The critical temperature finds definitions in different areas; in the case of fluid transition is defined as the critical temperature, the temperature above which a substance can not exist in a liquid state (not even being subjected to compression). In the case of the superconducting transition, it is defined as the critical temperature, the temperature below which the material becomes superconducting. More generally, the temperature at which a phase transition occurs can be called critical temperature. The value of a gas temperature related to its critical temperature explains the difference between gas and vapor:
- gas is aeriform at a temperature above its critical temperature (therefore, even if subjected to high pressures, can not condense);
- vapor is aeriform at a temperature below its critical temperature (consequently they can condense if subjected to pressure).
This is equivalent to saying: the gas is not liquefiable by compression; the vapor is liquefiable by compression. The same chemical substance can, therefore, behave as gas or vapor depending on whether it is at a temperature above or below its critical temperature. In daily practice, it is customary to attribute the name gas to those aeriform whose critical temperature is clearly below the average ambient temperature, and that of vapor to those aeriform whose critical temperature is clearly above the aforementioned average ambient temperature.
The existence of the critical temperature for the liquefaction of gas is understandable because the gas molecules can join together to form a liquid only if the energy of the interactions between the molecules is greater than their average kinetic energy. By increasing the temperature, the energy of the interactions remains practically constant (it depends solely on the electronic structure) while the kinetic energy increases; it is obvious that once the critical temperature at which the average kinetic energy = the energy of the interactions is exceeded, the gas cannot liquefy. Obviously, the weaker the interactions between the molecules of a gas, the lower the value of its critical temperature will be. Water, in which the binding energy between the molecules (hydrogen bond bridge) is significantly higher than those of helium (London forces) has a critical temperature (37.1 °C = critical temperature of water) much higher than that of helium (−267.9 °C).