JEE Main & Advanced Chemistry States of Matter / पदार्थ की अवस्थाएँ - गैस एवं द्रव The Critical State

(1) A state for every substance at which the vapour and liquid states are indistinguishable is known as critical state. It is defined by critical temperature and critical pressure.           

(2) Critical temperature (Tc) of a gas is that temperature above which the gas cannot be liquified however large pressure is applied. It is given by, \[{{T}_{c}}=\frac{8a}{27Rb}\]           

(3) Critical pressure (Pc) is the minimum pressure which must be applied to a gas to liquify it at its critical temperature. It is given by, \[{{P}_{c}}=\frac{a}{27{{b}^{2}}}\]           

(4) Critical volume (Vc) is the volume occupied by one mole of the substance at its critical temperature and critical pressure. It is given by, \[{{V}_{c}}=3b\]           

(5) Critical compressibility factor (Zc) is given by, \[{{Z}_{c}}=\frac{{{P}_{c}}{{V}_{c}}}{R{{T}_{c}}}=\frac{3}{8}=0.375\]         

A gas behaves as a Vander Waal?s gas if its critical compressibility factor \[({{Z}_{c}})\] is equal to 0.375. A substance is the gaseous state below \[{{T}_{c}}\]is called vapour and above \[{{T}_{c}}\]is called gas.