The behaviour of a real gas is usually depicted by plotting compressibility factor Z versus P at a constant temperature. At high temperature and high pressure, Z is usually more than one. This fact can be explained by van der Waals equation when

P + n2aV2 V-nb=nRTAt low pressures, ‘b’ can be ignored as the volume of the gas is very high. At high temperate ‘a’ can be ignored as the pressure of the gas is high.∴        PV-b=RT             PV - Pb = RT            PV = RT + Pb           PVRT=Z=1+PbRT