Question

(a) Give the main difference between physical changes and chemical changes.
(b) Which of the following are chemical changes and which physical ? Give reason.
(i) a glass bottle breaking
(ii) coal burning in air
(iii) making a cake
(iv) wool being knitted into a sweater

Answer 1

(a) Important differences between a physical and a chemical change are given below:

PHYSIAL CHANGE	CHEMICAL CHANGE
1. No new substance is formed.	1. A new substance is formed.
2. Physical change is reversible.	2. Chemical change is usually irreversible.
3. It is a temporary change.	3. It is a permanent change.
4. No heat energy is evolved or absorbed.	4. Heat energy is evolved or consumed in a large amount.
5. Examples: Melting of ice, formation of clouds, tearing of cloth etc.	5. Examples: Rusting of iron, ripening of fruits, cooking of food etc.

(b) (i) Breaking of a glass bottle is a physical change because each of the broken piece of bottle is still a glass and no new substance has been formed. In the process, only the shape and size of bottle change.

(ii) Burning of coal in air is a chemical change because the burnt coal cannot be obtained back and it is an irreversible change.

(iii) Preparation of a cake is a chemical change as all the raw materials that go inside the mixture get cooked after baking and the process cannot be reversed.

(iv) Knitting sweater from wool is a physical change as we can obtain back the wool from the sweater. So, it is a reversible process.