Question

${{\text{O}}_{\text{2}}}$ is denser than air and so, it collected over:
A. ${{\text{H}}_{\text{2}}}{\text{O}}$
B. spirit
C. Hg
D. kerosene oil

Answer 1

Hint:oxygen is a homo-diatomic molecule, so a non polar molecule. Due to non-polarity it will not react with a polar liquid such as water. Due to high density oxygen is also insoluble in a polar solvent.

Complete answer:
By the chemical reaction, a gas is produced. This gas is collected. The method used for the collection of gas depends upon the two factors:
1. The solubility of the gas
2. The density of the gas
The gases which are insoluble or very less soluble in water and are denser than air are collected over water.
In this method, the reaction mixture is taken into a flask. Then this mixture is passed over a container having water. The gas produced does not dissolve in water due to insolubility and high density so it goes upwards and removes through the channel and collects.
Oxygen is non-polar gas, so it is insoluble in water and it is given that oxygen is denser than air, so it is collected over water.
So, ${{\text{O}}_{\text{2}}}$ is denser than air and so it is collected over ${{\text{H}}_{\text{2}}}{\text{O}}$.

Therefore, option (A) ${{\text{H}}_{\text{2}}}{\text{O}}$, is correct.

Note:
The gases which are soluble in water and are less or more dense than air cannot be collected over water because they will react with water. The gases which are denser than air and are soluble such as carbon dioxide and hydrogen chloride are collected by downward delivery. The gases which are less dense than air and are soluble such as ammonia are collected by upward delivery.