Critical Phenomenon and Critical Constants
Gasses can be liquefied if the temperature decreases and the pressure increases continuously, But for every gas there is a characteristic temperature above which it cannot be converted to the liquid state no matter how high the pressure is. 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 Tc. Hence critical temperature is defined as the temperature below which the gasses can converted into liquid on continuously increase in pressure and above which no liquefication(i.e. a process of converting gas into liquid) is possible, no matter how high the pressure is.
At the critical temperature, a certain minimum pressure has to be applied to the gas to liquefy it. This pressure is called critical pressure and is denoted by Pc may be defined as the minimum pressure which must be applied to a gas at its critical temperature to liquefy it.
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 Vc.
At this stage or point, both gas and its corresponding liquid would occupy the same volume and therefore their densities are equal so, at this stage, it is not possible to distinguish between the liquid and gaseous states 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.
Tc, Pc and Vc are known as Critical Constants of the gas.
