The state of a gas is defined by a relationship between the four independent variables pressure (P), volume (V), temperature (T) and number of moles (n). The relationship between these parameters is governed by different gas laws as summarised below.
Gases that obey the equation PV=nRT under all conditions are called ideal gases. But in practice there is no ideal gas. Gases tend to behave ideally at high temperatures and at low pressures. For real gases, van der Waals modified the ideal gas equation as
Critical temperature (Tc) of a gas is defined as the temperature above which it cannot be liquefied at any pressure. Critical pressure (Pc) of a gas is defined as the minimum pressure required to liquefy 1 mole of a gas at its critical temperature. Critical volume (Vc) is defined as the volume occupied by 1 mole of a gas at its critical temperature and critical pressure. The critical constants are related to Van der Waals constants as follows
When a gas is made to expand adiabatically from a region of high pressure into a region of low pressure, the temperature of the gas is reduced rapidly and this is known as Joule-Thomson effect. This effect is used in the liquefication of gases.