WBBSE Class 8 Science Solutions Chapter 2.3 Chemical Reaction

Detailed explanations in West Bengal Board Class 8 Science Book Solutions Chapter 2.3 Chemical Reaction offer valuable context and analysis.

WBBSE Class 8 Science Chapter 2.3 Question Answer – Chemical Reaction

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

Question 1.
Which of the following is oxdising agent:
i) Cl₂
ii) H₂S
iii) H₂O
iv) NH₃
Answer:
i) Cl₂

Question 2.
Which of the following is not an enzyme
i) Amylase
ii) Lipase
iii) Carbohydrate
iv) Ranine
Answer:
iii) Carbohydrate.

Question 3.
Which gas is used for welding-
i) Cl₂
ii) O₃
iii) CH₄
iv) C₂ H₂
Answer:
iv) C₂ H₂

Question 4.
If limestone is heated we get-
i) slakelime
ii) quicklime
iii) carbon
iv) carbon-monoxide
Answer:
ii) quicklime

Question 5.
If kerosene oil is heated we get-
i) heat
ii) light
iii) heat and light
iv) none of the above
Answer:
iii) heat and light.

Question 6.
Carbon dioxide turns c!ear lime water-
i) red
ii) blue
iii) milky
iv) colourless
Answer:
i) milky.

Question 7.
Ionic compounds are more soluble in-
i) ether
ii) benzene
iii) chloroform
iv) water
Answer:
iv) water

Question 8.
Which of the following is true-
i) enzymes are indispensible in digestion of food
ii) enzymes are required to produce energy from food
iii) without enzymes no cell can survive
iv) all the above statements are true
Answer:
iv) all the above statements are true

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

1. Energy is required to initiate a chemical reaction.
2. Water is the only solvent.
3. Kerosene and benzene can dissolve ionic compounds.
4. The action of catalysts is called catalysis.
5. Most of the enzymes are protein in nature.
6. Rusting of iron is a physicsl reaction.
7. Combustion of yellow-coloured sulphur powder in the combustion spoon will produce sulphur dioxide.
8. For extraction of metal from its ore, a process of oxidation is taken help of.
9. Oxidation can not occur alone in any reaction, reduction must occur along with it.
10. A coating of synthetic enamel paint on iron can prevent rusting.
Answer:
1. true
2. false
3. false
4. true
5. true
6. false
7. true
8. false
9. true
10. true

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

1. Chemical reactions are done by ____.
2. Chemicals participating in a reaction are called ______.
3. Renine is a ______ breaking enzyme.
4. ______ is needed in combustion.
5. The supply of oxygen in atmosphere is due to ______.
6. Baking soda is an ______ compound.
7. Substances that can speed up specific chemical reactions are called ______.
8. Arhar dal or seeds of watermelon contains an enzyme called ______.
9. Ammonia makes the phenolphthalein solution ______.
10. Burning of any fucl is ______.
Answer:
1. atom
2. reactants
3. protein
4. Oxygen
5. photosyntesis
6. ionic
7. Catalysts
8. urease
9. pink
10. exothermic

Match the following : (1 mark for each question)

Column A	Column B
a) In acid phenolphthalein is	i) reducng agent.
b) Phenolphthalein is a	ii) exothermic
c) Carbon monoxide is a	iii) colourless
d) Cl2 is a	iv) exothermic reaction
e) Burning of any fuel is	v) melts iron
f) Burning of C2H2 (acetylene gas)	vi) indicator
g) Heat and light during welding come from	vii) oxidising gas

Answer:
a) In acid Phenolpthalein is colourless.
b) phenolpthalein is a indicator.
c) Carbon monoxide is a reducing agent.
d) Cl₂ is a oxidising gas.
e) Burning of any fuel is exothermic.
f) Burning of C₂ H₂ (acetylene gas) melts iron.
g) Heat and light during welding come from exothermic reaction.

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

Question 1.
Give ar example of a catalyst.
Answer:
Mar:ganese dicxide.

Question 2.
Give an example of positive catalyst.
Answer:
Manganese dioxide.

Question 3.
Give an example of negative catalyst.
Answer:
Concentrated H₂ SO₄.

Question 4.
Write a name of enzyme.
Answer:
Amylase.

Question 5.
Write a name of protein breaking enzyme.
Answer:
Tripsine.

Question 6.
Write a name of lipid breaking enzyme.
Answer:
Lipase.

Question 7.
Write a carbohydrate breaking enzyme.
Answer:
Amylase.

Question 8.
Carbohydrate type of food lastly turns into what?
Answer:
Glucose.

Question 9.
What we lastly get from protein type of food?
Answer:
Amino Acid.

Question 10.
What is end of fat breaking enzyme?
Answer:
Fatty acid and Glycerol.

Question 11.
Name some solvents.
Answer:
Water, kerosene, benzene, turpentine oil are uesd as solvents.

Question 12.
Define reactants.
Answer:
Chemicals participating in a reaction are called reactants.

Question 13.
Give an example of ionic compound.
Answer:
Baking soda is an example of ionic compound.

Question 14.
What are catalysts?
Answer:
Substances that can speed up specific chemical reactions are called catalysts.

Question 15.
What is endothermic reactions?
Answer:
Chemical reactions that absorb heat from the surroundings are called endothermic reactions.

Question 16.
What happens when parafin, kerosene, coosking gas are burnt?
Answer:
They produce heat and light.

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

Question 1.
Which living things can make food through photosynthesis?
Answer:
The green plants and some bacteria convert solar energy into chemical energy of food through photosynthesis.

Question 2.
Why do you think are there ‘dark rooms’ in studios?
Answer:
When light falls on a photographic film, many reactions occur and the film gets ciarkened. so to prevent the film from light ‘dark rooms’ are there in siudios.

Question 3.
Why solvent is necessary in a chemical reaction?
Answer:
Mere mixing of the two powdered solids is not sufficient for a chemical reaction. The solvent separates the ions and molecules in the reactants and thus the reaction begins.

Question 4.
No bubble is evolved if a mixture of dry NaHCO₃ and tartaric acid crystals is shaken with benzene or kerosene. Why?
Answer:
Baking soda is a ionic compound. Solvents like kerosene. benzene or turpentine oil can not dissolve ionic compounds. Moreover, tartaric or any other acid must give H^{+ions for reaction. Such an incident can occur in water, but not in a slovent like kerosene or benzene. So the reaction will not ensure in such solvents.

Question 5.
What are exothermic reactions?
Answer:
Chemical changes that evolve heat are called exothermic reactions. Examples of some exothermic reactions are:
Burning of carbon C + O₂ → CO₂ + heat
Combustion of methane CH₄ + 2O₃ → CO₂ + 2H₂O + heat.

Question 6.
What are endothermic reactions?
Answer:
Chemical reactions that absorb heat from the surroundings are called endothermic reactions. Example of endothermic reaction is-
Making quicklime from limestone:
CaCO₃ CaO + CO₂ – heat.

Question 7.
Why the welding personel have to use special spectacles?
Answer:
Heat and light produced during velding come from an exothermic chemical reaction: 2C₂ H₂ + 5O₂ → 4CO₂ + 2H₂O+ heat
This raises the temperature to above 200° C. Light emitted at such a high temperature contains some ultraviolet (UV) radiation that is very harmful to eyes. For this reason, the welding personnel have to use special spectocles that filter out UV radiation.

Question 8.
What is combustion?
Answer:
From our daily experiences we know that when wood, coal etc are set to fire in open air, heat and light are produced. Paraffin, kerosene, cooking gas also burn similarly in open air producing heat and light. Scientifically this phenomenon is termed as combustion.

Question 9.
What is reduction?
Answer:
In a reaction where oxygen gets eliminated from a compound is called reduction. A reaction? where hydrogen combines with an element or copound is also called reduction.

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

Question 1.
What steps can be taken to protect iron from rusting?
Answer:
i) A coating of coal tar or a synthetic enamel paint can be applied on iron, so that it can not come in contact with air and moisture. This coating prevents rusting.
ii) If zinc is electroplated over iron, it prevents rusting. This process is called galvanization.

Question 2.
What is Catalyst? Write down the characteristics of catalysts.
Answer:
There are many reactions that occur very slowly. However, the addition of minute quantities of some other substances results in much facter reaction. Substances that can speed up specific chemical reaction are called catalysts.

Question 3.
Write the caracteristics of catalysts.
Answer:
Caracteristics of catalysts-

1. Catalysts participate in reaction they catalyze but are regenerated in the end.
2. There is no universal catalyst that can enhance the rate of all reactions.
3. Catalysts must always be found by experiment. Just by looking at an equation one can not say which catalyst will be effective in that case.

Question 4.
How are enzymes important to our body?
Answer:
Enzymes are indispensable in digestion of carbohydrates, proteins and lipids present in food. In our body enzymes are required to produce energy from food. In a cell. there are different enzymes to make proteins, DNA. hormones, components of cell membrane etc. Specific enzymes destroy harmful components formed in the cell. Without enzymes, no cell can survive.

Question 5.
What happens during chemical reactions?
Answer:
During chemical reaction some chemical bonds are broken and new ones are formed; in some some cases ionic compounds are formed; in some cases crystals of ionic compounds are broken to form new subetances; in some others ions react to form new substances. In general. such changes is either evolution of heat or absorption of heat.