Deviation from Ideal Gas Behaviour

Deviation from Ideal Gas Behaviour is observed particularly at high pressures & low temperatures. A very convenient way of showing the deviation of real gases from Ideal behaviour is in terms of compressibility factor ‘Z’. Mathematically,

Z  =  PV/nRT         

(A).        In case of Ideal gas, Z = 1   (PV=nRT)

(B).         In case of real gas, Z ≠ 1   (PV ≠  nRT)

Thus for, real gas, Z ≠ 1, it can be greater than one or less than one.

(1).         If Z>1, it refers PV is greater than nRT, which implies that gas is less compressible than expected from Ideal behaviour. Such a deviation from Ideal behaviour is known as Positive Deviation.

(2).         If Z<1, it refers PV is less than nRT, which implies that gas is more compressible than it should if it behaves Ideal gas. Such a deviation is known as Negative Deviation.