Liquefaction of Gas
The liquefaction of a gas is a phenomena which takes place when the intermolecular forces of attraction increases to such an extent that they combine the gas molecules together forming a liquid state. Liquefaction of gas can be increased by increasing the intermolecular forces of attraction, which in turn can be increased either by increasing the pressure, which will reduce the distance between the molecules or decreasing the kinetic energy by cooling the gas making them slower. Hence, a gas can be liquefied by cooling or by application of pressure or the combined effect of both.
The temperature at which a gas can be liquefied is called liquefaction temperature. Above this temperature, the gas can not be liquedfied no matter how high is the pressure. Thus liquefaction temperature of a gas is the critical temperature (Tc). Above this temperature, gaseous state exists, at this temperature liquefaction occurs and below this temperature liquid state exists. Critical temperature depends on the attractive forces present in the gaseous molecules.
The volume and pressure corresponding to critical temperature is called critical volume(Vc) and critical pressure(Pc) respectively. Critical temperature, critical pressure and critical volume are called critical constants.
The value of Tc, Pc and Vc are-
Tc = 8a/27Rb
Pc = a/27b2
Vc = 3b
Gases can be liquefied by various method. Linde's method, Claude's Method and Faraday's method are used frequently for this purpose.
