Question

(A) 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.

(B) Define the following terms:
(i) Condensation
(ii) Sublimation

Answer 1

(A) 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.
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.
So, change A is a physical change while change B is a chemical change.

(B) (i) Condensation : The process by which a substance in gaseous state changes into its liquid state on cooling is called condensation.
(ii) Sublimation: The process by which solid changes into gaseous state directly on heating and vice-versa without passing through the liquid state is called sublimation.