The liquefaction behaviour of the temporary gases like CO2 approaches that of permanent gases like N2,O2 etc., as we go

Critical temperature of a gas is highest temperature at which liquefaction of gas first occur. Liquefaction of so called permanent gases (i.e. gases which show continuous positive deviation in Z value) requires cooling as well as considerable compression.But temporary gases like CO2 and CH4 approach that of permanent gases as we go above the critical temperature.