What is the process for the …

A gas can be converted into a liquid through a process called liquefaction.

Liquefaction of a gas can occur only when the intermolecular force of attraction between the gas molecules are increased.

Methods for liquefaction:

(i) Increase in pressure of the gas at room temperature: On increasing pressure the gas molecules come closer to each other, results in an increase in the intermolecular forces of attraction between the molecules, which leads to the liquefaction of gases.

(ii) Decrease in the temperature of the gas:

As the gas gets cooled the kinetic energy of the gas molecules decreases, results in the decrease of the speed of their random motion and in the increase of the force of attraction between them.

Carbon dioxide, ammonia, sulphur dioxide and hydrogen chloride can be liquefied by increasing the pressure and decreasing the temperature.

There are some gases, which cannot be liquefied at room temperature even at very high pressure. These gases are called as permanent gases.

Ex: H_2, He, O₂.