Question

Answer the following questions:
(1) Give two examples of changes that are both irreversible and desirable. [1 Mark]
(2) Justify the characteristics of physical change with the help of examples. [3 marks]

Answer 1

(1) The two changes that are both irreversible and desirable are the cooking of food and the growth of an infant. [1 mark]
(2) (i) No new substances are formed during physical change.
Eg: When ice is heated, it melts and turns into water. When the water is heated further, it turns into steam. The steam condenses and becomes water when it cools. The water forms into ice as it cools further. Ice, water, and steam molecules, always have two hydrogen atoms and one oxygen atom. As a result, we can conclude that no new substances are formed. [1 mark]
(ii) Physical changes are generally reversible.
Eg: When the wax is heated, it turns into a liquid. However, when liquid wax cools, it solidifies. [1 mark]
(iii) There is no change in weight during physical change.
Eg: Make a salt solution by dissolving 20g of salt in water. It will end up with 20g of salt after the water evaporates. [1 mark]