The liquefaction behavior 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 devotion in Z value) requires cooling as well as considerable cmpression. But temporary gases like CO2 and CH4 approach that of permanent gases as we go above the critical temperature.