Question

(A) 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]

(B) Liquefied petroleum gas is used in kitchens. When the gas is in a cylinder, it is in the liquid state. When it comes out of the cylinder it becomes a gas (change A), then it burns (change B). Identify the processes A and B with proper explanation. [3 Marks]

(C) If an object weighs 12 N on Earth, what will its weight on the Moon be? What is the change in its mass? [3 Marks]

Answer 1

(A) (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]

(B) $\bullet$ LPG is in a liquid state inside the cylinder. However, when it comes out of the cylinder it becomes a gas. So, it is a physical change, because no new substance is formed. It is only a change in state. (1 Mark)
$\bullet$ When the gas comes out and burns, it is a chemical change, because the LPG gas burns with oxygen and forms carbon dioxide. Heat is also released while burning the gas. (1 Mark)
So, change A is a physical change while change B is a chemical change.(1 Mark)

(C)

• Weight of body on the Moon = 1/6th of its weight on Earth​. [1 mark]
• Therefore, the body will weigh 1/6 of 12 = 1/6 x 12 = 2 N on the Moon​ [1 mark]
• Mass of a body does not change with change in gravity. So, mass of a body will remain the same on moon. [1 mark]