Liquefaction of gases

  • Discovered by Andrews for CO2
  • Above a certain temperature, it is impossible to liquefy a gas.
  • The temperature below which a gas can be compressed by applying only pressure is called critical temperature (Tc).
  • The pressure applied to liquefy gas is called critical pressure Pc
  • The volume occupied by one mole of the substance is called critical volume Vc
  • The pressure v/s volume plot for CO2 is
    Lquefaction of Gases
  • At low temperature, say 13.1˚C, the CO2 is entirely gaseous.
  • As pressure is increased, the volume decreases from A to B.
  • At B, deviations from Boyle’s law (P V) start. The volume decreases rapidly till C. between B & C, CO2 exists both as a liquid and gas and pressure remains constant.
  • After C, CO2 is completely liquefied and on increasing pressure further only slight decrease in volume is seen.
  • The pressure corresponding to the horizontal line BC is the vapour pressure of liquid.
  • As the temperature is raised, the horizontal portion becomes smaller till 31.1˚C where it reduces to a point Tc. At Tc the boundary between liquid and gas disappears.
  • Above 31.1˚C, no liquefaction is seen.
  • Thus, it can be seen that liquefaction of gases is possible only at lower temperatures.

Van der Waal’s Equation and the Critical Point:

The constant of Van der Waal’s equation can be obtained from the critical point data.

The Van der Waal’s Equation can be arranged as follow:

P = RT / (V – b) – a / V2

To investigate the horizontal point of inflection on a plot of P versus V, we obtain

dP/dV = -RT/ (V – b2) + 2a / V3

d2P/ dV = 2RT/ (V – b)3 – 6a/V4

At gas-liquid equilibrium, i.e. at critical-point,

P = Pc

V = Vc

T = Tc

And first order and second order derivative of P with respect to V are zero.

Therefore we can write;

Pc = RTc /(Vc – b) – a / Vc3

0 = RTc /(Vc – b2) + 2a / Vc3

0 = 2RTc / (Vc – b)3 – 6a / Vc4

These equations can be solved to get

a = 3PcV2c

b = Vc/3

R = 8 PcVc/ 3Tc

(PcVc) / (RTc) = 8/3 = 0.375


Zc = 0.375, which is a compressibility factor at critical point.

Since R is a gas constant, and the values of a and b are different for different gases.

They can be calculated in terms of critical-point quantities Pc and Tc by substituting

Vc = 3 RTc / (8Pc)

a = 27R2T2c / (64Pc)

b =  RTc / (8 Pc)