Question

(A) (i) Explain brownian motion. [1 Mark]
(ii) Define intermolecular force of attraction. [1 Mark]
(iii) Give reason why a gas can fill the whole vessel in which it is enclosed. [1 Mark]

(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) What part of fractional distillation apparatus makes it efficient and possess an advantage over a simple distillation process? Explain using a diagram. [4 Marks]

Answer 1

(A) (i) Brownian motion is the haphazard, random motion of suspended particles in a fluid as they constantly collide with other molecules. [1 Mark]
(ii) The molecules of matter are in constant motion and attract each other with a force called intermolecular force of attraction due to which they are held together. [2 marks]
(iii) Because, in gases, the molecules are free to move. The intermolecular force of attraction is the least in gases. So, a gas almost fills the whole vessel in which it is enclosed. [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) When the difference in the boiling points of the components is less than 25 K, they are separated using a special technique known as fractional distillation. ​[1 Mark]
This technique uses an apparatus named fractionating column, containing beads that separate the component at different heights depending on their boiling points. [1 Mark]
The components are then passed to the water condenser, where the water regulates the temperature. In the condenser, the vapours are converted to liquid state, separating the components to separate in a different container. [1 Mark]
[1 Mark]