It has been observed that when a gas is cooled below a certain temperature, it can be converted to its liquid state by the application of pressure alone. When the temperature of the gas is above that temperature, the gas can’t be compressed to liquid. This temperature for gas is called the critical temperature.
The pressure applied to change a gas to its liquid state at a critical temperature is called its critical pressure. Every gas has its characteristics and critical values of temperature and pressure.
Critical Temperature of Gas and Vapour:
A gas below its critical temperature is a vapour and vapours can be compressed to the liquid state. But when the temperature of a gaseous matter is above its critical temperature, it is a gas. Naturally, a gas can’t be liquified only by application of pressure.
Critical temperature of oxygen and hydrogen are -119°C and -24°C respectively. Hence, at room temperature, these are permanent gases and can’t be liquefied by applying pressure alone. But. the critical temperatures for carbon dioxide, ammonia, sulphur dioxide are 31°C, 132.2°C and 157.2°C respectively. So, they can be compressed to liquid at room temperature and are therefore called vapours.