Liquefaction of Gases

There are large empty spaces (voids) separating the tiny molecules of gases from one another. Each molecule enjoys an almost independent existence. Molecules are in a state of continuous rapid motion and have negligible attractive forces between them due to wide separation. This is particularly so, when temperature is high and pressure is low. When the temperature of the gas is lowered, both the volume of the gas and the kinetic energy of the molecules decrease. The molecular motion becomes slow and molecules become sluggish. The progressive decrease of temperature brings the molecules closer and closer because they are unable to resist the attractive force that starts operating between them. Ultimately, at sufficiently low temperature, the voids between the molecules become less than 10^-5cm and the gas changes into liquid state.

This process of liquefaction by bringing gas molecules closer can also be achieved by increasing the pressure of the gas: this also decreases the volume of the gas. For example, sulphur dioxide can be liquefied at 265 K if pressure is 760 mm of Hg. It can also be liquefied at 293K if the pressure is increased to 2470 mm of Hg.

Thus, liquefaction of gases can be achieved by either decrease of temperature or by increase of pressure.

Concept of critical temperature

Experimental observations have shown that effect of temperature is more significant in achieving liquefaction than that of pressure. This is due to the fact that for every gas there is a certain temperature above which it cannot be liquefied how so ever high the pressure may be. The kinetic energy of gas molecules above this temperature is sufficient enough to overcome the attractive forces. This temperature is referred to as the critical temperature of the gas. It may be defined as the temperature above which a gas cannot be liquefied by application of pressure. A gas will remain as gas above critical temperature and in order to liquefy it by compression, it has to be cooled to its critical temperature. Critical temperature is denoted by T_c.

For example,

T_c of CO₂ = 304.1 K.

Critical pressure (P

_c)

The pressure required to liquefy the gas at its critical temperature is called critical pressure.

For example, the value of P_c for CO₂ = 55328 mm of Hg.

Critical volume (Vc)

The volume of one mole of the gas at critical temperature and critical pressure is called critical volume.

For example, value of V_c for CO₂ = 94.0 cm³ mol^-1

The parameters T_c, P_c and V_c for a gas are collectively called 'critical constants'.