Critical Temperature

In a gas, the molecules are in a state of constant motion. Hence, they possess a definite amount of Kinetic energy. As the temperature of gas is lowered (i.e. the gas is cooled), the Kinetic energy of the molecules decreases. Hence the molecules become slower and come closer. As a result, the intermolecular force of attraction increases. Ultimately, if the cooling is continued, a stage is reached where the intermolecular forces of attraction becomes so large that the gas liquefies.

Similarly, if at a particular temperature, pressure is applied more and more on the gas, the molecules come closer. Hence the intermolecular forces of attraction will keep on increasing and ultimately a stage may reach where gas may change into liquid.

Out of the effect of temperature and pressure as discussed above, it has been observed that effect of temperature is much more significant. It has been observed that for number of gases, if the temperature is not below a particular value, the gas does not liquefy howsoever high pressure we may apply. This temperature is called Critical Temperature.

Hence critical temperature (T_c) of a gas may be defined as, “that temperature above which the gas cannot be liquefied, however high pressure we may apply on the gas”. The minimum pressure required to liquefy the gas at critical temperature is called Critical Pressure (P_c). The volume occupied by one mole gas at critical temperature and pressure is known as Critical Volume (V_c).