Explain the liquefication of gases.

Explanation

In order to liquefy a gas it must be cooled below its critical temperature. A gas can be liquefied by cooling into below its boiling at given pressure. But this type of technique is not possible for the liquefaction of gases like `N_2` and `O_2` which possess very low boiling points. To liquefy such type of gases a special technique based on intermolecular forces is to be used.
If we reduce the velocities of molecules to lower values then the neighbouring molecules attract each other, get cooled and condense to a liquid . In order to happen this , the gas is allowed to expand into available volume without supplying any heat from outside. In this process the attractions between the neighbouring molecules will be lessened. In doing so, the gas molecules convert some of their kinetic energy into potential energy and travel slowly. As a result of which the average velocity decreases and therefore the temperature of the gas decreases and the gas cools down. In order to happen this the gas is allowed to expand through a narrow opening called throttle. This way of cooling of low pressure is called Joule-Thomson effect. If the process is repeated several times by allowing the cooled gas again to mix up with the remaining gas by recirculation, finally the molecules get cooled to such a low temperature, and as a result of which the gas condense to a liquid.