]

(c) Oxalic acid decomposes to produce carbon dioxide and carbon monoxide.

(d) The blue crystals of CuSO₄, 5H₂O are dehydrated to white, anhydrous CuSO₄.

Indentification of H₂SO₄

• H₂SO₄ is a heavy oilly liquid. It chars sugar and paper.
• When barium chloride solution is added to dilute solution of H₂SO₄, a heavy white precipitate is formed. The precipitate is insoluble in HCl, HNO₃ and NH₄OH solutions.
  BaCl₂ + H₂SO₄ = BaSO₄ ↓ + 2HCl

Uses of sulphuric acid :

• It is used to prepare HCl₃, HNO₃, H₃PO₄ etc
• It is used to prepare the fertilisers like ammonium sulphate, superphosphate of lime etc.
• In dyes, drugs and explosive industries for the manufacture of paints, pig­ments, dyes, drugs, picric acid and explosives like TNT.
• For refining petroleum.
• It is used for a laboratory reagent and for drying of gases.
• It is used for cleaning the surface of metals before carrying out electroplating,
• Sulphuric acid is used for the manufacture of rayon, photographic films, rubber and synthetic detergents.

Pollution due to SO₂ : SO₂, obtained from different sources, such as — ex­haust gas from motor vehicle, petroleum refining plant, extraction of metals etc. mixes in our atmosphere and creates environmental pollution.

Effect:

• SO₂ produces problems in eyes and also in lungs.
• Possibility of cancer.
• Asthama, broncrities, allergy in our body.

Stone cancer : Due to the corrosive action of SO₂, SO₃, and H₂SO₄ on marble stone i.e. on calcium carbonate are the main cause of damaging effect of historical monuments like the Tajmahal.
CaCO₃ (Marble stone) + H₂SO₄ = CaSO₄ + CO₂ + H₂O

An insoluble layer of CaSO₄ is formed over monuments. As a result once layer is formed sulphuric acid does not come in contact with marble stone. The reaction is stopped and there is no further decay of the monuments. But if this layer is removed by any means then decay continuous. It is called stone cancer.

Possible remedy :

• Banning of acid (sulphuric) factories by an ordinance.
• Excessive uses of motor vehicles should be restricted.
• Petroleum refining plant should be kept 90 km apart.

Distinction of HCl, HNO₃ and H₂SO₄ :

Well structured WBBSE Class 9 Physical Science MCQ Questions Chapter 6 Heat can serve as a valuable review tool before exams.

Heat Class 9 WBBSE MCQ Questions

Multiple Choice Questions :

Question 1.
What is the unit of heat in SI system ?
(i) calorie
(ii) Joule
(iii) erg
(iv) Newton
Answer:
Joule

Question 2.
Who designed absolute scale of temperature ?
(i) Newton
(ii) Lord Kelvin
(iii) Celsius
(iv) Boyle
Answer:
Lord Kelvin

Question 3.
Which substance has the highest specific heat ?
(i) water
(ii) iron
(iii) gold
(iv) ice
Answer:
Water

Question 4.
The fundamental interval of celsius scale is divided into –
(i) 100
(ii) 180
(iii) 120
(iv) 102 division
Answer:
100

Question 5.
The fundamental interval of Fahrenheit scale is divided into
(i) 180
(ii) 100
(iii) 32
(iv) 212 division
Answer:
180

Question 6.
If C and F represent a particular temperature in Celsius and Fahrenheit scales respectively, then their relation is :
(i) \(\frac{C}{4}\) = \(\frac{F-32}{9}\)
(ii) \(\frac{C}{5}\) = \(\frac{F-32}{9}\)
(iii) \(\frac{C}{3}\) = \(\frac{F-32}{5}\)
(iv) \(\frac{C}{9}\) = \(\frac{F-32}{5}\)
Answer:
\(\frac{C}{5}\) = \(\frac{F-32}{9}\)

Question 7.
The temperature of a body is measured by the instrument :
(i) hydrometer
(ii) thermometer
(iii) voltameter
(iv) ammeter
Answer:
thermometer

Question 8.
How much calorie of heat would be produced by converting 42 joule of work completely into heat ?
(i) 100 calorie
(ii) 10 calorie
(iii) 1000 calorie
(iv) 90 calorie
Answer:
10 calorie

Question 9.
Normal body temperature of a man is –
(i) 98.4° C
(ii) 98.4° F
(iii) 98.4 K
(iv) 100° F
Answer:
98.4° F

Question 10.
Which substance has the highest specific heat?
(i) ethanol
(ii) water
(iii) acetone
(iv) ether
Answer:
water

Question 11.
If the specific heat of copper be 0.09, what will be the water equivalent of a block of copper of mass 50 g ?
(i) 45 g
(ii) 4.5 g
(iii) 450 g
(iv) 44 g
Answer:
4.5 g

Question 12.
What is the melting point of ice in Fahrenheit scale ?
(i) 0° F
(ii) 32° F
(iii) 180° F
(iv) 121° F
Answer:
32° F

Question 13.
What is the specific heat of water in SI system ?
(i) 4 2 joule
(ii) 4200 joule/kg
(iii) 4200 joule/kg/K
(iv) 42 erg
Answer:
4200 joule / kg / K

Question 14.
The dimension of heat is –
(i) M L T
(ii) M L² T^-2
(iii) M L^-1 T^-2
(iv) M L^-1 T^-1
Answer:
ML² T^-2

Question 15.
Generally body temperature of a man is measured in :
(i) Celsius scale
(ii) Fahrenheit Scale
(iii) Kelvin scale
(iv) none of these
Answer:
Fahrenheit

Question 16.
Heat reaches the earth from the sun by the process of –
(i) conduction
(ii) convection
(iii) radiation
(iv) expansion
Answer:
Radiation

Question 17.
The value of mechanical equivalent of heat in CGS system is :
(i) 4.18 × 10⁷ erg / calorie
(ii) 4.8 erg / calorie
(iii) 5.18 × 10⁷ erg / calorie
(iv) 5.18 erg / calorie
Answer:
4.18 × 10⁷ erg / calorie

Question 18.
How much work is to be done to produce 50 calorie of heat ?
(i) 210 joule
(ii) 200 joule
(iii) 201 joule
(iv) 310 joule
Answer:
210 joule

Question 19.
The water equivalent of a body is equal to :
(i) mass of the body x specific heat
(ii) mass of the body / specific heat
(iii) mass of the body + specific heat
(iv) mass of the body – specific heat
Answer:
Mass of the body × specific heat

Question 20.
Mercury remains in liquid state for long range of temperature :
(i) -39° C to 357° C
(ii) -57° C to 457° C
(iii) -10° C to 257° C
(iv) -12° C to 122° C
Answer:
-39° C to 357° C

Question 21.
-40° C is equal to :
(i) -50° F
(ii) -40° F
(iii) -60° F
(iv) -80° F
Answer:
-40° F

Question 22.
1 Calorie is equal to –
(i) 4.18 Joule
(ii) 5.18 Joule
(iii) 3.18 Joule
(iv) 4.18 erg
Answer:
4-18 Joule.

Question 23.
If a temperature in Kelvin scale is TK and the temperature in Celsius scale is t° C, the relation is :
(i) TK = (273 + t°C)
(ii) TK = (273 – t°C)
(iii) TK = (273 / t°C)
(iv) none of these
Answer:
T K = (273 + t°C)

Question 24.
Calorimetry relates to the measurement of –
(i) heat
(ii) temperature
(iii) mechanical energy
(iv) none of these
Answer:
heat

Question 25.
Quantity of heat of a body depends on its –
(i) temperature
(ii) mass
(iii) material of the body
(iv) all of them
Answer:
all of them

Question 26.
Quantity of heat is given by –
(i) \(\frac{m s}{t}\)
(ii) mst
(iii) \(\frac{m t}{s}\)
(iv) \(\frac{m s t}{w}\)
Answer:
mst

Question 27.
Work done W and heat produced H are related to each other as(where J is the mechanical equivalent of heat)
(i) WH = J
(ii) H = JW
(iii) W = J H
(iv) W J H = 1
Answer:
W = JH

Question 28.
The value of mechanical equivalent of heat (in erg/calorie) is –
(i) 4.2 × 10^-7
(ii) 4.2 × 10⁷
(iii) \(\frac{1}{4.2}\)
Answer:
4.2 × 10⁷

Question 29.
The latent heat of fusion of ice is –
(i) 80 cal g^-1
(ii) 3.36 × 105 cal g^-1
(iii) 80 joule kg^-1
(iv) 3.36 cal g^-1
Answer:
80 cal g^-1

Question 30.
Saturated vapours obeys –
(i) Boyle’s law
(ii) Charles’ law
(iii) pressure law
(iv) none of these
Answer:
none of these

Question 31.
Dews are formed at a temperature –
(i) greater than dew point
(ii) less than dew point
(iii) equal to dew point
(iv) at critical temperature.
Answer:
equal to dew point

Question 32.
The volume of water is minimum at a temperature –
(i) -4°C
(ii) 4°C
(iii) 0°C
(iv) 1°C
Answer:
4°C

Question 33.
Thermal capacity of a body of mass m and specific heat s is –
(i) \(\frac{\mathrm{m}}{\mathrm{s}}\)
(ii) \(\frac{\mathrm{m}}{\mathrm{s}}\)
(iii) \(\frac{\mathrm{m}}{\mathrm{s}}\)
(iv) ms
Answer:
ms

Fill in the blanks :

1. Heat is the cause and temperature is the _____.
Answer:
effect

2. Heat is a form of _____ a body possesses.
Answer:
energy

3. The range of temperature between the upper and lower fixed point is known as _____ interval.
Answer:
fundamental

4. Kelvin scale of temperature starts from the temperature corresponding to _____, taken as zero Kelvin.
Answer:
-273° C

5. Mean calorie is _____ of the heat necessary to raise the temperature of 1 gram pure water from 0° C} to 100° C.
Answer:
(\(\frac{1}{100}\)) th

6. 1 calorie = 4.18 or approximately 4.2 _____.
Answer:
joule

7. Specific heat of _____ in SI system is 4200 J/kg/K.
Answer:
water

8. Thermal capacity or heat capacity of a body is defined as the ratio of the heat supplied to a body to its consequent rise in _____.
Answer:
temperature.

9. In _____ process heat is transmitted in wave form.
Answer:
radiation

10. In radiation process _____ is not required.
Answer:
medium

11. Heat reaches the earth from the sun by _____ process.
Answer:
radiation

12. Normal body temperature of a man is _____ degree celsius.
Answer:
36.9

13. 94° F = _____ °C.
Answer:
34.4

14. The water equivalent of a body is = mass x _____.
Answer:
specific heat

15. A body becomes hot by _____ heat and becomes cold on giving away heat.
Answer:
taking

16. Temperature is the condition of a body.
Answer:
thermal

17. Melting point of _____ under normal atmospheric pressure is taken as the lower fixed point of a scale of temperature.
Answer:
ice

18. The quantity of heat required to raise the _____ of unit mass of a substance through one degree is called the specific heat of the substance.
Answer:
temperature

19. When work is completely converted into heat or heat is compietely converted into work, one is _____ to the other.
Answer:
equivalent

20. Work done to produce unit heat is known as _____ equivalent of heat.
Answer:
mechanical

21. Heat is a _____ quantity.
Answer:
scalar

22. In CGS system the unit of specific heat is _____
Answer:
Calorie/g/°C

23. The unit of specific heat in SI system is _____
Answer:
J/kg/K

24. -40°C = _____ °F
Answer:
-40

25. What is the dimension of heat ?
Answer:
The dimension of heat is ML² T^-2.

26. 1°C = ______ °F
Answer:
\(\frac{9}{5}\)

27. in CGS system heat is measured by the unit called _____.
Answer:
Calorie

28. Temperature of a body depends on the _____ contained in it.
Answer:
heat

29. In SI system the unit of thermal capacity is _____.
Answer:
JK^-1

30. 1 kcal = _____ cal
Answer:
1000

31. In case of change of state, heat is _____ or liberated.
Answer:
absorbed

32. Latent heat of fusion of ice is _____ Jkg^-1
Answer:
3.36 × 10⁵

33. Specific heat of _____ is highest than any other common substance.
Answer:
Water

34. The temperature at which dews are formed called _____point.
Answer:
dew

35. The density of water at 4°C is _____.
Answer:
maximum

36. In CGS system, latent heat of vapourization of water is _____.
Answer:
540 cal/g/°C

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 7 Crisis of Environment, Plants and Conservation offer valuable context and analysis.

WBBSE Class 7 Science Chapter 7 Question Answer – Crisis of Environment, Plants and Conservation

(Page no. 227)

Question 1.
What is happening in the pictures given above? Write down the reasons behind them.
Answer:
Storm, Oceanic high tide, melting of snow cap in polar regions, flood, landslides. These occur due to increasing environmental pollution.

Question 2.
Now write the difference between Weather and Climate.
Answer:

Weather	Climate
1. Weather is a day-to-day atmospheric condition.	1. Climate is the average of weather condition for a long period (years)
2. Weather reflects changes of every day every hour even in every moment.	2. Climate reflects condition of a definite specific period of time.
3. Weather is an atmospheric condition of a small place. Weather of two nearly places can be different.	3. Climate is the weather condition of a vast region.

(Page no. 228)

Question 1.
What are the important factors on which weather and climate depend?
Answer:

• Temperature of atmosphere.
• Humidity of atmosphere
• Rainfall.
• Snowfall.
• Movement of wind.
• Ocean current

Try to answer the following question:

1. When do buds appear in a mango tree?
Answer:
In Spring season.

2. When do migratory birds visit our country from abroad?
Answer:
During winter season.

3. When do the Husa fishes lay eggs?
Answer:
In Rainy season.

4. When does Palash bloom?
Answer:
In Spring season.

5. Write the name of some natural phenomena which occur in a definite time of the year.
Answer:

• Merrigold blooms in winter season,
• Sunflower blossoms in summer season.
• Suli flower blossoms autumn season.
• Cabbage and Cauliflower vegetables grow only in winter season.

6. In which time of the year, is our country very warm.
Answer:
Summer season.

7. Do you know the name of any other season apart from Summer, Monsoon, Autumn and Winter?
Answer:
Spring.

8. Does Monsoon arrive at a definite time in every year?
Answer:
Now dueto imbalance in ecosystem as a result of environmental pollution monsoon arrive late than the expected time.

9. How long does the winter stay?
Answer:
For three months.

What can happen if seasons stay for a longer or shorter period of time? Fill up the table below :
Answer:

Name of the season	Problems due to its longer or shorter stay
1. Summer	1. Due to longer stay of summer the water bodies dry up, there is scarcity of water, draught occurs, production of crop is hampered.
2. Rainy	2. Due to heavy rain flood and landslide occur, scanty rain affects the production of rice and other crops
3. Winter	3. If winter stays shorter then the production of vegetables will be affected mainly the production of potato will be hampered.
4. Spring	4. If Spring stays shorter ecological balance will be hampered as in spring many new plants grow and it is mating season of animal world.

(Page no. 129)

Worksheet

1. What is your name? Sufal Mahoto
2. How old are you ? 55 years
3. Write the name of your locality and mention the characteristics of weather of your locality –
Answer:
Midnapore, it is very hot in the month of May, monsoon comes here in June-July months, winter seasons are December and January.

Question 4.
What are the changes that you have experienced in the climatic conditions (temperature, rainfall etc.) in comparison with your childhood?
Answer:

• Average amount of rainfall has decreased
• Temperature had exceeded than before.
• Summer has become very severe than before and it lasts longer than past.
• Winter season remains for shorter period than before.

Question 5.
What are the differences of present day climate with that of 20 years earlier?
Answer:

• Duration of winter season has decreased than before.
• Due to deforestation summer lasts longer and it had become unbearable and severe.
• Average Rainfall is less than before.
• Spring season lasts for shorter time

Question 6.
What can be the reasons behind these changes?
Answer:

• Deforestation.
• Establishment of factories in large number has polluted the atmosphere.
• Excessive use of petrol and diesel vehicles.
• Brick built houses has increased which has lead to increase in temperature.

Question 7.
Do you know about any physical problem that arises due to these changes?
Answer:

• Indigestion and other diseases regarding stomach.
• Skin problem.
• Cancer in various parts of body.
• Asthma and rise in blood pressure.

Question 8.
Trace out different problems (other than physical problems) of your locality due to climatic changes.
Answer:

• Production of food crop has decreased.
• There is scarcity of drinking water as the water of lakes and ponds has become polluted.
• Temperature has increased.
• Flood occurs frequently.

Question 9.
What can be done so that the climatic change do not cause major disruptions?
Answer:

• Afforestation.
• Use of unbaded petrol and of CNG (Compressed Natural Gas) in automobiles.
• installation of tall chimneys in factories and fitting them with filters to prevent pollution.
• Decreasing the number of vehicles on road.

(Page no. 230)

(i) What do you see from the above pictures?
Answer:
Pollution of environment.

(ii) Where does the emitted gases from various sources go?
Answer:
In the atmosphere.

(Page no. 233)

Find out different news on global warming and write them down briefly.

Topic	News
1. Melting of the glaciers.	I. It will cause swage of water in the rivers nourished by the glaciers and then the water will decrease resulting in severe water crisis.
2. Rise in sea level.	2. The coastal areas may become flooded. There will be loss of life and huge economic loss.
3. Destruction of biodiversity.	3. Many marine animals and fishery resources in the oceans and seas will be come extinct, ecological balance will be hampered.

(Page no. 235)

Make a list of the organisms that live near your school or home. If you have seen any organism in other places, then include them in the following list.

Nature of habitat	The plants that you saw (Herb/Shrub/tree)	The animals that you see (vertibrate/invertebrate)
1. Wet land	grass, water-lily, moss.	snake, frog, snail.
2. Wet dyke	moss, small bushes.	Earth-warm, Centipede, Scorpion

Nature of habitat	The plants that you saw (Herb/Shrub/tree)	The animals that you see (vertibrate/ invertibrate)
3. Dense bush near a pond	Sandhyamalati, Aakando. Dhatura	Mongoose, Rat, water bird.
4. Rathole	—	Rat
5. Hole in the thick trunk of a tree.	creepers, mosses	owl. squirrel
6. Termite Hill	grass	Termite, Ant
7. Cracks on the bricks of an old house	Peepal, planting, climbers.	bat; pigeons
8. Roof top of house	—	lizard, behive.
9. Garden	Rose plant, Jasmine, Hibiscus grass, coconut tree.	Cameleon, squirrel, crow, cuckoo, wood pecker

(Page no. 237)

Question 1.
Do we get any benefit from all these animals and plants? Let’s write them down. You can add some other plants and animals to this list.
Answer:

Name of plant	Benefits animal	Name of animal	Benefits
1. Paddy	Services as main food crop for human beings.	1. Panchax fish	serves as food for acquatic animals and birds.
2. Banyan	Gives cool shade, leaves are eaten by animals, birds builts nests.	2. Snake	Eats different harmful insects.
3. Neem	leaves have medicinal value, wood is very useful.	3. Bat	Eats different harmful worms and insects.
4. Coconut	fruit is eaten and also used to make different dishes leaves are useful to make broom, rope, wood is useful	4. Crow	Eats rotten things, dead things and helps to keep our environment clean

(Page no. 241)

Question 1.
Reasons for loss of bio-diversity of earth: Reason for loss of habitat
Answer:

• Deforestation to meet the requirement of wood.
• For increasing demand of agricultural land.
• To increase the habitat area of human being.
• Establishment of industries and factories.

Question 2.
What will happen if The Sundarban is wiped out to meet the requirement of wood?
Answer:
The Sundarbans is natural habitat of many wild lifes especially of Royal Bengal tigers. They will become extinct. We get wax and honey from this forest area that will also not be available. It is a famous tourist spot. So a great source of income from tourism will also come to and end.

Question 3.
Let’s make a list of different animals who fall prey to the greed of the poachers.
Answer:

Substance obtained by killing the animal	Animal killed for getting the  substance	Use of those parts	How can we obtain these subtances without killing the animals.
1. Teeth	Elephant	decorative items	when the animal dies.
2. Hide	Tiger, Lion, Rhinocerous	garments, bag, drum	when the animal dies naturally
3. Horns	Deer	decorative items	can be created artificially
4. Fur	Bear, Sheep	garments, shawl	can be made artificially
5. Musk	Deer	perfume	When the animal dies naturally

(Page no. 242)

Question 1.
What will happen if The number of tiger or other big carnivores decreases significantly in a forest:
Answer:
The ‘food web’ and the flow energy in the different level of animal will get hampered. This will destroy ecological balance.

Question 2.
What will happen if some fishes were introduced in a pond which eat other small fishes.
Answer:
Small fishes will become extinct. These fishes are valuable source of nutrients to our body

(Page no. 243)

Question 1.
What will happen if The amount of rainfall decreases in a certain region?
Answer:
The trees and plants of that area will dry up, agricultural production will be hampered, soil will become dry leading to soil erosion, there will scarcity of drinking water.

Question 2.
What will happen if Industrial waste is poured into the river water?
Answer:
The water will become contaminated, this can spread different diseases, the fishes and other acquatic animals will die.

(Page no. 244)

Question 1.
What will happen in case of large-scale exploitation of a plant that is a source of medicine
Answer:
Then a time will come when that plant will not be available to us any more, so the medicine will not be available as a result the disease will become uncurable.

(Page no. 245)

Wastes and Risks to Human Health :

• Contaminated water.
• Smoke from factories.
• Pesticides in fruits and vegetables.
• Unbygenic condition inside factories.
• Municipal waste causing choking hazard in street animals.
• Plastic bottles and packets daily waste in our day-to-day life.

Mention the nature and components of different wastes shown in the picture.

SI. No	Source of waste	Nature of waste (solid/liquid)	Component of waste
1.	Sewage	liquid waste from domestic activities	Kitchen waste, toilet and other household waste waters.
2.	Factories	gaseous pollutants	CO2, SO2, and CO and also oxides of nitrogen.
3.	Pesticides	solid	pesticides in veg­etables and fruit.
4.	Factories	Solid, gaseous	pieces of glass, plastic, chemicals
5.	Rural and Urban areas	Solid, liquid, gas	rotten things bottle, plastic
6.	Dumping ground of waste.	Solid, liquid	plastic bags, glass, bottles, plastic, rotten things:

(Page no. 246)

Make a list of different wastes from different sources and trace their nature.
Answer:

Source	Name of the waste product	Nature of waste product
1. School	torn paper, broken pen, pencil, polythene.	snake, frog, snail.
2. Rice Mill	Husk of paddy, ashes	solid
3. Pond	dirty water, cleaning agent like detergent, soap.	solid; liquid
4. Agricultural	chemical fertilizer, pesticide, straw, husk	solid
5. Hospital	Syringe, cotton. wrapped of medicine, bottle.	solid
6. Hospital	Syringe, cotton, wrapper of medicine, bottle.	solid
7. Playground	plastic bottle, torn papers	solid
8. Bus stand	shell of ground nut, torn paper, bottles	solid
9. Houses	peel of fruit and veg­etable, torn clothes paper, bottle.	solid

(Page no. 247)

Diseases are as follows :

1. Infectious diseases spread by vectors (Mosquito fly, rat, cat, dog)-Malaria. Filaria. Dengue, Diarrhoea, Plague, Diptheria. Rabies)
2. Infectious diseases spread by used clothes of patients of hospitals – Hepatitis, Jaundice, Phenuiuonia, Malaria, Tuberculosis
3. Infectious diseases spread by different untreated compound or metals of factories – Cancer, nervous disorder, pain in bones, skin disease, lungs problem, Asthma.

Have you noticed symptoms of such disease in people known to you? Write it down.
Answer:

Where does he work	What can be the reason	What is the problem	What to do
1. Asbestos factory	Asbestos	Pulmonary problem	Should consult doctor
2. Cement factory	powder of cement	Pulmonary problem	Should consult  doctor
3. Pesticide factory	Smell of chemicals	Breathing Problem, consult Cancer	Should consult doctor
4. Alcohol factory	Alcohol	problem relating to stomach, problem of indigestion	Should consult doctor

Now find out what unhealthy habits or reasons are responsible for these diseases or risks of diseases
Answer:

Disease/Risk	Cause	Which unhealthy nature/lack of precaution is responsible	What to do
Hepatitis	Viral infection through water	Drinking polluted water	One should drink boiled water
Malaria	Bite of Mosquito	Sleeping without mosquito net	One should sleep inside mosquito net instead of only using mosquito coil
Cholera	Virus in drinking water	Drinking polluted water, coming in contact with disease person	One should boil water before drinking
Plague	Rats	Not killing rats inside shops and houses.	We should keep our house and shop clean,

(Page no. 249)

Teacher showed these pictures to the students and asked to complete the following table on role of plant in one’s life.
Answer:

What will happen if there are no plants	What will happen if plants are present
1. Environmental pollution.	1. Rainfall will occur.
2. Draught and flood occur.	2. Oxygen supply increase so the greenhouse effect will decrease.
3. Green house effect increases.	3. Supply of food remains.
4. Scarcity of food.	4. Ecological balance remains.
5. Ecological balance becomes disturbed.	5. Stops soil erosion.
6. Rainfall will decrease.	6. We gel life saving medicine.

(Page no. 250)

Describe how do the following animals depends on forest plant for food.
Answer:

Name of the animal	Food and their sources	Does they get same amount throughout the year	What will happen if sources are lost
Dear	grass, leaves	No. it decreases during summer	supply of food will be hampered
Elephant	leaves	No, decreases in summer	supply of food will be hampered
One horned Rhino	grass, small shrubs	No	may die in absence of food.
Bat	fruits	Yes	may extinct
Squirrel	fruits and nuts	Yes	shortage of food
Hornbill	Cereals	Yes	will die
Red Ponda	leaves of Bamboo	No	shortage of food
Monkey	leaves, fruit flower, root	No	supply of food will be hampered
Goat	leaves, gram	Yes	shortage of food

Answer:
1. Nectar of flower → Bee → Lizard → Eagle
2. Leaf → Deer → Tiger
3. Rotten leaves → Fish → Civet cat
4. Twig → Elephant
5. Grain → Small bird → Hawk

Now construct different food chain from the following plants and animals.
Grass. goat. small fishes. boal fish. Kingfisher. Crane. Kite. Rat. Hawk. Rice plant, Stem borer, Hen. Leopard, Zebra. Lion, Rhino, Tiger.

SI. Organisms of food chain No.	Shape of food chain
1. Grass → goat → Lion	Forest food chain
2. Rice plant → Rat → Kite	Grassland food chain
3. Stem borer → Zebra → Leopard	Forest food chain
4. Small fish → boal fish → Crane	Acquatic food chain
5. Rice plant → Hen → Hawk	Grassland food chain

(Page no. 253)

If there is any mango/banyan/peepul/simul/tamarind tree near your home, then notice that which animals are living there.

1. Insects — Grasshopper. Honey bee
2. Birds — Bulbul, owl, crow
3. Mamais — Bat, Monkey
4. Reptiles— Snake, Chameleon, Lizard

(Page no. 254)

If one plant can give shelter to all these animals then you can understand that how many animals can live in a forest. Now can you say which animals live in the following forests —

1. Coniferous forest — Snow Leopard, Elephant, Snake, Tiger.
2. Deciduous forest — Tiger, Lion, Rhinoceros, Elephant.
3. Mangrove forest — Tiger, Deer, Crocodile, Bee, Birds.
4. Grassland forest — One-horned Rhino, Deer, Zebra, Giraffe
5. Weed forests of wetland — Mongoose. Civet cat, Iguana.
6. Bushes and trees around your house — Iguana, Cat, Frog, Snake. Squirrel.

Many animals are becoming extinct on earth due to loss of plant. Name some of these animals which depend exclusively on plants.

Name of the animal	Depends on plant for food/shelter
1. Adjutant stork 2. Owl 3. Civet Cat 4. Langur	peepal Mango Tamarind Arjun, Banyan

Class 7 Science Chapter 7 Question Answer West Bengal Board – Crisis of Environment, Plants and Conservation

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
The ability to absorb CO₂ in different plants is ……………….
i) same
ii) different
iii) may be same, may be different
Answer:
ii) different

Question 2.
Corals become affected due to the increase in ……………….
i) acidity
ii) temperature
iii) acidity and temperature
Answer:
iii) acidity and temperature

Question 3.
Indiscriminate use of chemicals can bring about the destruction of ……………….
i) animals
ii) plants
iii) animals and plants
Answer:
iii) animals and plants

Question 4.
Trees of a jungle are felled ……………….
i) to produce luxury materials
ii) for increasing agricultural land
iii) to increase the habitable areas
iv) all are true
Answer:
iv) all are true

Question 5.
Musk is obtained by killing ……………….
i) deer
ii) elephant
iii) bear
iv) sheep
Answer:
i) deer

Question 6.
Hide is obtained by killing ……………….
i) elephant
ii) bear
iii) tiger
iv) sheep
Answer:
iii) tiger

Question 7.
Horns of deer are used for……………
i) decorative items
ii) medicine
iii) perfume
iv) garments
Answer:
i) decorative items

Question 8.
Reserpine is received from ……………
i) Penicillium
ii) Rauwolfia
iii) Basil
iv) Cinchona
Answer:
ii) Rauwolfia

Question 9.
The habitate of grass, moss, water-lily is ……………
i) near a pond
ii) termite hill
iii) wet dyke
iv) wet land
Answer:
iv) wet land

Question 10.
The ice cover of Alaska Bay adjoining the North Pole has reduced by 40% in the last ……………
i) 10 years
ii) 30 years
iii) 20 years
iv) 40 years
Answer:
ii) 30 years

Question 11.
The average temperature of earth had increased by 1°C in last ……………
i) 100 years
ii) 50 years
iii) 200 years
iv) 150 years
Answer:
i) 100 years

Question 12.
More than 99% of the glacieric are present in ……………
i) North poles
ii) South poles
iii) North and South poles
Answer:
iii) North and South poles

Fill in the blanks : (1 mark for each question)

1. …………… is an atmospheric condition which changes every day, every hour even in every moment.
Answer:
Weather

2. …………… is the average of weather of a particular area for a long period (years).
Answer:
Climate

3. Climatic changes have an important correlation with ……………
Answer:
global warming.

4. CO₂ is one of the important gases produced by …………… activities.
Answer:
anthropogenic

5. The biggest source of fresh water on the earth is ……………
Answer:
glaciers

6. More than …………… of the glaciers are present in North and South poles of earth.
Answer:
99%.

7. The …………… nourishes nine big rivers of Asia.
Answer:
Himalayas

8. …………… is the shelter for the largest clan of the world.
Answer:
Sunderbans

9. …………… is a type of marine invertebrate animal.
Answer:
Coral.

10. The biodiversity of our country is so rich that India is termed as a …………… Nation.
Answer:
mega diversity.

11. We get Quinine from ……………
Answer:
Cinchona.

12. Ranwolfia medicine is derived from ……………
Answer:
Keserpine.

State whether True or False : (1 mark for each question)

1. Climate is the average for small period of time.
Answer:
Flase

2. Hilsa fish lay eggs in the rainy season.
Answer:
True.

3. Climatic changes does not have any relation with global warming.
Answer:
False.

4. Some glaciers are also found in Himalayas.
Answer:
True.

5. Sea level may fall due to the melting of these glaciers.
Answer:
False.

6. The coastal areas may become flooded if the sea level rises.
Answer:
True.

7. Glaciers reflect nearly 20% of the sunlight and absorbes 80 of it.
Answer:
False.

8. Coral exoskeleton is made up of sodium bicarbonate and it protects the body.
Answer:
False.

9. Dead corals and other organic materials help in forming the coral island.
Answer:
True.

10. The temperature of Indian Ocean have risen due to global warming.
Answer:
True.

11. India is the habitat of 30% of all the plants of the world.
Answer:
False.

12. Introduction of Tilapia and giant African magur in the ponds of rural Bengal posed a huge problem for the existence of indigenous fishes.
Answer:
True.

Match the column A With column B.

Column A	Column B
a) Coral reefs are in danger b) Glaciers reflect nearly 80% c) 2005 was the warmest year d) Weather of two nearby places c) Some gases are naturally produced f) Some glaceries are found g) In 1988 nearly 16% of corals h) Bengal vulture is i) Hepatitis is a j) The plant with greater surface area	i) in the atmosphere ii) of the world were lost forever iii) of leaf absorb more CO2 iv) due to global warming v) viral infection vi) on the verge of extinction vii) of sunlight viii) can be different ix) in the Himalayas x) in the last century

Answer:
a) Coral reefs are in danger due to global wanning.
b) Glaciers reflect nearly 80% of sunlight.
c) 2005 was the warmest year in the last century.
d) Weather of two nearby places can be different.
e) Some gases are naturally produced in the atmosphere.
f) Some glaceries are found in the Himalayas.
g) In 1988 nearly 16% of corals of the world were lost forever.
h) Bengal vulture is on the verge of extinction,
i) Hepatitis is a viral infection.
j) The plant with greater surface area of leaf absorb more CO₂

Very Short Questions & Answers : (1 mark for each question)

Question 1.
Which animal eat the egg of the turtle in Galapagos Island?
Answer:
Dogs and pigs used to eat the eggs of the turtle.

Question 2.
From which country Tilapia and giant Magur came?
Answer:
They came from Africa.

Question 3.
Name three fishes which are on the verge of being extinct?
Answer:
Mourala, Punti, Khalse.

Question 4.
The polar ice caps are melting, why?
Answer:
Due to global warming.

Question 5.
Name some animals that live in the Coral reefs.
Answer:
Fishes, molusca, echinodermata, sponge, Crustacea etc animals live in the coral reefs.

Question 6.
What is Global Warming?
Answer:
Due to unplanned human activities the amount of carbon dioxide and other gaseous compound in the atmoshere are increasing. This gaseous substances in turn retain the excess heat on earth. As a result, the temperature of earth increases than what is normal. This is Global Warming.

Question 7.
Name some gases which are present in smoke emitted from different sources.
Answer:
Carbon dioxide (CO₂), Methane (CH₄), Nitrous Oxide (N₂O). Chloro-Fluoro-Carbon. Sulpher di-oxide (SO₂) and particles of dust, Carbon etc.

Question 8.
Name some animals which are on the verge of extinction?
Answer:
Royal Bengal Tiger, Bengal Vulture. One-horned Rhinoceros. Gangetic Dolphin. Cheetah, Chimpanzee, Orangutan and Gorilla.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is Coral? Write a short note on it.
Answer:
Coral is a type of marine invertebrate animal. It be longs to the Phylum Cuidaria. They live together by forming colonies. They form an exoskeleton surrounding their body. This exoskeleton is made up Calcium Carbonate which protects the body. The exoskeletons are very colourful and have high demand in the fields of making of fancy items and jewelleries.

Question 2.
What is Food Chain?
Answer:
Green plants trap solar energy and make their food by photosynthesis. Animals cannot make their own food, so they obtain energy by eating plants or by eating animals which have eaten plants. This predator-prey relationship is called food chain.

Question 3.
What do we get from biodiversity?
Answer:

• Diversity of food crops helps us to get all kinds of nutrients necessary to our body and also helps to increase agricultural production.
• Plants and micro-organisms can absorb CO₂, and maintain O₂ – CO₂ balance in environment.
• Rooted plants increases the water retention capacity of the soil and also sustain the rainfall by returning back the excess water in the form of water vapour.
• Fungi and bacteria help in maintaining the level of nutrients present in the soil.
• Biodiversity of plants is a source of life saving medicine, wood, and other industries like silk industry, Lac industry etc.

Question 4.
What are the reasons of loss of biodiversity of earth?
Answer:
The reasons behind the loss of biodiversity of earth are as follows —

• Deforestation is done to meet the requirement of wood agricultural land and habitat area for human beings, so other organisms are losing their habitat.
• Illegal hunting or poaching is done to get teeth, hides, horns, fur and other body parts of animals. This greed is responsible for endangering the lives of different animals.
• If an organism cannot adapt with the climatic changes its existence will be in question. Animals like Emperor Penguin, Polar Foxes, Polar Bears are endangered due to melting of ice in polar region.
• Increasing environmental pollution is another reason for destruction of animals and plants.
• If some plants or animal is economically important then excessive use of them can be dangerous for their existance.

Question 5.
Name four Biodiversity hot spots of India.
Answer:

• Eastern Himalayas : Sikkim, Darjeeling, Dooars and Terai region.
• Western Ghat and Srilankas: The hilly forest region along the western coast of India.
• Indo-Burma: North-eastern states of India (e.g.: Meghalaya, Arunachal Pradesh)
• Sundaland : Andaman -Nicobar region of India.

Question 6.
How is plant helpful to us?
Answer:

• Due to plants water cycle remains on earth.
• Temperature of environment is controlled by plant.
• Velocity of storm is checked by plant.
• Environmental Pollution is prevented by plants.
• Plant stops soil erosion.
• Plants provide shelter of organisms.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 3 Atom, Molecule and Chemical Reaction offer valuable context and analysis.

WBBSE Class 7 Science Chapter 3 Question Answer – Atom, Molecule and Chemical Reaction

(Page no. 85)

Try to write the symbols from names of elements given below in the first two tables.

table 1

Name of Element	Symbol
Aluminiun	Al
Nickel	Ni
Arsenic	As
Silicon	Si
Zinc	Zn
Boron	B

table 2

Name of Element	Latin name	Symbol
Tin	Stannum	Sn
Mercury	Hydrogirum	Hg
Lead	Plumbum	Pb

Now we will learn the names and symbols of elements that originate from the names of famous scientists, the place of discovery, the native country of the discoverer or some planet.

(Page no. 87)

Now fill up the following table using the diagram for atomic structure of Helium atom as shown on the previous page.

Impact the diagrams above and fill up the following table :

(Page no. 88)

Fill up the following table with help from your teacher, if necessary.

(Page no. 89)

Discuss among yourselves and fill up the following table :

Write the valencies of radicals from the number of Sodium or Potassium atom.

(Page no. 90)

Now we shall determine the valencies of different metals and non-metals from their compounds with Chlorine. The number of Chlorine atoms which combine with an atom of an elements is the valency of that element.

(Page no. 92)

Symbols and valencies of some elements are given below. Write down the formula of the compounds by using the above procedure.

Discuss among yourselves and fill up the following table using the valencies of elements and radicals.

(Page no. 93)

Now discuss among yourselves and fill up the following table.

(Page no. 97)

Discuss among yourselves and balance the chemical equations by putting suitable numbers in the blank places (. . .) in them

Write the names and formula of products and balanced equations of the reactions by discussing among yourselves.

Discuss anong yourselves and write the formula (e) of missing product (s) in each case.

(Page no. 100)

Discuss among yourselves and write the type of reaction against each equation in the table.

Class 7 Science Chapter 3 Question Answer West Bengal Board – Atom, Molecule and Chemical Reaction

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
Symbol of Arsenic is-
i) Ar
ii) As
iii) An
iv) Ac
Answer:
ii) As

Question 2.
Symbol of Mercury is-
i) Mc
ii) Me
iii) Mu
iv) Hg
Answer:
iv) Hg

Question 3.
Symbol of Lead-
i) Pb
ii) Ld
iii) Le
iv) Pm
Answer:
i) Pb

Question 4.
Formula of Lead Chloride is-
i) pbCl
ii) pbCl³
iii) pbCl₂
iv) pbCl₃
Answer:
iii) pbCl₂

Question 5.
Formula of Ferric Chloride is-
i) FeCl₂
ii) FeCl₃
iii) FeCl³
iv) FeCl²
Answer:
ii) FeCl₃

Question 6.
Formula of Potasiam Chloride-
i) KCl²
ii) KCl₂
iii) PCl
iv) KCl
Answer:
iv) KCl

Question 7.
Formula of Cupric Chloride-
i) CuCl₂
ii) CuCl
iii) CuCl²
iv) CuCl₃
Answer:
i) CuCl₂

Question 8.
Formula of Sodium Sulfate is-
i) NaSO
ii) NaSO₄
iii) Na₂ SO₄
iv) Na₂ SO₂
Answer:
iii) Na₂ SO₄

Question 9.
Formula of Aluminium Oxide is-
i) AlO₃
ii) Al₂ O₃
iii) Al₂ O₂
iv) AlO
Answer:
ii) Al₂ O₃

Question 10.
Formula of Silver Chloride is-
i) AgCl₃
ii) AgCl₂
iii) Ag₂ Cl
iv) AgCl
Answer:
iv) AgCl

Fill in the blanks : (1 mark for each question)

1. Formula of Magnesium Chloride is ______.
Answer:
MgCl₂

2. The atomic number of an element is 6 and mass number is 12 then the number of neutrons in nucleus would be ______.
Answer:
6.

3. In L orbit maximum _____ electrons can revolve.
Answer:
8 .

4. In compound P₂ O₅ the valency is ______.
Answer:
5.

5. Symbol of Aluminium Oxide is ______.
Answer:
Al₂ O₃.

6. Atomic weight of carbondioxide is ______.
Answer:
44.

7. In chemical equation the atomic number of both sides are ______.
Answer:
equal.

8. Symbol of Silver is ______.
Answer:
Ag.

9. Valency of Carbon is ______.
Answer:
4.

10. Mass of one electron particle is ________ of one Hydrogen atom.
Answer:
1 / 1837.

State whether True or False : (1 mark for each question)

1. Hydrogen reacts with nitrogen to produce ammonia.
Answer:
true

2. Chemical reaction is of one type.
Answer:
false

3. Ammonia is formed by the reaction of nitrogen and hydrogen.
Answer:
true

4. Mercury and oxygen are produced when mercuric oxide is heated.
Answer:
true

5. Formula of Ferric Oxide is Fe₂ O.
Answer:
false

6. Slacked lime is produced by the reaction of water with quicklime.
Answer:
true

7. Water is produced by chemical combination of oxygen and hydrogen.
Answer:
true

8. The formula of hydrogen sulfide is H₂ S₂.
Answer:
false

Short Questions & Answers : (1, 2 marks for each question)

Question 1.
What is the meaning of Greek word ‘Atom’?
Answer:
Meaning of the word ‘Atom’ is indivisible.

Question 2.
What is atom?
Answer:
The smallest particle of an element that retains the properties of the element is called an atom.

Question 3.
What is the mass number of an atom?
Answer:
The total number of protons and neutrons in the nucleus of atom is called the mass number of that atom.

Question 4.
What is Atomic Number?
Answer:
The number of Protons present in the nucleus of an aton of an element is known as the Atomic Number of the element.

Question 5.
What is Mass Number?
Answer:
The total number of Protons and Neutrons present in the nucleus of an atom of an element is known as the Mass Number of the element.

Question 6.
What is the difference between H₂ and 2H ?
Answer:
H₂ means 2 atoms of Hydrogen and 2H mears 2 molecules of Hydrogen.

Question 7.
Write the name of an element whose atom has no neutron.
Answer:
Ordinary Hydrogen atom or Protium.

Question 8.
What is the largest number of electron in K orbit?
Answer:
The largest number of electron in K orbit is 2 .

Question 9.
What is the atomic number of Helium?
Answer:
The atomic number of Helium is 2 .

Question 10.
How do the atoms exists in molecules?
Answer:
Ator exists in combined state.

Question 11.
Write the symbols of Potassium and Mercury.
Answer:
Symbol of Potassium is K and Mercury is Hg.

Question 12.
Who founded the theory of Molecules?
Answer:
Amedeo Avogadro.

Question 13.
What is total number of Natural Element?
Answer:
92.

Question 14.
What is ti.e molecular atomicity of Phosphorus.
Answer:
4.

Question 15.
What is tlit valency of Phosphate (PO₄) ?
Answer:
Valen y is 3.

Question 16.
If the orbit gues away from nucleus what would happen, will the number of electron increase 0 – decrease?
Answer:
Nurber c elect:on would increase.

Question 17.
W at is :ine charge of Nucleus?
Answer:
Pusitive.

Question 18.
A + B = Carbond 1 oxice, write the reacting element A and B of this reaction.
Answer:
A is Carbon (C) and : 3 is Oxygen (O₂)

Question 19.
Why the symbol of iron is Fe?
Answer:
The Latin nam: of iron is Ferrum. So the symbol is taken as Fe.

Question 20.
What is the total number of atoms in molccule of Ammonia?
Answer:
In Amonia (NH₃) molecule, the number of atoms are 4.

Question 21.
Which elements have zero valency?
Answer:
Non-reacting elements like inert gases (He, Ni, Ar etc.) have zero valency.

Question 22.
Name the element and the radical whcih have same valency.
Answer:
The element Sodium (Na) and radical Nitrate (NO₃) have same valency.

Question 23.
Can we say Neutron that is the main constituent of fromation of atoms of all substances?
Answer:
No, we cannot say that neutron is the main constituent.

Question 24.
What is the valency of Nitrogen in compound Ammonia?
Answer:
Valency of Nitrogen is 3 in Ammonia- (NH₃).

Question 25.
What is the charge of Electron?
Answer:
Negative.

Question 26.
When we can say an equation is balanced?
Answer:
If the number of atoms of each element on the left hand side is equal to the number of atoms of that element on the right hand side, then we can say that the equation is balanced.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is symbol? Give example.
Answer:
When the name of an element is represented by a single letter or two letters of english alphabet it is called symbol of that element. By using symbol we can express briefly an atom. As example – symbol of Aluminium is Al, symbol of Mercury is Hg.

Question 2.
What is sign?
Answer:
The way of expressing briefly the molecules of elements and compounds is called sign. For example –
Carbon-di-oxide (CO₂), Sulphur-di-oxide (SO₂). Nitric Oxide (NO).

Question 3.
Explain the process of writing symbols.
Answer:
(1) For some elements the first letter of the name of the element written in capital. is used as Symbol.
Example: H – Hydrogen, O – Oxygen,
S-Sulphur, \quad N-Nitrogen.

(2) When there are two or more than two elements whose names start with the same alphabet. the elements are given a two-letter symbol. The first letter of the symbol is the first letter of the name of the element, written in small letters.
Examples –
B – Boron, Be- Beryllium, Br- Bromine

(3) The symbol of each element is written in capital letter.
Example –
Carbon -C, Oxygen -O, Nitrogen -N.
(4) The symbols of a few elements are derived from their Latin names.
Examples –
Sodium – Na (derived from latin name Natrium); Iron – Fe (latin name Ferrum).

Question 4.
What is an element?
Answer:
A element is a pure substance that is made up of identical minute particles called atoms. Elemelits cannot be split into two or more simplers substances by chemical methods. There are only about 114 elements known till date, out of which 92 are found in nature.

Question 5.
What is molecule?
Answer:
Atoms sometimes exist alone but more often they combine with other atoms of the same or different kind. A group of atoms formed by the combination of atoms is called a molecule. A molecule can exist by itself. A molecule may contain similar or dissimilar atoms.

Question 6.
How many particles are there in an atom?
Answer:
Three types of particles are present in an atom –
i) Proton,
ii) Electron,
ii) Neutron.
(-) Indicates particles called Proton. These are charged with positive electricity or positively charged particles.
(O) Indicates particles called electrons. These are negatively charged particles.
( ) Indicates particles called Neutron. They have no electricity.

Question 7.
What is nucleus? Where do the electrons revolve?
Answer:
The Protons and Neutrons are grouped at a tiny place in the center of an atom. This place is called nucleus of an atom.
Electrons revolve in differenrt paths around the nucleus. The paths around the nucleus in which electrons can revolve are called orbits.

Question 8.
What are ions?
Answer:
The atoms whcih attain positive or negative charges due to gain or loss of one or more electrons are called ions.
The positively charged ions formed by the loss of electrons from an atom are called cations. The negatively charged ions formed by the gain of electrons by an atom are called anions. As example Na^{+is a cation and Cl^{-is an anion.

Question 9.
What are radicals?
Answer:
Sometimes one or more than one atom of same or different elements assemble of to form ions. These are called radicals. These radicals participate in chemical reaction in such assembled condition.
As example – Radical Lead (Pb⁺²)., Radical Nitrate (NO₃ ^–)

Question 10.
What is Valency?
Answer:
The capacity of elements to combine with each other is called their valency.
Valency is equal to the number of electrons of an atom can donate or accept to attain the stability of a complete valence shell. The elements with valency 1,2 and 3 are respectively called monovalent, divalent and trivalent element.
Example –
Valency of Sodium(Na) is 1.
Oxygen (O) is 2.
Nitrogen (N) is 3.

Question 11.
What is variable valency?
Answer:
Various elements like Iron, Copper etc. possesses more than one valency. These elements can form different compounds by changing their valencies when they combine with different number of atoms of the same element. These elements have variable valencies. For example, iron forms two compounds with chlorine, FeCl₂ (Ferrous Chloride) and FeCl₃ ( Ferric Chloride). The valency of Iron in the two compounds is 2 and 3 respectively.

Question 12.
What is a chemical equation? Define reactants and products.
Answer:
If we express the chemical reaction in brief by using the formulae of reactants and products, then it is called a chemical equation.
The elements or compounds that take part in the chemical reaction are called the reactants.
The elements or compounds that form as a result of the chemical reaction are called the products.

Question 13.
How many types of chemical reactions are there?
Answer:
There are four types of chemical reactions –

1. Combination reaction.
2. Dissociation reaction.
3. Displacement reaction.
4. Double displacement reaction.

Question 14.
What is combination reaction? Give an example.
Ans:
The chemical reaction in which two or more elements or compounds combine to form a single new compound is called a combination reaction.
For example :-
Water is produced by combination of oxygen and Hydrogen.

Question 15.
What is dissociation reaction? Give example.
Answer:
When a single compound is dissociated into more tharrone subtance by the influence of heat or electricity then such type of reaction is called dissociation reaction.
For example –
When Potassium Chlorate is heated, oxygen is liberated.

Question 16.
What is Displacement reaction? Give example.
Answer:
Such a reaction, where one element takes its place by replacing another element of the compound is called displacement or substitution reaction.
For example – Iron displaces Copper from Copper Sulphate solution

Question 17.
What is double displacement reaction? Give example.
Answer:
The chemical reaction in which two compounds exchange their radicals is called a double displacement reaction.
For example –
If dilute hydrochloric acid (HCl) is added to a solution of Silver Nitrate, a white insoluble solid (Silver Chloride) is formed.

Question 18.
What information we can get from symbol CO₂ ?
Answer:

1. Carbondioxide is a compound.
2. It is a molecule of Carbondioxide.
3. In a molecule of Carbondioxide there is one atom of carbon and two atoms of oxygen.
4. Molecular weight of carbondioxide is 44.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 8 Environment and Public Health offer valuable context and analysis.

WBBSE Class 7 Science Chapter 8 Question Answer – Environment and Public Health

(Page no. 256)

1. Which instrument do we use to measure temperature?
Answer:
thermometer.

2. Which kind of street lamps are now being used instead of electric bulbs and tube lights?
Answer:
Fluroscent bulb

3. A special kind of metal, liquid in nature is used to make thermometer and fluroscent bulb. Name it.
Answer:
mercury.

4. Now, let us find out the other sources from which mercury can enter the human body.
Answer:

• mirror
• light
• vegetables
• water.

(Page no. 257)

1. Now, let us find out the other sources through which Arsenic and Fluoride can enter our bodies.
Answer:

• water
• pesticide
• food crops.

2. Write the effects of X-ray on different organs of human below.
Answer:

• Oesophagous – Tendency of vomiting, Diarrhoea
• Lung-Breathing trouble.
• Bone marrow-Improper formation of blood cell. Haemorrhage, Cancer.

(Page no. 260)

Besides many other physical factors increase the risks and cause many diseases. Discuss with your friends or teacher and complete the chart.
Answer:

Physical factors causing disease in the professional field	Symptoms/disease
1. Electricity	Shock, Heart problem, Nerve disorder.
2. Heat	Burn, boils, injury
3. Cold	Frost bite, fever, cough and sneezing
4. Sound	Deafness, Headache.
5. Radioactive element	Anaemia, Cancer, skin disease.

B. 1. Compounds of which metal did ancient Egyptians used to colour their potteries? (Iron/ Copper/Zinc) Copper.
2. Which gas can cause death in closed mine? (Hydrogen/Oxygen Carbon monoxide) Carbon- mono-oxide.
3. Which chemicals are used in colouring cosmetic cotton and toilet articles? (water/ Sodium chloride/Synthtic organic dies) Synthetic organic dies.
4. Which metal or metallic compounds are used in wall paints, making toys or making batteries? (Iron, Sodium, Lead) Lead.
We came to know about many components of chemical environments from the above discussion.

As per chemical nature these are-

Chemical Nature	Name of the component
1. Gas	Carbon-di-oxide, Carbon monoxide, Sulphurdioxide.
2. Metal	Mercury, Flouride.
3. Synthetic compound              ‘	Organic dies, chemicals.

Now discuss with your friends/teachers/doctors what diseases may occur in being engaged in above mentioned professions.

• Diabetes
• High blood pressure
• Pain in neck
• Insomnia
• Sugar
• Arthritis.

(Page no. 262)

Try to write down the diseases on problem related to the following professions :
Answer:

Professions	Related diseases/problems
1. Those who pick up tea leaf	decay of fingertips
2. Those who are traffic police.	breathing problem, poor eyesight, restlessness.
3. Those who are carpenter.	breathing problem, muscle cramp
4. Those who carry goods	pain in neck, anaemia
5. Rickshaw-puller	Arthritis, muscle cramp
6. Those who cook in front of oven	ache in eye, breathing problem, diz ziness
7. Those who work in cotton factory	breathing problem, anaemia
8. Those who are hospital worker	dizziness, vomiting tendency
9. Those who are worker in mines	breathing problem, dizziness, wound in skin
10. Those who are worker in construction industries.	Arthritis, cancer, wound in skin, muscle cramp, brittle bone
11. Those who work in laboratories.	ache in eye, poor eye sight, vomiting tendency, less or more hunger
12. Those who works in different factories	deafness, restlessness, cancer, heat cramp
13. Those who work in Jute factories	faint, anaemia
14. Those who are hawker	heat cramp, varicose vein of legs, less or more hunger

(Page no. 273)

Which of the following picture will you choose to control your anger and why ?

Listening Music, Do exercise, singing, Reading Books, Playing, Face to Face talking, Dancing with Music, Watering in the garden, Meditation or Yoga.

Way out of controlling anger	Why did you choose this method
Reading books.	It gives pleasure and calms down our mind.
Meditation or Yoga.	relieves mental tension and increases concentration.
Dancing with Music.	It gives joy and relieves tension.
Playing.	Increases concentration, calms mind.
Watering in the garden.	It gives pleasure and increases love for nature

(Page no. 274)

Name of the disease	How it spreads
1. Common cold	1. By air
2. Dysentery	2. By water
3. Malaria	3. By mosquito
4. Plague	4. By rats
5. Rabies, Hydrophobia	5. By dogs.

(Page no. 275)

Can you tell me how diseases spread through air? Let us write how common cold spreads.

How does common cold spread?	What precautions should a person suffering from common cold take so that the disease does not spread?
1. By sneezing	1. We should use handkerchief while sneezing.
2. By spitting or coughing	2. We should not spit or cough in front of any person.
3. Talking with affected person	3. We should keep distance or cover our mouth while talking
4. By air	4. We should not move freely in pub­lic places.

(Page no. 277)

Now discuss with your friends. Try to find out who among you suffer from the same problems.

Cause behind the problem.	Nature of the problem	How did you get rid of the problem
I. Eating eggs	rashes in body, sneezing, vomitting	take medicine
2. Dust entering the nose.	burning sensation in eyes and nose sneezing, water comes out from nose.	use handkerchief
3. Eating Prawn.	red rashes, itching sensation	take medicine
4. Inhaling pollen of flower.	sneezing, burning sensation in nose.

Measures to prevent air-borne diseases :

• When ill, at the time of sneezing/ coughing cover your mouth and nose with a handkerchief.
• Do not spit here and there.
• Do not come in close contact with affected person.

(Page no. 278)

Fill up the following table after bearing about some other air borne diseases.

(Page no. 285)

Fill up the following table after barning about some water-borne disease.

(Page no. 288)

Try to write the name of some other disease which are spread by animals.

Name of the disease	The organism that spreads the disease
1. Malaria 2.  Dengue 3.  Typhoid 4.  Plague 5. Cholera 6.  Yellow Fever 7.  Rabies 8.  Filaria (Elephantiasis)	bite of female Anopheles mosquito bite of Aedes aegypti mosquito, rod-shaped bacteria Salmonella Typhi bite of rat fliesAedes mosquito biting of dogs and foxes small worm Wuchereria

Class 7 Science Chapter 8 Question Answer West Bengal Board – Environment and Public Health

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
Which one of the following in water-borne disease?
i) Diabetes
ii) Cholera
iii) Allergy
iv) Scurvy
Answer:
ii) Cholera.

Question 2.
Which of the following disease has been eradicated?
i) Small Pox
ii) Plague
iii) Polomyelities
iv) Kala-zar
Answer
i) Small-Pox

Question 3.
Well known effective drug for Malaria is ………….
i) Penicilin
ii) Paracitamol
iii) Quinine
iv) Cough syrup
Answer:
iii) Quinine.

Question 4.
A viral disease is …………..
i) Cancer
ii) Leukaemia
iii) Typhoid
iv) Yellow fever
Answer:
iv) Yellow fever

Question 5.
Dehydration results from …………..
i) Pneumonia
ii) Measles
iii) Rabies
iv) Cholera
Answer:
iv) Cholera.

Question 6.
Name of the British army doctor who proved that Malaria is a mosquito-borne disease is ………
i) Louis Pasteur
ii) Ronald Ross
iii) Dr. Edward Jenner
iv) Joseph Lister
Answer:
ii) Ronald Ross

Question 7.
Aedes mosquito spreads ………
i) Dengue
ii) Typhoid
iii) Malaria
iv) Kala-zar
Answer:
i) Dengue

Question 8.
Which is a beneficial fungi?
i) Neurospora
ii) Claviceps
iii) Penicuilium
iv) Rhizopus
Answer:
iii) Penicillium.

Question 9.
A harmful chemical added to chowmin and chilly chicken to impart a meaty flavour is ……………….
i) Clove
ii) Turmeric
iii) Ajinomoto
iv) Chilly
Answer:
iii) Ajinomota

Question 10.
Which seed is mixed with mustard seed—
i) Poppy seed
ii) Rapeseed
iii) Gramseed
iv) Argemone seed
Answer:
iv) Argemone seed.

Fill in the blanks : (1 mark for each question)

1. The…………. look after our mental health.
Answer:
psychologists.

2. The menance of …………. has been fully eradicated from the world by vaccination alone by the year 1977.
Answer:
small pox.

3. Tuberculosis is a …………. disease.
Answer:
communicable

4. The substances that cause allergy in our body are termed as ………….
Answer:
allergen.

5. …………. spreads from contaminated water.
Answer:
cholera.

6. Diseases which spread through water are termed as ………….
Answer:
water-borne diseases.

7. …………. is one type of malarial parasite.
Answer:
Plasmodium Vivax.

8. Mosquito is the …………. host and other vertebrate animals are the host.
Answer:
primary or definitive, secondary or intermediate.

9. …………. spread diseases like Typhoid and Diarrhoea.
Answer:
House fly.

10. Usually the …………. grow on food items rich in Carbohydrates.
Answer:
fungi.

State whether True or False : (1 mark for each question)

1. Powdered khesari dal is mixed with besan because khesari dal is very cheap.
Answer:
true

2. Lathyrism has been observed in America.
Answer:
false

3. Argemone seeds are darker than mustard seeds.
Answer:
true

4. Coloured sweets and icecreams may contain colourants that can cause cancer.
Answer:
true

5. Various dyes used during Holi can cause problems of heart.
Answer:
false

6. Sometimes immediately after eating food items like prawn and brinjal uneasiness sets in the body.
Answer:
true

7. Poultry products, raw vegetables unpasteurized milk often contain harmful mi¬crobes.
Answer:
true

8. Usually the fungi grow on food items rich in fats.
Answer:
false

9. The mosquito is a mechanical vector.
Answer:
false

10. Spoilage of various types of food is due to the microbial degradation by various microorganisms.
Answer:
true

11. House flies spread Kala-azar.
Answer:
false

12. Legs of sand flies are long and thin.
Answer:
true

Match the column A with column B

Column A	Column B
a) Lathyrism has been observed	i) attacks the poultry birds
b) Some safe colours are now being	ii) may be present in food
c) A type of influenza virus	iii) made from flowers
d) Yeast is required for	iv) spread Malaria
e) Some very dangerous types of bacteria	v) make no sound while flying
f) Aedes mosquitoes	vi) in Ethiopia, Bangladesh and India
g) Anopheles mosquitoes	vii) spread dangue
h) Culex mosquitoes	viii) making bread

Answer:
a) Lathyrism has been observed in Ethiopia, Bangladesh and India.
b) Some safe colours are now being made from flowers.
c) A type of influenza virus attacks the poultry birds.
d) Yeast is required for making bread.
e) Some very dangerous types of bacteria may be present in food.
f) Aedes mosquitoes spread dangue.
g) Anopheles mosquitoes spread Malaria.
h) Culex mosquitoes make no sound while flying.

Very Short Questions & Answers : (1 mark for each question)

Question 1.
Name some chemical which can cause harm if they enter our body.
Answer:
Arsenic and Fluoride.

Question 2.
Define the term health.
Answer:
Health means physical, mental and social healthiness, it does not only mean fitness of body.

Question 3.
Mention some steps of prevention of disease.
Answer:
Vaccination, to add nutritious ingredients from outside (Iodine, iron, Vitamin)

Question 4.
Name some diseases which spread through air.
Answer:
Common cold, Cough, Flu, Chicken Pox, Scarlet fever, Diptheria etc.

Question 5.
Name some water-borne diseases.
Answer:
Cholera, Diarrhoea, Jaundice, Typhoid, Tuberculosis and Dysentery.

Question 6.
Name the organs mosquitoes use to drink blood.
Answer:
Labium, Mandible, Maxilla, Hypopharynx and Labrum.

Question 7.
What kind of mosquitoes spread Malaria?
Answer:
Anopheles mosquitoes spread Malaria.

Question 8.
What kind of mosquitoes spread Filaria?
Answer:
Culex mosquitoes spread Filaria.

Question 9.
What kind of mosquitoes spread Dengue?
Answer:
Aedes mosquitoes spread Dengue.

Question 10.
Mention an identifying feature of Asdes mosquitoes.
Answer:
While taking rest they sit almost paralleliy to the ground.

Question 11.
Mention an identifying feature of Anopheles mosquitoes.
Answer:
While taking rest they sit making an acute angle with the surface.

Question 12.
Mention an identifying feature of Culex mosquitoes.
Answer:
While taking rest they sit paralleliy as the surface.

Question 13.
Mention a symptom of Polio.
Answer:
Unnatural fatigue in the muscles of hands and legs.

Question 14.
What is the name of pathogen of Tuberculosis?
Answer:
Bacteria is the pathogen of Tuberculosis.

Question 15.
Write a measure to prevent airborn diseases.
Answer:
When ill, at the time of sneezing/coughing one must cover his mouth and nose with a handkerchief.

Question 16.
What is the name of pathogen of common cold.
Answer:
Virus is the pathogen of common cold.

Question 17.
When Cholera appeared in form of epidemic in bengal?
Answer:
In the year 1817 Cholera appeared in form of epidemic in bengal.

Question 18.
In what type of food items bacteria grow?
Answer:
Generally Bacteria grow in food items rich in protein (e.g. meat, egg, fish, dairy products)

Question 19.
In what type of food items fungi grow?
Answer:
Usually the fungi grow on food items rich in Carbohydrate.

Question 20.
Name some Bacterial Pathogen caused disease.
Answer:
Salmonellosis. Campylobacteriosis, Food-borne toxicity, Botlism.

B Short Questions & Answers : (2 marks for each question)

Question 1.
How are fungi beneficial?
Answer:
Fungi is used to produce food like bread, cheese and alcoholic beverages. Some antibiotics are produced from fungi, penicilium and yeast is required for making bread.

Question 2.
Why House fly is called a Mechanical vector and mosquito a Biological Vector?
Answer:
House fly only does the work of carrying the germ, so it is called Mechanical Vector. On the other hand the mosquitoes allow the germ to breed inside their body, so the mosquito is a Biological Vector.

Question 3.
What are Parasites?
Answer:
Some worms infest various organs of animals. These worms are parasites. If we take raw or under cooked meat of such animals, the parasites enter our bodies causing many diseases.

Question 4.
What is Bird-flu? How it effects other birds and human beings?
Answer:
Often a type of influenza virus (H5N1) attacks the poultry birds, this is called Bird-flu. The saliva catarrhal discharge or excreta of affected birds can infect other birds too. If human beings come in contact with skin, excreta or blood of virus-infected chicken, they can catch the diseases.

Question 5.
How microbial contamination of food occurs?
Answer:
Poultry products (egg, milk), raw vegetables unpasteurized milk often contain harmful microbes. If these food items are not properly processed or if they are taken raw, various types of infections may arise.

Question 6.
What is Allergy?
Answer:
Our immune system sometimes reacts to different proteins or other substances present in food. Allergy is basically the response generated by our immune system towards foreign proteins. This may cause itching sensation, rashes on skin and uneasiness in the body.

Question 7.
what can happen if food items with harmful additives is taken for a long time?
Answer:
Prolonged use of food items containing such harmful additives can lead to serious problems of the degestive and nervous system. It can also cause kidney diseases and may even cause cancer.

Question 8.
What is radioactive substances?
Answer:
Some invisible rays are emited continuously from elements like uranium and plutonium and other compounds. These are called radioactive substances.

Long Questions & Answers : (3 marks for each question)

Question 1.
Mention some causes of mental problem.
Answer:

• Disorder by birth: malnutrition of foetus, various infection after birth prevents the growth of mind and intelligence of the child.
• Mental pressure: various violent activities, unexpected incidents some times causes mental pressure beyond tolrance.
• Many family problems.
• Conflict and being unable to take decision.
• Addiction.

Question 2.
What is the proboscis? Did it help in male mosquito in sucking blood? How it is used by the mosquito to spread diseases?
Answer:
Various mouth parts of a mosquito get transformed into a proboscis for their special food habit. The mosquito has a long tubes like proboscis. In case of female mosquitoes the proboscis is slender and pointed just like an injection needle. But the proboscis in a male mosquito is blunt, so they can suck only different plant juice like necter, juice of fruits etc.

At the time of sucking blood a female mosquito infuses to saliva into the body and that animal through proboscis. They do so for preventing the blood from getting coagulated while sucking. Through this saliva the germs from the blood of a female mosquito enter into the body of that animal.

Question 3.
Which mosquito spread Malaria? Write down its identifying features.
Answer:
Female Anopheles spread Maleria. Its identifying features are-

• Black spot on the wings.
• At the time of taking rest they sit making an acute angle with the surface
• Make sound while flying.
• Come out in the evening.
• Lay eggs in clean water.

Question 4.
Name the diseases spread by Sand Fly. How can you identify it?
Answer:
Sand Fly spreads Kala-zar and Sand Fly fever.

• Sand flies are smaller than mosquitoes in size.
• Body is very hairy.
• Legs are long and thin.
• Come out after dark.

Question 5.
Describe the identifying features of Culex Mosquito. Name the diseases Culex can spread ?
Answer:
Identifying features of Culex mosquitoes are

• No spot on the wings.
• Sit parallelly as the surface while taking rest.
• Make no sound while flying.
• Come out at night.
• Lay eggs in dirty water. Culex mostquitoes spread Filaria and Encephalitis.

Question 6.
Name the diseases Aedes mosquitoes spread. How can we identify them?
Answer:
Aedes mosquitoes spread Dengue and Chickungunia diseases. Each identifying features are —

• White and Black stripes on the abdomen and legs.
• While taking rest sit almost parallely to the ground.
• Make no sound while flying.
• They come out in day time.
• Lay eggs in clean water.

Question 7.
What are the identifying features of House Fly? Name the diseases they can spread.
Answer:
A House Fly has ……..

• Grey-coloured thorax.
• 4 long black stripes on the back.
• Whole body is covered with hair.
• They come out during day time and remain inactive at night. House Flies spread Typhoid and Diarrhoea.

Question 8.
Onchocerciasis disease is spread by a fly, name it and describe its features.
Answer:
Black Fly spreads on Onchocerciasis

• Black Fly is black in colour.
• Proboscis is small in size and like a toothed dagger.
• Body is stout
• Come out during day time in groups and are very active just before down and dusk.

Question 9.
How fly spreads diseases?
Answer:
The fly spreads diseases in three different manners.
(i) Whenever they sit on stool, urine, pus, spittle etc. then minute particles of those things get stuck to their legs and antennae. When this fly sits on food items, germs are mixed with those food stuff and infection occurs.

(ii) The fly can not take solid food. So they vomit on solid food. As a result, parts of the solid food gets liquified and the fly takes the food with the help of the proboscis. With that vomit the germs from the body of the fly get mixed with our food items and infection occurs.

(iii) House flies excrete throughout the whole day at the interval of five minutes wherever
they sit. Many germs are present in the stool of the fly. When they defecate on our food, these germs get mixed with our food.

Question 10.
Tell the difference in the ways of transmission of diseases by the flies and the mosquitoes
Answer:

Question 11.
Describe some resources to control mosquito.
Answer:

• Water in reservoirs, buckets, flower vases should be changed every 2-3 days.
• Fishes like (Guppy, Panchax, Shol, Lata, Gambusia etc.) should be allowed to grow in stagnants waters because they eat the larvae of mosquitoes.
• Kerosene, diesel, bleaching powder should be spiaiged in the drains.
• Mosquito killer spray or liquid water should be used inside house to keep it free from mosquitoes.

Question 12.
Why food stuff gets spoiled?
Answer:
Spoilage of various types of food is due to the microbial degradation of food by various micro organisms. These micro-organisms are generally bacteria and fungi. When the microbes present in a food grow, they degrade the food with their enzymes and absorb the simple compounds thus obtained into their cells.

Question 13.
Name some Bacteria which are beneficial.
Answer:
Lactobacillus Bacteria help to-make curd. We obtain many life saving drugs from several species of streptomyces Bacteria. Some special Bacteria residing in the stomach of cattle help them in digesting cellulose. Some intestinal Bacteria help human beings by producing the much-needed vitamin Bp.

Question 14.
What is pasteurization?
Answer:
In simple words, pasteurization is the process of destroying the pathogenic microbes along with to their spores in food, especially liquid food (mild and dairy products, fruit juice etc.) This method of making foods and beverage free from microbes was discovered by the french Louis Pasteur, so to honour him the method is named as Pasteurization.

Question 15.
What are the common symptoms of allergy from food?
Answer:

• Rashes on skin.
• Itching sensation all over the body.
• Swelling of lips, throat and tongue
• Uneasiness in throat respiratory problems.
• Redness in eye and face,
• Vomiting
• Abdominal cramp
• Diarrhoea.

Question 16.
When a food can be said to be adultered?
Answer:

• When something is added to food that lowers its nutritional value.
• When some cheap additive is mixed with food instead of some necessary ingredients.
• When some harmful additive is added to the food to make the food look more attractive and to bring desirable flavour.

Question 17.
What is ‘dropsy’? How dropsy affects us?
Answer:
Argemone seeds closely resemble mustard seeds are cheaper than mustard. The oil extracted from argemone seeds is deliberately mixed with mustard oil. Long term use of such idulterated mustard oil. cause a disease called ‘dropsy’. Dropsy severely damages skin and organs like liver, lungs, kidney and heart. It can even cause failure of heart and the lungs, leading to death.

Question 18.
What is Lathyrism?
Answer:
Often Khesari daal which is very cheap (seeds of Lalhyrus Sativus) mixed with Arhar daal. Continued intake of substantial amount of Khesari daal for 2-3 months can cause progressive spastic paralysis. This is Lathyrism.

Question 19.
How fungi can be beneficial?
Answer:
We take the help of fungi to produce food items like bread, cheese and alcoholic beverages. The affinity of fungi for carbohydrate-rich food help the fungi in making those food items and beverages. For example, some antibiotics are produced from the fungus penicillium yeast is required for making bread.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 2 Time and Motion offer valuable context and analysis.

WBBSE Class 7 Science Chapter 2 Question Answer – Time and Motion

(Page no. 70)

Select the correct option from below and fill in the blanks:
1. Rectilinear motion.
2. Motion in a circular path.
3. Rotational motion.
4. Combination of Rotational and Rectilinear motion.
5. Curvilinear motion
Answer:

Examples of different types motion	Name of the motion
1. The motion of the pencil tip while drawing a straight line with a ruler.	Rectilinear motion
2. Motion of the tips of clock-hand	Motion in a circular path.
3. Motion of a stone which is thrown away from the roof.	Curvilinear motion.
4. Motion of merry-go-round.	Motion in a circular path.
5. Motion of a spinning top at a fixed point.	Combination of rotational motion and rectilinear motion.
6. Motion of a clock’s pendulum.	Rectilinear motion.
7. Motion of a car along a straight road.	Rectilinear motion.
8. Motion of a cycle along a straight line.	Rectilinear motion.
9. Motion of a screw driver.	Curvilinear motion.
10. Motion of an electric fan’s blade.	Rotational motion.

Class 7 Science Chapter 2 Question Answer West Bengal Board – Time and Motion

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
The rate of change of displacement with time is called-
i) speed
ii) retardation
iii) velocity
Answer:
iii) velocity

Question 2.
Distance travelled by a moving object-
i) can be zero
ii) cannot be zero
Answer:
ii) cannot be zero

Question 3
If the velocity of the body decreases with time, the motion is said to be
i) acceleration
ii) retardation
iii) displacement
is Answer:
ii) retardation

Question 4.
In SI system unit of retardation is-
i) MS²
ii) MS^-1
iii) MS^-2
Answer:
iii) MS^-2

Question 5.
The state of motion and rest of an object is-
i) absulute
ii) relative
Answer:
ii) relative

Question 6.
The average velocity of an object-
i) could be zero
ii) could not be zero
Answer:
ii) could not be zero

Question 7.
A rotating fan take sometime to stop when it is suitched off due to
i) interia of motion
ii) momentum
iii) air current
Answer:
i) interia of motion

Question 8.
The distance travelled by the body in a unit time interval is called
i) velocity
ii) speed
iii) acceleration
Answer:
ii) speed

Question 9.
If force is applied from outside on a moving body then its momentum
i) decrease
ii) increase
iii) does not change
Answer:
ii) increase

Question 10.
If a rubber ball is thrown on a wall it comes back, this can be explained by Newton’s-
i) First law
ii) Second law
iii) Third law
Answer:
iii) Third law

Question 11.
The action and the reaction both act on-
i) two different objects
ii) same object
Answer:
i) two different objects

Question 12.
We come to know about the rate of change of momentum when force is applied, from the Newton’s-
i) first law
ii) second law
iii) third law
Answer:
ii) second law

Question 13.
We know about definition of force from Newion’s-
i) first law
ii) second law
iii) third law
Answer:
i) first law

Question 14.
In CGS unit of force is-
i) Newton
ii) gram-weight
iii) dyne
Answer:
iii) dyne

Question 15.
The property of a body to remain at rest forever is called-
i) inertia of motion
ii) inertia of rest
iii) momentum
Answer:
ii) inertia of rest

Question 16.
Th property of inertia of a body is-
i) fundaniental
ii) not fundamental
Answer:
i) fundamental

Fill in the blanks : (1 mark for each question)

1. The tendency of a body to keep itself at rest for ever is known as _______ of rest.
Answer:
inertia.

2. Speed is a _______ quantity.
Answer:
scalar.

3. Two vector quantities with same units are _____ and ________.
Answer:
acceleration, retardation.

4. ____ is the rate of change of velocity with time.
Answer:
Acceleration.

5. Dimensional formula of retardation ______.
Answer:
LT^-2

6. To change the state of inertia we apply ______.
Answer:
force.

7. Dimensional formula of velocity _____.
Answer:
LT^-1

8. A body which changes is position with time is called a ____ object.
Answer:
moving.

9. Force = mass × ________.
Answer:
acceleration.

10. 1 Kgf = ______.
Answer:
9.8 N (Newton).

11. One newton is the force which when acts on a body of mass 1 kg produces an acceleration of _____.
Answer:
1 m/ sec^-2

12. Dimensional formula of momentum _______.
Answer:
MLT^-1

13. In SI system the gravitational unit of force is ______.
Answer:
Newton.

14. 1 Newton = 1 kg × _________.
Answer:
1 m/sec²

15. In SI system the unit of momentum is _______.
Answer:
Kilogram-meter/second.

16. The action and the reaction both act on ______ bodies.
Answer:
two different.

17. The rate of change of momentum is proportional to _______.
Answer:
force.

18. Momentum is a _____ quantity.
Answer:
Vector.

State whether True or False : (1 mark for each question)

1. If a body has zero speed it can have acceleration.
2. Acceleration is a scalar quantity.
3. Velocity is a vector quantity but speed is not.
4. In speed magnitude and direction both are to be mentioned.
5. A body may have uniform speed but it may not have uniform velocity.
6. The rate of increasing velocity is called uniform velocity.
7. Motion of the tips of clock-hand is called rotation.
8. When a moving object change its place it is called displacement.
9. If an object have zero displacement, it can have distance.
10. To express velocity we have to mention both its derection and magnitude.
11. We got defination of force from Newton’s second law.
12. In SI system unit of force is Newton.
13. Measure of force is known from Newton’s third law.
14. Action and reaction cannot suppress each other.
15. If a moving bus suddenly stops the passengers moves in front ward direction.
16. If 1 Newton force works on 1 gm mass then 1cm / sce² acceleration is produced the body.
17. Reaction remains as long as action persists.
18. If a body has less mass then it has less accleration.
19. If a vehicle suddenly starts to move it’s pas: engers lean backward direction.
20. Force applied = mass × retardation.
Answer:
1. true
2. false
3. true
4. false
5. true
6. false
7. true
8. false
9. true
10. true
11. false
12. true
13. false
14. true
15. true
16. false
17. true
18. false
19. true
20. false

Match the column A with column B.

Column A	Column B
a) Capacity of doing work	i) more work is done
b) Increase in the rate of respiration	ii) was the president of Royal Society
c) Increase in the rate of heartbeat	iii) to do work
d) When we spend more energy	iv) is called energy
e) Energy’ is the ability	v) is Newton
f) To every action there is an	vi) we apply force
g) Sir Issac Newton	vii) generates more heat
h) In SI system the unit of force	viii) increase in the rate of respiration
i) To change the state of inertia	ix) is Dyne
j) In CGS system the unit of force	x) equal and opposite reaction

Answer:
a) Capacity of doing work is called energy.
b) Increase in the rate of respiration generates more heat.
c) Increase in the rate of heartbeat increase in the rate of respiration.
d) When we spend more energy more work is done.
e) Energy is the ability to do work.
f) To every action there is an equal and opposite reaction.
g) Sir Issac Newton was the president of Royal Society.
h) In SI system the unit of force is Newton.
i) To change the state of inertia we apply force.
j) In CGS system the unit of force is Dyne.

Short Questions & Answers : (1,2 marks for each question)

Question 1.
How many types of motion are there?
Answer:
There are two types of motion.
i) Rotation and ii) Revolution.

Question 2.
What is Revolution?
Answer:
When a moving object travels in an orbit while rotating on its own axis then the motion of that object is called revolution.

Question 3.
What type of motion is seen in a moving car?
Answer:
Moving car has mixed motion.

Question 4.
Why displacement is a vector quantiny?
Answer:
Displacement has both magnitude and direction. So it is a vector quantity.

Question 5.
What type of motion has a moving electric fan?
Answer:
It has rotational motion.

Question 6.
What is the unit of speed in SI system?
Answer:
In SI system the unit of speed in meter/second.

Question 7.
Is speed vector or scalar quantity?
Answer:
Speed is vector quantity.

Question 8.
Which physical quantity has unit as meter/second² ?
Answer:
In SI system acceleration has unit as meter/second².

Question 9.
What is the unit of velocity? Is it a scalar or vector quantity?
Answer:
In CGS system unit of velocity in centimeter/sec. and in SI system unit of velocity is meter/second. Velocity is a vector quantity.

Question 10.
A body after moving in a circular part comes back to its starting point, what is its displacement?
Answer:
If a body after travelling in a circular part comes back to its starting point, the displacement is zero.

Question 11.
What we come to know from Newton’s Fast law?
Answer:
From Newton’s fast law, we come to know about- (i) Inertia and (ii) force.

Question 12.
From which law of Newton we come to know about force?
Answer:
We come to know about force from Newton’s fast law.

Question 13.
What is the unit of force in CGS system?
Answer:
In CGS system the unit of force is Dyne.

Question 14.
What is the unit of force in SI system?
Answer:
In SI system the unit of force is Newton.

Question 15.
One Newton = ? Dyne.
Answer:
One Newton = 10⁵ Dyne.

Question 16.
Name the types of Inertia.
Answer:
Inertia are of two types – i) Inertia of rest and ii) Inertia of motion.

Question 17.
Is force vector quantity or scalar quantity?
Answer:
Force is vector quantity.

Question 18.
What is’ the unit of momentum in SI system?
Answer:
In SI system the unit of momentum is Kilogram-meter/second.

Question 19.
1 gm. force = how much Dyne?
Answer:
lgf = 980 dyne.

Question 20.
What is the unit of work in SI system?
Answer:
A unit of work in SI system is Joule.

Question 21.
What is energy? How we get energy?
Answer:
The energy of a body is its capacity to do work.
We get energy from food. We get something from food which gives us ability for doing work.

Question 22.
How can we feel that energy is being spent inside our body?
Answer:
Fatigue, increase in the rate of heart beat, increase in the rate of respiration, increase in the temperature of the body etc help us to understand that energy is being spent inside our body.

Question 23.
Sometimes you may have noticed that birds glide some disatance without moving its wings. Why does the birdmove in this manner?
Answer:
A flying bird is a moving object. It tries to remain in uniform motion when there is no force on it. So, without moving its wings it goes farther distance because of inertia of motion.

Question 24.
What is inertia?
Answer:
The tendency of a body of remaining at rest or in uniform motion is known as ‘inertia’ of the body.

Question 25.
What is ‘inertia of rest’ and ‘inertia of motion’?
Answer:
The tendency of a body to keep itself at rest forever is known as ‘inertia of rest’.
The tendency of remaining in uniform motion of the body is called as ‘inertia of motion’.

Question 26.
What is force?
Answer:
What we apply to change the state of inertia is called as force.

Question 27.
Explain Newton’s Third law of motion?
Answer:
Newton’s third law of motion is – To every action, there is an equal and opposite reaction.

Question 28.
When we can say a work is done?
Answer:
Work is said to be done only when body move (i.e. there is a displacement of the body).

Long Questions & Answers : (3 marks for each question)

Question 1.
Explain the concept of Newtons first law of motion.
Answer:
If there is no external force acting on a body, then-
i) The body at rest will remain at rest forever,
ii) The moving body will go with constant speed in the same direction of its motion.

Question 2.
Explain Newton’s second law of motion.
Answer:
Newton’s second law of motion states that-
i) If we apply more force on a body of fixed mass, its change of velocity will be more in one second. If we double the force, the acceleration of the body also doubles. That is to say there is a linear relation between force and acceleration.
ii) The direction to which the froce acts, the direction of acceleration is also the same. That is the velocity of the body increases along the direction of applied force.

Question 3.
Write down the equation for the magnitude of applied force, according to Newton’s second law of motion.
Answer:
Applied force = mas of the body × change in velocity in 1 second. = mass of the body × acceleration produced (change in velocity in one second = acceleration)

F = m × a[F = force, m = mass, a = acceleration]
The unit of force in SI system is 1 Newton.
1 Newton =1 kg × 1 m / sec²

Question 4.
Who was Sir Issac Newton?
Answer:
Sir Issac Newton was an English physicist, mathematician and astronomer. He proposed the Three Laws of Motion, Spectrum of Sunlight. Universal Law of Gravitation, Binomial Theo rem. Differential calculus etc. On December, 1642. Newton was born in a peasant family of Willsthrope in England. He got BA degree from Trinity college of Cambridge University in 1669 at the age of 27 . He died in 1727.

Question 5.
Why was Sir Issac Newton famous?
Answer:
Sir Issac Newton invented laws of Motion, Spectrum of sunlight, Binomial theorem, Differential Calculus etc expanded a new era of science and mathematics. He was the president of Royal society from 1672 for next 25 years. He wrote the famous book “Principia Mathematica”.

Question 6.
When a bus suddenly starts moving from rest, the passengers in the bus leans in backward direction, explain why?
Answer:
When the bus is not in motion, the passenger with the bus are at rest. When the bus suddenly starts moving the situation change. The foot of the passengers starts moving with the bus. but the upper part of the body tries to remain at rest. So the legs move away in forward direction. the body leans in reverse direction.

Question 7.
Explain how we can find out the quantity of work done in physics.
Answer:
Word done = ( applied force) × displacement in the direction of the force) W = F × d[W = Work, F = Force, d = displacement]
If the displacement of the body is 1m in the direction of a force of 1N. Then the work done is I Joule or 1J.
& W = F × d
1J = 1N × 1M = 1N.m

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 1.5 Use of Environment Friendly Energy offer valuable context and analysis.

WBBSE Class 7 Science Chapter 1.5 Question Answer – Use of Environment Friendly Energy

(Page no. 63)

Write some other uses of thermal energy in the gaps given below :
Answer:

(Page no. 64)

Mention some of other uses of electric energy to the gaps given above. Similarly fill in the gaps given below about the uses of fossil fuels known to you.
Answer:

(Page no. 65)

What are directly related to population explotion? (Fill up the blanks) :
Answer:

(Page no. 67)

Foreign countiies are using solar energy since long time back.
We have also started using solar energy extensively in our state.
Fill in the blanks if you have noticed it.

Where is solar energy used?
Answer:
Solar energy is used in homes and on streets.

What purpose is it used for?
Answer:
Solar cooker is used to prepare food and solar light is used on streets.

(Page no. 68)

Write the names of the places in our state or other states where you have seen such wind mills.
Answer:
Rajasthan and Tamil Nadu.

(Page no. 68)

Write the uses of bio-gas.
Answer:
1. For cooking
2. To lit bulbs.

Discuss the potentials of degradable organic municipal waste or weeds like water hyacinth. How can we use them?
Answer:
Energy can be obtained from wastes. It can be used to lit bulb, run fans and cook food.

Class 7 Science Chapter 1.5 Question Answer West Bengal Board –Use of Environment Friendly Energy

Fill in the blanks : (1 mark for each question)

1. We need _______ to do any kind of work.
Answer:
energy.

2. To run power tiller ______ is used.
Answer:
diesel.

3. ______ is used as Jet fuel.
Answer:
Kerosene.

4. ______ is used in motor cycle.
Answer:
Petrol.

5. ______ is used to run fan and use light.
Answer:
electrical energy.

6. ______ fuels are formed from the remains of plants and animals over a span of millions of years.
Answer:
Fossil.

7. The large portion of electrical energy is produced in our country by burning ______.
Answer:
coal.

8. _______ of years are required for the formation of energy sources like coal or petroleum under the soil of earth.
Answer:
Millions and millions.

9. Demands for the conventional forms of energy are increasing day by day due to ______.
Answer:
Population explosion.

10. Petroleum is obtained by ______ holes in the Earth crust, where the oil is found.
Answer:
drilling.

11. Due to excessive use of fossil fuel ______ is increasing.
Answer:
pollution.

12. ______ is an environment friendly energy.
Answer:
Solar energy.

13. Coal, petroleum and natural gas are _____ fuels.
Answer:
fossil

14. Solar energy can be used to generate electricity in _______ cells.
Answer:
solar.

State whether True or False: (1 mark for each question)

1. Coal and petroleum is formed from the remains of plants and animals buried under the earth.
2. Animals depend upon sun indirectly.
3. Drilling of petroleum first started in Asia continent.
4. Solar energy is being used in our country since long time back.
5. Solar panel didn’t cause any harm to our environment.
6. Solar panel lasts for 100 years.
7. There are large wind mills in Bakkhali in our state.
8. Natural gas is obtained from organic waste.
9. Remains of plants and animals are called fossils.
10. Bio-gas is produced from the various organic waste.
Answer:
1. true
2. true
3. true
4. false
5. true
6. false
7. true
8. false
9. false
10. true

Short Questions & Answers : (1, 2 marks for each question)

Question 1.
What are the uses of fossil fuels?
Answer:
Fossil fuels are used to – a) run generator. b) run power tiller. c) for running launch, aeroplane.

Question 2.
What are directly related to population explosion?
Answer:
With the growth of population – a) production of more food, b) Construction of more accomodation, c) increase of transport and d) establishment of more factories are necessary.

Question 3.
In early days how people used to go from one place to another?
Answer:
In early days people used bullock cart, horse, ass, elephant, stagecoach, boat etc or walked on foot to go from one place to another.

Question 4.
Nowadays how people travel from one place to another?
Answer:
Nowadays people use train, tram, bus, aeroplane, motorcycle, ship, launch etc to travel from one place to another.

Question 5.
What are environment-friendly forms of energies?
Answer:
Conventional forms of energies are exhausting day by day, they have also detrimental effects on our environment. Hence, we have to explore the posibilities of some new sources of energy. These energies would have less harmful effect on us. These are environment friendly forms of energies. Some such energies are – solar energy, wind energy, energy from biomass.

Question 6.
How solar energy is produced?
Answer:
The solar cell generates electricity during the day-time using the sunlight directly. The energy thus generated, is being accumulated in the batteries connected with solar pannels. It has got its use at night or during dimlight of day.

Question 7.
What are the various uses of solar energy?
Answer:
The various uses of solar energy are solar light, solar cap, solar cooker, solar bike, solar water-heater, solar signal, solar mobile charger

Question 8.
What are the advantages of using wind energy?
Answer:
The advantages are –
i) Their is no shortage of air flow.
ii) Once installed, the wind mills serve for a long time.
iii) The wind mills can be used as local generators for areas where installation of long transmission cable is not possible.

Question 9.
How energy from Bio-mass is produced?
Answer:
The excreta of animals the various organic wastes and different materials of plant and animal origin can be gathered in a large ditch. After anacrobic digestion, it produces bio-gas which can be used for different purposes.

Question 10.
Name some other eco-friendly sources of energy?
Answer:
Some other eco-friendly sources of energy are Tidal energy, Geo-thermal energy etc.

Long Questions & Answers : (3 marks for each question)

Question 1.
Mention some of the uses of heat energy.
Answer:
Heat energy is used –
a) for cooking.
b) electricity generation.
c) sterilising of knife, scissors before operation.
d) making bricks.
e) driving the steam engine.
f) burning of clay made things.
g) in smithy to make different kinds of tools.

Question 2.
Write down some uses of electrical energy.
Answer:
Electrical energy is used –
a) to operate the machine in industry.
b) for using computer.
c) for playing loudspeaker.
d) for running rail, tram.
e) to operate washing machine.
f) charging of mobile phone.
g) X-ray.

Question 3.
Is the source of underground fossil fuels in exhaustible?
Answer:
Yes, the source of under guound fossil is exhaustible. It took many millions and millons of years for the formation of the energy sources like coal or petroleum under the soil of earth. We are spending the resources in very short time comparable to the time of their formation. In this way a day will come when all the sources of coal or minerals would be exhausted.

Question 4.
Discuss the ways how the use of energy could be done as minimum as possible?
Answer:
a) The over use and wastages of energy have to be scaled down.
b) The life of coal and petroleum can be lengthened by producing less-polluting fuel in an appropriate way.
c) The gases emitted from the bed of rocks underneath or from the crevices of the hills, are generally wasted, we have to make use of such gases.
d) Instead of pumping out underground water for irrigation or drinking purposes, we can use the surface water from rivers etc.
e) If possible use of non-machanised boat for transportation or use of bicycles would be encouraged, it will save fossil-fuel.
f) Unnecessary use of energy in various types of amusements is to be curtailed down.
g) We have to ensure the awareness of everyone about the use of non-conventional sources of energy.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 1.4 Electricity offer valuable context and analysis.

WBBSE Class 7 Science Chapter 1.4 Question Answer – Electricity

(Page no. 55)

Complete the table.
Answer:

Class 7 Science Chapter 1.4 Question Answer West Bengal Board –Electricity

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
The fish that can produce electricity in their body-
i) electric eel fish
ii) cat fish
iii) trout fish
Answer:
i) electric eel fish

Question 2.
Inside electric iron there is wire made of-
i) Copper
ii) Silver
iii) Nichrome
Answer:
iii) Nichrome.

Question 3.
Filament is made up of –
i) Aluminium
ii) Tungsten
iii) Nickel
Answer:
ii) Tungsten

Question 4.
At a relatively low temperature fuse wire-
i) melts
ii) remains it is
iii) changes slightly
Answer:
i) melts

Question 5.
A combination of number of cells forms-
i) dry cell
ii) filament
iii) battery
Answer:
iii) Battery.

Question 6.
Positive terminal is marked by the sign-
i) +
ii) –
iii) ×
Answer:
i) +

Question 7.
Good conductor of electricity is-
i) wood
ii) plastic
iii) copper
iv) paper
Answer:
iii) copper.

Question 8.
In the cell chemical energy is converted into-
i) mechanical energy
ii) chemical energy
iii) magnetic energy
iv) electrical energy
Answer:
iv) electrical energy

Question 9.
Other name of disposal cell is-
i) secondary cell
ii) primary cell
iii) button cell
iv) terminal
Answer:
ii) primary cell

Question 10.
If the circuit is not disconnected any where then that circuit is called-
i) closed circuit
ii) open circuit
iii) blocked circuit
Answer:
i) closed circuit

Question 11.
A bad conductor of electricity-
i) copper
ii) iron
iii) China clay
iv) silver
Answer:
iii) China clay

Question 12.
In electric calling bell there is-
i) static electricity
ii) electromagnet
iii) natural magnet
Answer:
ii) electromagnet.

Question 13.
In solar calculator the energy of light from the sun is transformed into-
i) electrical energy
ii) magnetic energy
iii) electromagnetic energy
Answer:
i) electrical energy

Question 14.
The sfficiency of solar panel is-
i) high
ii) very high
iii) low
iv) very low
Answer:
iv) very low

Fill in the blanks : (1 mark for each question)

1. ________ is formed by combination of a number of cells.
Answer:
Battery.

2. ________ is called ‘primary cell’ or ‘disposable cell’.
Answer:
Dry cell.

3. There are ________ substances in the cell of torch.
Answer:
chemical.

4. The inner part of the bulbs which glows when switch is on, is called ________.
Answer:
filament.

5. Plastic, china clay, wood ete are all ________ of electricity.
Answer:
bad conductor.

6. In electric calling bell ________ is used.
Answer:
electromagnet.

7. In electric iron there is ________ wire.
Answer:
Nichrome.

8. The filament of bulb is made of ________ metal.
Answer:
tungsten.

9. ________ wire is used for the safety of any electrical circuit.
Answer:
Fuse.

10. In telephone ________ is used.
Answer:
electromagnet.

11. The body fluids of all organisms contain various charged ________.
Answer:
particles.

12. The positive end of LED is ________ than the negative end.
Answer:
larger.

State whether True or False : (1 mark for each question)

1. Dry cells are used in torch.
2. There are physical substances in the cell of a torch.
3. Battery is formed by combination of a number of cells.
4. If the number of turns of wire increases then the magnetic strength of electromagnet also increases.
5. If the quantity of electricity increases, the strength of the electromagnet decreases.
6. Fuse wire is used for the safety of any electrical circuit.
7. The filament of bulb is made of Aluminium metal.
8. The bodies of all living organisms consist of molecules and atom.
9. There is a system to generate electricity in bodies of Hilsa, Trout fish.
10. Brain consists of numerous nerve cells.
Answer:
1. true
2. false
3. true
4. true
5. false
6. true
7. false
8. true
9. false
10. false

Match the column A with column B.

Column A	Column B
a) Electro magnet is used	i) electronic wristwatch
b) The number of cells increases	ii) has two terminals
c) Electricity can not pass	iii) in solar calculator
d) Button cell is used in	iv) relatively low tremperature
e) A cell	v) through plastic goods
f) Primary cell is known as	vi) inside the electric iron
g) Solar panel supplies electricity	vii) to make motor
h) Fuse wire melts at a	viii) is made of tungsten
i) Nicrome wire is used	ix) strength of electreomagnet increases
j) The filament of a bulb	x) disposable cell

Answer:
a) Electro magnet is used to make motor.
b) The number of cells increases strength of electreomagnet increases
c) Electricity can not pass through plastic goods.
d) Button cell is used in electronic wristwatch.
e) A cell has two terminals.
f) Primary cell is known as disposable cell.
g) Solar panel supplies electricity in solar calculator.
h) Fuse wire melts at a relatively low tremperature.
i) Nicrome wire is used inside the electric iron.
j) The filament of a bulb is made of tungsten.

Short Questions & Answers : (1,2 marks for each question)

Question 1.
What is ‘primary cell’ or ‘disposable cell’?
Answer:
There are chemical substances in the cell of torch. In the cell chemical energy is converted into electrical energy. This cell is called ‘pimary cell’ or ‘disposable cell’.

Question 2.
How many terminals are there in a ‘cell’?
Answer:
A ‘cell’ has two terminals-a) ‘Metallic cap’ terminal or (+) signed terminal. b) ‘Metallic disk’ terminal or (-) signed terminal.

Question 3.
Do you know any device which can be run by solar energy?
Answer:
Solar cooker. solar table fan, solar tubelight. solar street light etc. are run by solar energy.

Question 4.
What is Button cell?
Answer:
The cell present in the electronics wrist watch looks like a button. This is called Button cell.

Question 5.
What is secondary cell?
Answer:
There are six or more than that cells in the powerful battery of a car. Such type of cell is called secondary cell.

Question 6.
What is filament? Explain how it is connected inside a bulb?
Answer:
The inner part of the bulb which glows when switch is on, is called filament.
Filament is connected between two thick metallic wires. One of that two thick wires is connected with the positive (+ signed) terminal of the cell and other is connected with the negative (- signed) terminal of the cell.

Question 7.
What is circuit?
Answer:
To make the bulb glow, both the terminals of the bulb are connected with the opposite terminals of the cell. This arrangement is called circuit.

Question 8.
What is open and closed circuit?
Answer:
An open circuit is one where the continuity has been broken by an interruption in the path for current to flow. A closed circuit is one that is complete, with good continuity throughout.

Question 9.
Which subtances are called ‘good conductors of electricity’?
Answer:
The subtances through which electricity can pass easily are called good conductor of electricity. As example – Copper, Iron, Aluminium.

Question 10.
Which substances are called ‘bad conductors of electricity’ or insulators?
Answer:
The subtances through which electricity cannot pass are called bad conductors of electricity or insulators. As example-plastic, wood, glass.

Question 11.
What is electromagnet?
Answer:
If wire is bound around a magnetic substance (Iron, Nickel, Cobalt) and electricity is passed through it, the magnetic substance transforms into a magnet. This type of magnet is called electromagnet. When flow of electricity is stopped, it loses its magnetic property.

Question 12.
How the strength of a electromagnet could be increased?
Answer:
(a) If the number of turns of wire increases then the magnetic strength of electromagnet also increases.
(b) If the number of cells increases i.e. the quantity of electricity increases, the strength of the electromagnet increases.

Question 13.
Name some organisms which can generate electricity in their body. How they use it?
Answer:
There is a system to generate electricity in the bodies of the Jellyfish and electric eel fish. They can generate sufficiently strong electricity. This electricity stuns the enemies and keeps them away.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is LED?
Answer:
The full form of LED is Light Emitting Diode.
It is an electronic device that glows even by a little amount of electricity. It has no filament. Its positive end is larger than the negative end. An LED is not destroyed even in 20 years. Different coloured LEDs are available in the market.

Question 2.
Mention some applications of heating effect of electric current.
Answer:
Some applications of heating effect of electric current are –
a) Electric Iron : Here Nichrome wire is wrapped on Mica. Nichrome wire is heated when electricity is passed through it.
b) Electric Bulb : The filament of bulb made of Tungsten generates heat when electricity is passed through it. Light energy is generated due to transform of heat.
c) Fuse wire : Fuse wire melts at very low temperature. So it is used for the safety of any electrical circuit. When an excess amount of electricity passes through fuse wire due to any reason, it is heated very much and melts. As a result the circuit is cut off and flow of electricity is stopped.

Question 3.
What is solar calculator?
Answer:
Solar calculator is a special type of electronic device. This device runs by electrical energy. Actually solar panel supplies electricity in solar calculator. This panel is constituted by some solar cells. The energy of sunlight is transformed into electrical energy when it falls on solar panel.

Question 4.
Discuss the relation between electrical energy and living world.
Answer:
The relation between electrical energy and living world is very close. The bodies of all living organisms consist of molecules and atoms. Sometimes they can be ionised. At the time of staying in different liquids of living organisms ionised atoms or radicals participate in different physiological activities.

Question 5.
Discuss the influence of electrical energy in the body of human being.
Answer:
a) A special type of component is present in the muscles of heart. The electrical stimulation made by them generates heart beats which is spread in the body in the form of waves.
b) Nerve impulse of brain is also electrical. Brain consists of numerous nerve cells. Different informations are conducted through nerve cells with the help of electrical stimulations. As a result, contraction and expansion of muscles are possible. We can move and do various types of activities.

Question 6.
What is resting potential?
Answer:
In normal situation due to the presence of excess positive charge in the outside of membrance and excess negative charge inside of membrance a potential difference is developed. This is called resting potential. Nerve cells conducts the stimulation only when changes in resting potential occur.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 1.3 Magnet offer valuable context and analysis.

WBBSE Class 7 Science Chapter 1.3 Question Answer – Magnet

(Page no. 38)

Now take the bar magnet near each of these obfects and note which of these are being attracted by the magnet.
Answer:

Class 7 Science Chapter 1.3 Question Answer West Bengal Board –Magnet

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
A magnetic substance-
i) plastic
ii) wood
iii) iron
iv) stone
Answer:
iii) iron

Question 2.
A non-magnetic substance-
i) plastic
ii) cobalt
iii) iron
iv) nickel
Answer:
i) plastic

Question 3.
Magnetities are called-
i) natural magnets
ii) artificial magnets
iii) man-made magnets
Answer:
i) natural magnets

Question 4.
The attractive power of a bar magnet is strongest at its-
i) north pole
ii) midpoint
iii) both poles
Answer:
iii) both poles

Question 5.
Two opposite poles of magnets-
i) repulse
ii) attract
iii) remain neutral
Answer:
ii) attract

Question 6.
Which is incorrect-
i) induction occurs before attraction
ii) like poles repel each other
iii) a magnet has directional property
iv) single magnetic pole can exist
Answer:
iv) single magnetic pole can exist

Question 7.
Heating destroys magnetic property of-
i) ATM card
ii) hard disk of a computer
iii) loudspeaker
iv) all of these
Answer:
iv) all of these

Question 8.
There is a magnetite in pigeon’s body between-
i) neck and head
ii) neck and breast
iii) skull and brain
iv) between two wings
Answer:
iii) skull and brain

Fill in the blanks : (1 mark for each question)

1. About 2500 years ago a shepherd named _____ discovered magnet.
Answer:
Magnes

2. Material that are not attracted by a magnet are called _____ substances.
Answer:
non-magnetic.

3. Materials that are attracted by a magnet are called _______ substances.
Answer:
magnetic

4. ______ is a non-magnetic substance.
Answer:
Rubber

5. One end of a magnet is north-pole and another end is ______.
Answer:
south-pole

6. Magnetites are called ______.
Answer:
natural magnets

7. A _____ has the property to attract iron.
Answer:
magnet

8. Unlike poles of magnets _____, whereas like poles _____ each other.
Answer:
attract, repel

9. The earth itself behaves like a huge _______.
Answer:
magnet

10. ______ are used in dynamo of a bicycle.
Answer:
magnets

11. _____ birds and ______ travel following the lines of force of earth’s magnetic field.
Answer:
Migratory, turtles.

12. There is a small black magnetic substance between the skull and brain of ________.
Answer:
pigeons

State whether True or False : (1 mark for each question)

1. Electromagnets are used in electric calling bell.
2. A blade is attracted by magnet.
3. A rubber is attracted by magnet.
4. Cobalt is not attracted by magnet.
5. North poles of two magnets attı act each other.
6. A magnet suspended freely always comes to rest only north-south direction.
7. Only one pole exists in a magnet.
8. Magnetic poles always exist in pairs.
9. The earth itself behaves like a huge magnet.
10. Magnets are not used in loudspeakers.
11. Magnetic trips are used in ATM cards and credit cards.
12. Attraction occures before induction.
Answer:
1. true
2. true
3. false
4. false
5. false
6. true
7. false
8. true
9. true
10. false
11. true
12. false

a. Match the column A with column B.

Column A	Column B
a) A magnet has	i) in electric meters
b) A magnet attracts	ii) before attraction
c) Heating destroys	iii) can not exist
d) Magnets are used	iv) two opposite poles
e) An animal can	v) kept near loudspeakers
f) Induction occurs	vi) magnetized strip of steel
g) Single magnetic poles	vii) magnetic materials
h) Strong magnet should not	viii) two types
i) Magnetic needle is small	ix) magnetic property
j) Magnets are of	x) recognize earth’s magnetic field

Answer:
a) A magnet has two opposite poles.
b) A magnet attracts magnetic materials.
c) Heating destroys magnetic property.
d) Magnets are used in electric meters.
e) An animal can recognize earth’s magnetic field.
f) Induction occurs before attraction.
g) Single magnetic poles can not exist.
h) Strong magnet should not kept near loudspeakers.
i) Magnetic needle is small magnetized strip of steel.
j) Magnets are of two types.

Very Short Questions & Answers : (1 mark for each question)

Question 1.
What type of magnet is magnetite?
Answer:
Magnetite is a natural magnet.

Question 2.
Who and how many years ago discovered magnet?
Answer:
About 2500 years ago a shepherd named Magnes discovered magnet.

Question 3.
What are Magnetic substances? Give example.
Answer:
Materials that are attracted by a magnet are Magnetic subtances. Example-iron, nickel, cobalt.

Question 3.
What are Magnetic substances?
Answer:
Materials that are not attracted by a magnet are called non-magnetic substances. Example-plastic, rubber, wool.

Question 5.
What type of magnet is produced if electricity is passed through iron?
Answer:
Electro-magnet is produced.

Question 6.
Name two things where electromagnets are used.
Answer:
Electromagnets are used in electric calling bell and electric motors.

Question 7.
What are Artificial Magnets?
Answer:
Artificial Magnets are prepared by magnetising some suitable magnetic subtances.

Question 8.
What is the directive property of magnet?
Answer:
The property by which a floating or hung magnet is able to rotate freeely orienting itself North-South is called directive property of magnet.

Question 9.
What is bar magnet?
Answer:
A bar magnet is a magnetised sample of steel having a rectangular paralleopipod shape.

Question 10.
What are non-magnetic substances?
Answer:
Materials that are not attracted by a magnet are called non-magnetic substances. Example – plastic, wood, paper.

Short Questions & Answers : (2 marks for each question)

Question 1.
What are Natural Magnets or Magneite?
Answer:
Magnets which are available in nature are Natural Magnets or Magnetite. However, these are not commercially available. Such materials are available in specific areas only

Question 2.
Where do you think the attracting power of the magnet is (i) the strongest, (ii) the weakest.
Answer:
i) The attracting power of a bar magnet is strongest at its both ends. These regions are called the ‘pole of the magner’.
ii) Attracting power of a magnet is practically zero at its mid point. The region is called the ‘neutral zone’ of a magnet.

Question 3.
What is magnetic length? How it can be calculated?
Answer:
The distance between the north and the south poles of a magnet is called magnetic length of a bar fisagnet. This length is about 0.86 times the geometric length of the magnet.
So magnetic : ingth = 0.86 × geometric lentgth of the magnet.

Question 4.
What is magnetic axis?
Answer:
The line segments obtained by joining the geometric poles of a magnet, is called the magnetic axis.

Question 5.
Mention some roles of Magnetic field on the Physiology of Animals.
Answer:
The effect of magnetic field on some organisms had been studied by scientists. Beginning their journey from their native habitats, migratory birds and animals reach their winter habitats by following the lines of force of earth magnetic field.

Question 6.
What is magnetic field?
Answer:
The region around a magnet where its attractive and repulsive effects are felt, is called the magnetic field of the bar magnet.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is magnetic induction?
Answer:
When a magnet attracts a magnetic material, the material is magnetised by induction, then it becorves a temporary magnet and under the influence of the bar magnet it can attract other magnetic materials. This phenomenon is known as ‘magnetic induction’.

Question 2.
Write down the properties of a magnet.
Answer:
The properties of a magnet are –
a) A magnet attracts magnetic materials.
b) A magnet has directive property.
c) Like poles repel each other, unlike poles attract each other.
d) A magnet can induce magnetism in a magnetic substance.
e) Attraction follows induction.
f) A magnet always has two opposite poles.
g) Single magnetic poles cannot exist.

Question 3.
Is the Earth a magnet?
Answer:
The earth itself behaves like a huge bar magnet. We have learnt that the unlike poles attract each other. If a bar magnet is suspended from a support and allowed to rotate freely, it rests itself in North-South direction. This is because the magnetic North Pole of the earth is near the geographical South Pole, whereas the magnetic south pole is near the geographical North Pole.

Question 4.
What the North Pole and South Pole of a magnet actually means?
Answer:
Actually the so-called North and South Poles of a magnet should properly be called ‘Nouth-seeking pole’ and ‘South-seeking pole’ respectively. However, in our everyday ‘North pole’ and ‘South pole’ respectively.

Question 5.
What are Electro-magnets?
Answer:
When an iron bar is wrapped with an electric wire and electricity is passed through it. it behaves like a magnet. This type of magnet is called electromagnet.

Question 6.
Write down some uses of magnet.
Answer:
Uses of magnet –

1. Crews of ship use marine conpass that help them navigate their vessel.
2. The spools of audio and video cassettes are coated with a magnetic substance.
3. Magnetic strips are used in ATM cards and credit cards.
4. The hard disks of computers contains plastic disk coated with a magnetised material.
5. Many toys employ magnets.
6. Doctors use a special type of magnet to extract finer dust of iron accidentally fallen in someone’s eyes.
7. Magnets are used in loudspeakers.
8. Magnets are used in dynamo of a cycle.
9. Magnets are used in electric meters.
10. The door of refrigerators have magnets attached with them.

Question 7.
What precautions should be used in handling magnets?
Answer:
Instruments in which magnets are used must be handled with care. Such instruments must not be heated. Heating destroys magnetic property. The magnetic strips of two ATM cards kept together must not face each other. No strong magnet should not be kept near loudspeakers, TV, radio or hard disk of a computer.

Question 8.
How Aurora is formed?
Answer:
A special kind of rays called cosmic rays, enter the earth’s atmosphere. The charged particles in cosmic rays interact with the earth’s magnetic field and two radiation zones (Van Allen radiation belt) are formed. Auroral is also fromed. Living organisms utilise this aurora for their survival.

Question 9.
How can an animal recognize earth’s magnetic field?
Answer:
There is a black structure between the skull and brain of pigeons. This contains magnetic substance, called magnetite. In the dusk, pigeons return to their nests without mistake. However. if small magnets are fixed to their head. they fail to do so and fly in wrong direction. Magnetites have been found in some bees and animals.

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 1.2 Light offer valuable context and analysis.

WBBSE Class 7 Science Chapter 1.2 Question Answer – Light

(Page no. 16)

1. Fill in the blanks in the following table. Put a [✓] mark in the appropriate place.
Answer:

Class 7 Science Chapter 1.2 Question Answer West Bengal Board –Light

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
A luminous object-
i) umbrella
ii) brick
iii) star
iv) wood
Answer:
iii) star

Question 2.
A non-luminous object-
i) sun
ii) firefly
iii) burning candle
iv) stone
Answer:
iv) stone

Question 3.
A transparent object-
i) air
ii) wood
iii) iron
iv) wall
Answer:
i) air

Question 4.
An opaque object-
i) air
ii) wood
iii) water
iv) transparent glass
Answer:
ii) wood

Question 5.
A translucent object–
i) air
ii) water
iii) glass
iv) oil paper
Answer:
iv) oil paper

Question 6.
Light can propagate-
i) without medium
ii) with medium
iii) both are true
iv) both are false
Answer:
i) without medium

Question 7.
Ultra violet rays of sun-
i) can cause skin cancer
ii) is harmful to the DNA molecules
iii) can cause harm lens of eye.
iv) all are true
Answer:
iv) all are true

Question 8.
Number of colours present in sunlight are-
i) five
ii) three
iii) seven
iv) six
Answer:
iii) seven

Question 9.
Flowering is possible in wheat if daylight length-
i) exceeds 12 hours
ii) less than 12 hours
iii) exceeds 8 hours
iv) less than 8 hours
Answer:
i) exceeds 12 hours

Question 10.
Fireflies produce light in their own body-
i) to search food
ii) to communicate between themselves
iii) to attack others
Answer:
ii) to communicate between themselves

Question 11.
When reflection occures from uneven surfaces, it is called-
i) regular reflection
ii) diffuse reflection
iii) none of the two
Answer:
ii) diffuse reflection

Question 12.
In letter M Lateral inversion-
i) will not occur
ii) will occur
iii) may occur, may not occur
Answer:
i) will not occur

Fill in the blanks : (1 mark for each question)

1. The reflection from any smooth surface is ______ reflection of light.
Answer:
regular

2. Tracing paper is an example of ______ object.
Answer:
translucent

3. Objects on the other side of ______ objects can not be seen at all.
Answer:
opaque

4. A ______ camera is a simple camera without a lens but with a tiny aperture.
Answer:
pinhole

5. The sunlight is a mixture of rays of different ______ .
Answer:
colours

6. In a plane mirror if the angle of reflection is 60° then the angle of deviation would be ______ .
Answer:
60°

7. In nature a ______ is formed due to dispersion of sunlight.
Answer:
rainbow

8. Movement of migratory birds from colder regions to warmer regions depends on the intensity of ______.
Answer:
sunlight

9. The ray that represents the light reflected by the surface is known as ______ .
Answer:
incident ray

10. The angle between the normal and incident ray is known as ______ .
Answer:
angle of incidence

11. In a pinhole camera the smaller the hole, the finer is the optical ______ .
Answer:
impression

12. When the source of light is ______ both penumbra and umbra are formed.
Answer:
large

State whether True or False : (1 mark for each question)

1. Firefly can emit light, so it is called source of light.
2. When the size of the light source is very small, we call it a point source.
3. When the source of light is large, only the umbra is formed.
4. The ray of light that bounces back from the mirror is called the index ray.
5. The angle between the normal and incident ray is known as angle of incidence
6. The angle of incidence and the angle of reflection are equal:
7. When reflection occurs from uneven surfaces, it is called regular reflection.
8. Rainbow is formed due to dispersion of sunlight.
9. The sunlight is a mixture of eight rays of different colours.
10. Sir Issac Newton discovered dispersion in 1866.
11. The process of seperation of different colours of light from a polychromatic light is called dispersion.
12. Sunlight is a polychromatic light.
13. Layer of Ozone gas prevents ultraviolet rays.
14. X-ray is one type of invisible light which can not pass through our skin and flesh.
Answer:
1. true
2. true
3. false
4. false
5. true
6. true
7. false
8. true
9. false
10. false
11. true
12. true
13. true
14. false

a. Match the column A with column B.

Column A	Column B
a) Ultraviolet light very harmful	i) under our skin
b) Fingerlings of Salmon fishes	ii) is known as spectrum
c) Movements of migratory birds	iii) straight line
d) Plants can not prepare food	iv) is regular reflection of light
e) We have melanin	v) the umbra is formed
f) The band of seven colours	vi) depends on the intensity of sunlight
g) The reflection from smooth surface	vii) to the DNA molecules
h) Light moves along a	viii) without any medium
i) When we use a point source	ix) without sunlight
j) Light can propagate	x) die if they receive less light

Answer:
a) Ultraviolet light very harmful.to the DNA molecules.
b) Fingerlings of Salmon fishes die if they receive less light.
c) Movements of migratory birds depends on the intensity of sunlight.
d) Plants can not prepare food without sunlight.
e) We have melanin under our skin,
f) The band of seven colours is known as spectrum.
g) The reflection from smooth surface is regular reflection of light.
h) Light moves along a straight line.
i) When we use a point source the umbra is formed.
j) Light can propagate without any medium.

Very Short Questions & Answers : (1 mark for each question)

Question 1.
How many kinds of reflection are there?
Answer:
According to the nature of reflector reflection of light are of two types – a) regular reflection and b) diffuse reflection.

Question 2.
What kind of reflection helps us to see objects?
Answer:
Diffuse reflection helps us to see objects.

Question 3.
Which reflection creates image?
Answer:
Regular reflection creates image.

Question 4.
What type of image is formed in a plane mirror?
Answer:
Virtual image is formed in a plane mirror.

Question 5.
State an example of internal full reflection in nature.
Answer:
Mirage is an example of internal full reflection.

Question 6.
Is mirage a real image or virtual image?
Answer:
Mirage is a virtual image.

Question 7.
Which image cannot be formed on a screen?
Answer:
Virtual image cannot be fromed on a screen.

Question 8.
What type of reflection occurs in opaque glass?
Answer:
Diffuse reflection occurs in a opaque glass.

Question 9.
Which of these letters would have lateral inversion – D, T, A, P, B, C, N, W.
Answer:
D, P, B, C and N – these letters would have lateral inversion.

Question 10.
Give a natural example of internal full reflection.
Answer:
Air bubble in water dazzles – this is an example of internal full reflection.

Question 11.
How a ray of light changes its path when it travels from denser medium to rarer medium?
Answer:
A ray of light travelling from a denser medium to a rarer medium bends away from the normal. The angle of refraction is more than the angle of incidence.

Question 12.
Give an example of natural dispersion of light.
Answer:
A rainbow is an example of natural dispersion of light.

Question 13.
What is pure dispersion?
Answer:
In a dispersion where every colour is clearly and distinctly visible, it is called pure dispersion.

Question 14.
Give two uses of magnifying glass.
Answer:
Magnifying glass is used to read small letters in a book. It is used by watchmakers and repairers to see the minute parts of the watches.

Question 15.
At what distance an object should be placed from a convex lens to form image? What kind of image would be formed diminished or enlarged image?
Answer:
If an object is placed nearer than the length of focus of a convex lens then the object would have enlarged virtual image.

Question 16.
Is the velocity of red light and blue light same in glass? If not which one is faster?
Answer:
In glass the velocity of red and blue light are not same. In glass the velocity of red light is the greatest.

Question 17.
How we can distinguish between planet and a star?
Answer:
The heavenly bodies whose light seems to be constant are planets, and those whose lights seems to twinkle are stars.

Question 18.
What is the velocity of light in vaccum?
Answer:
The velocity of light in vaccum is = 3 × 10⁸ m/sec.

Question 19.
What is the velocity of light in water?
Answer:
The velocity of light in water is = 2.25 × 108 m/sec.

Question 20.
What is complementary colour?
Answer:
If the two colours mixed create white colour then they are called complementary colours. As example – yellow and blue are complementary colours.

Question 21.
In which medium water or glass, velocity of light is greater?
Answer:
Light travels faster in rarer medium than in denser medium. So light travels faster in water than in glass.

Question 22.
What type of wave is light?
Answer:
Light is a form of electromagnetic wave that needs no medium to travel.

Question 23.
What is the similarity between reflection and internal full reflection?
Answer:
The similarity is both of them follow rules.

Question 24.
At which position of an object convex lens forms real image?
Answer:
From convex lens if an object is placed at the length same or greater to its focus length then the object will form real image.

Question 25.
What is reflection?
Answer:
When light falls on a mirror, it comes back, this incident is caled reflection.

Question 26.
What is ‘point of incidence’.
Answer:
The point at which the ray of light touches the mirror is known as ‘point of incidence’.

Question 27.
What is reflector?
Answer:
The line which represents the position of the mirror is known as ‘reflector’.

Question 28.
What is normal?
Answer:
The line drawn perpendicular to the reflector at the point of incidence is called the normal.

Question 29.
What is angle of incidence?
Answer:
The angle between the normal and incident ray is known as angle of incidence.

Question 30.
What is angle of reflection?
Answer:
The angle between the normal and the reflected ray is known as angle of reflection.

Question 31.
Write the names of few flowers that bloom during summer days.
Answer:
Sunflower, Balsam, Tulip.

Question 32.
Write the names of few flowers that bloom during winter days.
Answer:
Marrigold, Dahlia, Rose.

Question 33.
Write the names of two plants whose process of flowering – requires day-length more than 12 hours.
Answer:
Wheat. Radish.

Question 34.
Write the names of two plants whose process of flowering requires day length less than 12 hours.
Answer:
Sunflower, Tomato.

Short Questions & Answers : (2 marks for each question)

Question 1.
What is luminous object? Give example.
Answer:
The object which smits light from its own source internally is called luminous object. Sun. stars. glowworm etc. are luminus objects.

Question 2.
What is non-luminous object?
Answer:
The objects which emit light from other source externally or through reflection of the luminous body is called non-luminous object. Bricks. wood, stone etc. are non-luminous object

Question 3.
What is extended luminous source?
Answer:
When the size of a luminous source is larger than the ‘point source’, the source is known as extended luminous source, like – Sun, torch, electric bulb etc.

Question 4.
What is point source?
Answer:
When the size of the light source is very small, we call it point source, like – light passing through a pinhole in black cardboard.

Question 5.
What is transparent medium?
Answer:
The medium through which light can pass easily is called transparent medium. Air, transparent glass. water etc. are known as ‘transparent objects”.

Question 6.
What is ‘Incident Ra’ and what is ‘Reflected Ray’?
Answer:
The ray of the light approaching the mirror is known as ‘Indcident Ray’.
The ray of light that leaves the mirror is called ‘Reflected Ray’.

Question 7.
What is opaque medium?
Answer:
The objects or medium through which light cannot pass at all are called opaque medium. Objects like wood, wall, iron etc. are known as opaque objects as light cannot pass through them.

Question 8.
What is Translucent media?
Answer:
Through some objects light couldn’t pass well. it passes in a mild and faded way, those sorts of objects or media are called translucent objects or translucent media. Ground glass. fog. tracing paper etc are translucent media.

Question 9.
Write down a property of light.
Answer:
Light moves along a straight line. This is one of the properties of light.

Question 10.
What is light ray?
Answer:
The imaginary rectilinear path with an arrow representing a path of light is known as light ray. Light ray does not exist in reality.

Question 11.
What is Beam of light? How many types are they?
Answer:
A collection of light rays is called as beam of light. Beam of light are of three types –
a. Parallel beam of light.
b. Divergent beam of light.
c. Convergent beam of light.

Question 12.
When umbra and penumbra is formed?
Answer:
When the source of light is large, then both the penumbra and umbra are formed, but when we use a point source only the umbra is formed. The darker portion of a shadow is umbra and the partially dark portion is penumbra.

Question 13.
State the laws of reflection.
Answer:
a) The angle of incidence and the angle of reflection are equal.
b) Incident ray, reflected ray and the normal on the reflector at the point of incidence must lie on the same plane.

Question 14.
What is Regular reflection?
Answer:
When reflection occurs on the polished surface it is called Regular reflection. The reflection from any smooth surface is regular reflection of light. The reflection on a plane mirror or curved mirror is an example of Regular reflection.

Question 15.
What is the surface of the separation of the two media?
Answer:
When two liquids of different density is mixed, then the surface of contact of the two liquidshaving different density, is called surface of separation of the two media.

Question 16.
What is the refraction of light?
Answer:
When light ray pases from one medium to another medium, it changes its path from the surface of separation of two media. This phenomenon of light is called refraction of light.

Question 17.
Which letter from A to Z suffer lateral inversion?
Answer:
The letters which suffer lateral inversion are – B, C, D, E, F, G, J, K, L, N, P, Q, R, S, Y and Z.

Question 18.
Why is word ‘AMBULANCE’ written in a reverse way on the car?
Answer:
The letters on the front of an ambulance are written laterally inverted like. The driver of the vehicle moving on road ahead of the ambulance reads these words laterally inverted as AMBULANCE in his rear view mirror and so he gives side to pass the ambulance first.

Question 19.
What is compound light?
Answer:
The Sunlight is actually a compound of number of rays of different colours. This type of light is called compound light.

Question 20.
What is ‘Spectra’? What is ‘dispersion’?
Answer:
When the Sunlight passes through a prism, the lights of different colours are separated from each other. We distinguish the light in seven colours. This band of seven colours is known as ‘Spectra’. The process of separation of different colours of light from a compound light is called as “dispersion”.

Question 21.
Who discovered ‘dispersion’? What are seven colours in Sunlight?
Answer:
The famous scientist Sir Issac Newton discovered ‘dispersion’ in 1666. The seven colours in sunlight are 1. Violet
2. Indigo
3. Blue
4. Green
5. Yellow
6. Orange
7. Red

Question 22.
How the living world is saved from the ultra-violet ray of Sun?
Answer:
in the atmosphere high above the Earth, there is a layer of Ozone gas. This is known as Ozonosphere. This layer absorbs Ultra-violet rays otherwise the living world would have been in great danger.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is Deffuse reflection?
Answer:
When reflection occurs in uncven surfaces. then it is called diffuse reflection of light. In diffuse reflection, when a parallel beam of light falls on a rough surface. they donot remain parallel after reflection. The diffused rays, after reflection goes in different directions. For example, the reflection on the wall, trees, soil, screen of the cinema hall etc.

Question 2.
What is a pinhole camera and how does it work?
Answer:
A pinhole camera is a simple camera without a lens but with a tiny apperature. A pinhole camera – effectively a light-proof box with a small hole in one side. Light frpom a scene passes through the aperture and projects an inverted image on the opposite side of the box, which is known as the camera obscure effect.

Question 3.
Why the ozone layer is being progressively destroyed? State its ill-effect.
Answer:
Various human activities produce such gaseous compound that reach the ozone layer and destroy ozone molecules and prevent their formation. As a result of this. the ozone layer is being progressivly destroyed. Progressive destruction of the Ozone layer leads to the rise in the number of skin cancer patients all over the world day by day.

Question 4.
Write a short note on X-ray.
Answer:
X-ray is one type of an invisible light. It can pass through our skin and flesh, but it cannot pass through the bones. So we can have a photograph of the damaged portion of the bone. But prolonged use of X-ray can also cause cancer. For that reason, a baby in the womb must not be exposed to X-ray.

Question 5.
Why cannot a plant survive in dark conditions?
Answer:
It is not possible for plants to prepare food with sunlight. The green parts of plants absorb solar energy and convert it into energy of food. Different parts of plant body are able to respond to external stimuli by utilising the energy derived from food. That is why, plants cannot survive when placed in dark conditions.

Question 6.
What is Rainbow? How is it formed?
Answer:
The Rainbow is formed in nature by the process of natural dispersion of sunlight. Rainbow is generally seen in the afternoon or evening sky after a shower. There are some floating water droplets in the sky. The sunrays while passing through them undergoes dispersion and finally it produce rainbow.

Question 7.
Name one harmful invisible light. Explain how this light is harmful.
Answer:
One harmful invisible light is Ultra-Violet light. It is extremely harmful to cells of living organism. It can cause harm to lens of eye and also to the photosensitive cells in the inner layer of eyeball (ratina). Besides, the ultra-violet ray of sun can cause skin canser.

Question 8.
Distinguish between reflection of light and refraction of light.
Answer:
When light rays move from one medium to another medium. a part of the light after undergoing reflection on the surface goes back to the former medium. This phenomenon is known as reflection of light. The remaining part of the light rays enter into the second medium and changes its path. Then each ray of light moves along a different straight line. This phenomenon is called refraction of light.

Question 9.
When a pencil or stick is placed in a glass of water, it appears to be bent, why?
Answer:
This is due to refraction of light in passing from water to air. The rays of light coming from tip of pencil or stick inside water, bend away from the normal at the surface of separation of the two media. So the immersed part of the pencil or stick appears to be bent on the surface of water.

Question 10.
When water is filled in a bucket and seen from above the base of the bucket seems to have risen, why this happen?
Answer:
This happens due to refraction of light. When the light ray from the base of the bucket after passing through water (the denser medium) enters into air (the rarer medium), at that time it deviates from the normal at the surface of separation. When it reaches our eye we can see the image of the base at a height above its original position.

Question 11.
Now try to find the examples of following influences in different animals –
a. Movement of animals under the influence of light.
Answer:
Locust stop their movement when sun remains covered by dark cloud.

b. Laying of eggs of animals under the influence of light.
Answer:
Salman fishes lay eggs in sufficient light as the Fingerlings of Salman fishes die if they receive less light.

c. Changing of colour of eye under the influence of light.
Answer:
The eyes of dog, cat. tiger glow at night in darkness.

d. Changing of colour of skin under the influence of light.
Answer:
If some cave-dwelling animals are brought into sunlight, coloured pigments start to be synthesized in their skin.