WBBSE Class 9 Physical Science Solutions Chapter 4.6 Water

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 4.6 Water offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 4.6 Question Answer – Water

Very short answer type questions

Question 1.
On which basis water is called hard and soft?
Answer:
Water is classified in hard and soft according to its ability of forming lather with soap.

Question 2.
Chemical activity of which is greater — potassium and magnesium ?
Answer:
Potassium has greater chemical activity than magnesium.

Question 3.
What type of water is used in a boiler ?
Answer:
Soft water

Question 4.
Which gas is produced when calcium reacts with water ?
Answer:
Hydrogen.

Question 5.
Which one has greater affinity towards water — Zn or Hg ?
Answer:
Zn.

Question 6.
In the activity series of the metals, which one has upper position – Ai or Au ?
Answer:
Al.

Question 7.
What is temporary hardness of water ?
Answer:
Temporary hardness of water is caused by the presence of soluble bicarbonates of calcium, magnesium and iron in it.

Question 8.
What is the cause of hardness of water ?
Answer:
Hardness of water is caused due to dissolved bicarbonate, chloride, sulphate salts of Ca, Mg and Fe in water.

Question 9.
State two causes of water crisis.
Answer:
Causes of water crisis :

•  Misuse of water
• Water pollution.

Question 10.
What are the products obtained from the reaction of sodium and water ?
Answer:
The two products are Sodium hydroxide and hydrogen.

Question 11.
What is the cause of black foot disease ?
Answer:
Black foot disease is caused by arsenic poison.

Question 12.
What is fluorosis disease ?
Answer:
Excess fluoride in water is responsible for fluorosis disease.

Question 13.
What are the types of hardness of water ?
Answer:
Two types of hardness of water :

• Temporary hardness
• Permanent hardness.

Question 14.
Which one is more pure ……….. deionised water or distilled water ?
Answer:
Distilled water.

Question 15.
What is electrochemical series ?
Answer:
Electrochemical series is arranged the position of metals according to their standard oxidation potential values.

Question 16.
Which metal reacts with water at ordinary temperature ?
Answer:
Potassium.

Question 17.
What is the process by which both types of hardness are removed ?
Answer:
Removal of both types of hardness is done by ion-exchange process

Question 18.
Give one cause of water crisis.
Answer:
Wastage of water.

Question 19.
What amount of potable water is present in total water of earth ?
Answer:
0.3%.

Question 20.
Which metalloid is the cause of water pollution ?
Answer:
Arsenic (As).

Question 21.
What amount of water is present in human body ?
Answer:
70%.

Question 22.
What is the value of specific heat of water ?
Answer:
4200  kg^-1 K^-1.

Question 23.
What is the latent heat of fusion of ice ?
Answer:
80 cal g^-1

Question 24.
What is the latent heat of vaporisation of water ?
Answer:
540 cal g^-1

Question 25.
What is potable water ?
Answer:
Potable water — Natural water free from suspended impurities and dissolved poisonous impurities, germs, bacteria, etc., so that it can be safely used for consumption of human beings and animals, is called potable water.

Question 26.
What is the method by which coliform count is made ?
Answer:
Most probable number index method (MPNI).

Question 27.
Dissolved oxygen is measured in which unit ?
Answer:
mg L^-1

Question 28.
What is Algal bloom ?
Answer:
An Algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system.

Question 29.
Which plant can reduce the amount of fluoride in drinking water ?
Answer:
Tulsi plant leaf can reduce the amount of fluoride in drinking water.

Question 30.
What is the function of AIAA ?
Answer:
Alumina impregnated activated alumina (AIAA) is an important adsorbent for removal of fluoride.

Short answer type questions 

Question 1.
What are the sources of natural water ?
Answer:
Sources of natural water: Natural water is available from various sources, such as seas, rivers, lakes or ponds, wells, springs and rain.

Question 2.
What is soft water ?
Answer:
Soft water: The class of water that easily forms lather or foam with soap is called soft water.

Question 3.
What is hard water ?
Answer:
Hard water: The class of water that does not easily form lather with soap, or does so after much wastage of soap is called hard water.

Question 4.
What is the cause of hardness ?
Answer:
Causes of hardness of water: The hardness of water is due to the presence of dissolved salts of metals (except Na, K) particularly calcium, magnesium and iron.

Question 5.
What is soap ?
Answer:
Soap : Soap is a salt of sodium or potassium of giant acid. Fatty acids are normally stearic acid (C₁₇H₃₅COOH), palmitic acid (C₁₅H₃₁COOH), oleic acid (C₁₅H₃₃COOH).

Question 6.
What is meant by temporary hardness of water ?
Answer:
Temporary hardness of water: Temporary hardness is caused by the presence of soluble bicarbonates of calcium, magnesium and iron in water.

Question 7.
What is meant by permanent hardness of water ?
Answer:
Permanent hardness of water: Permanent hardness is caused by the presence of chloride and sulphate salts of calcium, magnesium and iron in water.

Question 8.
What is meant by removal of hardness of water ?
Answer:
Removal of hardness of water: Hardness is due to the dissolved calcium, magnesium and iron salts. Now if these soluble salts are converted to insoluble salts by any process, then hardness of water is removed.

Question 9.
What is permutit ?
Answer:
Permutit : Permutit is a trade name for artificially prepared zeolite which is sodium aluminosilicate (Na₂O, Al₂O₃, 3SiO₂ ,2H₂O). Permutit is used to remove the hardness (both types, temporary and permanent).

Question 10.
What is deionised water ?
Answer:
Deionised water : The water that contains no cation other than H⁺ ion and anion other than OH^– ion is called deionised water. Deionised water is prepared only when hard water is allowed to pass through both cation ion-exchange and anion exchange resins.

Uses :

• In chemical laboratory for qualitative as well as quantitative analysis of presence of ions in a substance, other than OH^– and H⁺ ions.
• In batteries having lead electrodes.

Question 11.
Discuss the action of water on sodium and calcium metals ?
Answer:
Action of sodium and calcium on water: Sodium and calcium react promptly with water at ordinary temperature with fire and sound. At the end of the reaction metallic hydroxides and hydrogen are formed :
2Na + 2H₂O = 2NaOH + H₂ ↑
Ca + 2H₂O = Ca(OH)₂ + H₂ ↑

Question 12.
What is water pollution ?
Answer:
Water pollution: Some soluble and insoluble matters in water which produce some harmful effect to men and aquatic animals are called water pollution.

Question 13.
What is black foot disease ?
Answer:
Black foot disease: Ground water sometimes is contaminated with arsenic acid and arsenius acid as a deprotonated form. Consumption of these arsenic contaminated water for a long period has some poisonous effects in the form of black spots on palms and feets, skin become rough. The disease caused by arsenic poison is called black foot disease.

Question 14.
What is Fluorosis ?
Answer:
Fluorosis: Sea-water, river water contain fluoride from natural source. Excess fluoride in water is responsible for fluorosis disease which is a cause of dental decay and bone decay.

Question 15.
Write down two processes of getting arsenic free water.
Answer:
Two processes of getting arsenic free water :

• Arsenic is removed from water if water is allowed to pass through a bag made of cloth containing alumina or ferric hydroxide replacing the candle used in the filter. Arsenic or arsenious ions are absorbed in the alumina bed or ferric hydroxide bed and thus are removed.
• Take 20 liter of water, i- amount of tea-spoonful of bleaching powder and the same amount of ferrous sulphate in a bucket. After stirring well, arsenic is removed.

Question 16.
What is water crisis ?
Answer:
Water crisis: Within 2010 AD we are to face a critical situation for potable water. This is because of population is gradually increasing though storage of sweet water is fixed. This crisis can be overcome by avoiding the wastage of water and by judicious use of available water.

Collection of rain water and its uses for certain purposes like gardening, car-washing, floor-washing and even bathing can also mitigate water crisis. Scientists in different countries are engaged in finding a solution how saline water in sea be desalinated.

Question 17.
How is harvesting of water done for agriculture ?
Answer:
Harvesting of water for agriculture : In India and also in some other parts of world excessive rain water is stored in underground reservoirs. Such subsurface stores of water is freed from huge loss of water by evaporation as it happens in surface reservoirs. This stored water may be used for agricultural works

Board Answer Type Question 

Question 1.
Describe a method of removal of temporary hardness of water.
Answer:
Removal of temporary hardness by boiling: Temporary hardness of water is due to the presence of soluble bicarbonates of calcium, magnesium and iron in water. It is removed by boiling when soluble bicarbonate precipitate as insoluble carbonates and carbon dioxide gas evolves. On filtration soft water is obtained.

Question 2.
Describe the process by which both the temporary and the permanent hardness of water are removed.
Answer:
Removal of both temporary and permanent hardness by ion-exchange resin process :

Principle: Cations present as soluble salts in hard water are exchanged with the cations present in suitable resins producing insoluble salts containing the resin. After that step acids are produced, anions of which are exchanged with the anion present in other type of suitable resins.
Nature and function of resin : Resins are artificially prepared organic polymers widely used to remove hardness of water. These are of two types :

(a) Cation-exchanger resins carrying radicals like, -SO₃H, – COOH contain exchangeable W ions. These cations can be exchanged for equivalent positive ions like Ca₂, Mg₂, etc. present in soluble salt in water.

(b) Anion-exchanger resins carry – NHOH, NH₃OH containing exchangeable OH ions.

Process:
(a) On passing hard water through a chamber containing the cation-exchanger resin, exchange of Ca₂ and Mg₂ with the W of the  – COOH or – SO₃H group of the resin takes place.

The reactions are :

Now the water obtained after passing through the cation exchanger is acidic.

(b) Water after passing through cation exchange resin is now allowed to flow over anion exchange resin (expressed as RNH) due to exchange of anion, resin gets precipitated. Water thus obtained is completely deionised.

After these two steps, hardness of water is completely removed,the water contains no cation other than H⁺ ions and no anion other than OH^– ions.

Useful-exchangers :
(a) Cation-exchangers :

•  Amberlite IR-120
• Dowex-50
• Zeocarb 225

(b) Anion-exchangers :

• Amberlite IRA-400
• Dowex I
• Dowex II

Regeneration of resins: Cation exchange resin is regenerated by passing dilute H₂SO₄ and anion exchange resin is generated by using dilute solution of NaOH solution.

Question 3.
Why is water called versatile solvent?
Answer:
Water is a versatile solvent: Water molecule is amphiprotic, i.e. it behaves both as an ionic compound and a covalent compound. As a result, water can dissolve both ionic and covalent compound. Thus, on the one hand, acids, bases and salts (ionic compounds) easily dissolve in water and on the other hand, it can dissolve covalent compounds like alcohol, (methanol, ethanol), acetone, glycerol, sugar, urea, etc.
For this reason water is called versatile solvent.

Question 4.
How is delonised water prepared?
Answer:
Preparation of deionised water : Water is treated with some resins which contain carboxylic acid radical  – COOH or  – SO₃H
(suiphonic acid radical). The cations, Ca²⁺, Na⁺, Fe²⁺, etc. present in water are removed on exchange of these with H⁺ ions of – COOH or – SO₃H of resins.
If the resin is represented as RH, where R is the resin polymer,
H is hydrogen of acid radicals present in the resin.
CaCl₂ + 2RH =  R₂Ca↓ + 2HCl
NaCl + RH = RNa + HCl
After removal of cations, water contains some acid radicals like, HCl. To remove the acid radicals, water is treated with another type resin of basic nature. In this way, the water is freed from all types of ions, except H’ and OH- ions and this is deionised water.

Question 5.
(a) What is fluoride ?
(b) Name one harmful fluoride and state some of its bad effects on human.
(c) State some injurious effects of presence of fluoride in water.
Answer:
(a) Fluoride is meant by any binary compound of fluorine.
(b) The most harmful compound of fluorine is hydrogen fluoride (HF). It violently attacks skin, forming sores. Due to inhalation of vapour of this fluoride, loss of voice takes place and finally death results.
(c) Presence of traces of some fluorides in water is very dangerous.
Some of the injurious effects of fluoride are given below.

• Loss of calcium from bones causing skeletal weakness.
• Carries and discolouration of teeth.
• Bending of the knees of sides, known as, Genu Valgum.

Question 6.
(a) When the water in a soil layer may be considered arsenic-polluted?
(b) State two harmful effects of drinking water that contains arsenic beyond permissible limit.
(c) State one arrangement for supply or arsenic-free water.
Answer:
(a) According to the World Health Organisation (WHO), the permissible limit of arsenic in drinking water is 0.05 mg/L. So, the water in a soil layer that contains more than this specified quantity of arsenic is polluted.

(b) Harmful effects of arsenic : On drinking water containing arsenic beyond the permissible limit, diseases like skin pigmentation, organ failure may occur.

(c) Supply of arsenic-free water :

• Arsenic is removed from water, if water is allowed to pass through a bag made of cloth containing alumina or ferric hydroxide replacing the candle used in the filter. Arsenic or arsenious ions are absorbed in the alumina bed or ferric hydroxide bed and thus are removed.
• Take 20 liter of water, \(\frac{1}{4}\)th amount of tea-spoonful bleaching powder and the same amount of ferrous sulphate in a bucket. After stirring well, arsenic is removed.

Question 7.
Discuss the difference of activity of the metals with water on the basis of the position of the metals in Electrochemical series.
Answer:
Difference of activity of the metals in reaction with water:
(a) A metal with highest electron releasing ability is placed in the top most position. Metals placed in the electro-chemical series react with water according to their positions in the series.

(b) Chemical activity of metal entirely depends on their position in the electro-chemical series. Metal which occupies higher position than any other metal has greater value of

• Electropositive character
• Reducing capacity
• Chemical reactivity.

Question 8.
What is Permutit process?
Answer:
Permutit process: Both types of hardness can be removed cheaply in this process. Permutit is a trade name for artificially prepared zeolite which is sothum aluminosiicate (Na₂O, Al₂O₃, 3SiO₂, 2H₂O). It is a natural hydrated large-sized crystalline substance. A bed of granules of permutit is kept in a porous shelf slightly upper middle of a big drum made of iron.

Other porous shelves staged with stone and sand are placed above and below the shelf containing bed of permutit. When hard water kept in a tank is allowed to pass through the bed of permutit due to exchange of ions the hardness of water is removed and soft water is obtained. Let the ion-exchange material permutit be formulated as Na₂Ze, where Ze is the zeolite radical.

After the use of permutit for sometime when the permutit gets exhausted and loses its activity. It is regenerated by passing an aqueous solution of sodium chloride by which permutit gets back its exchange capacity.
CaZe + 2NaCl = Na₂Ze + CaCl₂
MgZe + 2NaCl Na₂Ze + MgCl₂
Permutit water-softening plants are used in private houses and factories and also for softening public water supplies of towns.

Question 9.
Discuss water pollution. How it is caused ? What are the dangers of water pollution ?
Answer:
Water pollution : Some soluble and insoluble matters in water which produce some harmful effect to men and aquatic animals are called water pollution.

Cause of water pollution : Pollution of water may be natural or man-made. Natural water coming from underground sources like tube-wells, wells, etc. and that flowing over earth’s surface often give rise to natural pollution due to presence of fluorides of alkali metals and arsenic compounds.

Man-made water pollution arises from the indiscriminate use of fertilizers, insecticides, pesticides in agriculture, and by the dumping of chemical effluents like metal salts, acids, alkalis, etc. from factories into the river or lakes. The washing of clothes and cooking utensils, bathing men and animals make water of ponds and lakes polluted.

Apart from these, the sub-soil water are contaminated with worms, bacilli amoeba and several other pathogenic bacteria and virus.

Dangers of water pollution :

• Appreciable amounts of fluorides in water give rise to gum diseases, decay of the enamel of teeth and also deseases of bones.
• Presence of considerable amount of arsenic compounds in water causes ailments of the digestive system and severe skin diseases. Consumption of arsenic contaminated water for a long period has some poisonous effects in the form of black spots on palms and feets. Black foot disease is due to arsenic poison.