Chapter 5 Physical and Chemical Changes Question/Answers with Important Points

Important terms to Learn In while learning Physical and Chemical Changes in Short Words

Chemical change – A change that results in the formation of new substances. (रासायनिक परिवर्तन)
Chemical reaction – A process in which substances interact to form new substances. (रासायनिक अभिक्रिया)
Crystallisation – The process of forming solid crystals from a solution or melt. (स्फटिकीकरण)
Galvanisation – The process of applying a protective zinc coating to iron or steel to prevent rusting. (जस्तीकरण)
Physical change – A change in which no new substances are formed and only the form or appearance changes. (भौतिक परिवर्तन)
Rusting – The process of iron reacting with oxygen and water to form rust. (जंग लगना)

Exercise Question With Answers:👉

1. Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis – Chemical change

(b) Dissolving sugar in water – Physical change

(d) Melting of wax – Physical change

(e) Beating aluminium to make aluminium foil – Physical change

(f) Digestion of food – Chemical change

2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

(a) Cutting a log of wood into pieces is a chemical change. (False)

Corrected statement: Cutting a log of wood into pieces is a physical change.

(b) Formation of manure from leaves is a physical change. (False)

Corrected statement: Formation of manure from leaves is a chemical change.

(d) Iron and rust are the same substances. (False)

Corrected statement: Iron and rust are different substances.

(e) Condensation of steam is not a chemical change. (True)

3. Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.

(b) The chemical name of baking soda is sodium bicarbonate.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called chemical changes.

4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

When baking soda (a base) is mixed with lemon juice (an acid), bubbles are formed with the evolution of carbon dioxide gas. This is a chemical change because a new substance (carbon dioxide) is produced.

5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

When a candle burns:

Physical change: The melting of wax.
Chemical change: The burning of wax to produce carbon dioxide, water vapor, and heat.

Another example of a process involving both physical and chemical changes is cooking an egg. The physical change is the solidification of the egg white and yolk (denaturation of proteins), and the chemical change is the chemical reaction of proteins in the egg.

6. How would you show that setting of curd is a chemical change?

Setting of curd is a chemical change because it involves the conversion of lactose in milk into lactic acid by the action of bacteria. This process results in the formation of new substances, including lactic acid, and changes the taste, texture, and properties of the milk.

7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Burning of wood is a chemical change because it involves combustion, producing new substances like carbon dioxide, water, ash, and heat.

Cutting wood into small pieces is a physical change because it changes only the size and shape of the wood without altering its chemical composition.

8. Describe how crystals of copper sulphate are prepared.

To prepare crystals of copper sulphate:

Dissolve a small quantity of copper sulphate powder in hot water to make a saturated solution.
Filter the solution to remove impurities.
Allow the solution to cool slowly.
As it cools, copper sulphate crystals will form at the bottom of the container.
Collect and dry the crystals.

9. Explain how painting of an iron gate prevents it from rusting.

Painting an iron gate prevents it from rusting by creating a barrier that prevents oxygen and moisture from coming into contact with the iron. This inhibits the oxidation process that leads to rust formation.

10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Rusting of iron objects is faster in coastal areas than in deserts because coastal areas have higher humidity and salt content in the air, both of which accelerate the rusting process. Deserts have lower humidity, which slows down the rusting process.

11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes.

Choose the correct one. (i) Process – A is a chemical change. (ii) Process – B is a chemical change. (iii) Both processes A and B are chemical changes. (iv) None of these processes is a chemical change.

Ans: (ii) Process – B is a chemical change.

Explanation: Process A (liquid to gas) is a physical change, while Process B (burning of gas) is a chemical change.

12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes.

Choose the correct one.

(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Ans: (iii) Both processes A and B are chemical changes.

Explanation: Anaerobic digestion of animal waste to produce biogas is a chemical change, and burning the biogas is also a chemical change as new substances are formed in both processes.

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