Importance of Critical Temperature

The critical temperature of a gas is a measure of strength of the intermolecular forces of attraction of that gas. Weaker the intermolecular attraction (forces), more difficult it is to liquefy and hence lower will be its critical temperature. For example: H₂ and He have weak intermolecular forces of attraction. Thus, they are difficult to liquefy and hence low critical temperatures

H₂   =   33.2 K (T_c)

He  =   5.2 K (T_c)

On the other hand, CO₂ & NH₃ have strong intermolecular forces of attraction, they can be easily liquefied and their critical temperatures are high.

CO₂  = 31°c or 304K (T_c)

NH₃ =132°c or 405K (T_c)

Thus, higher the critical temperature (T_c) easier to liquefy the gas, lower the critical temperature (T_c) difficult to liquefy the gas.