Explain what is meant by 'critical phenomenon'? What are 'critical constants' of a gas?

There are three critical constants for each gas namely critical temperature (T_c), the critical pressure (P_c) and the critical volume (V_c). If we go on cooling a gas to a sufficiently low temperature and increase the pressure on it, the gas can be liquefied. But for every gas there is a characteristic temperature above which it cannot be converted to the liquid state, however, we may increase the pressure on it. This limiting value of temperature is not the same for all gases but is different for different gases. This limiting value of temperature is called the critical temperature and is denoted by T_c. Hence critical temperature is defined as the temperature below which the continuous increase of pressure on a gas ultimately leads to its liquefication and above which no liquefication is possible, however, great the pressure be applied.

At the critical temperature, a certain minimum pressure has to be applied to the gas to liquefy it. It is called critical pressure and denoted by P_c. This critical pressure may be defined as ‘the minimum pressure which must be applied to a gas at its critical temperature to liquefy it.’

When the gas is at both critical temperature (T_c) and critical pressure (P_c), it is said to be under a critical state or at a critical point. At this stage, both gas and its corresponding liquid would occupy the same volume and therefore their densities are equal. In other words, we can say that at this stage, it is not possible to distinguish between the liquid and gaseous states of the substance as the two forms are existing in equilibrium. The phenomenon of a smooth merging of a gas into its liquid state under a critical state or critical point is referred to as ‘critical phenomenon’. The density of the gas at the critical point is called the critical density.

The volume occupied by one mole of a gas at its critical temperature and critical pressure is called the critical volume and is denoted by V_c.
T_c, P_c and V_c are known as critical constants of the gas.