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 \[\mathbf{(}{{\mathbf{T}}_{\mathbf{c}}}\mathbf{)}\] 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 \[\mathbf{(}{{\mathbf{P}}_{\mathbf{c}}}\mathbf{)}\] 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 \[\mathbf{(}{{\mathbf{V}}_{\mathbf{c}}}\mathbf{)}\] 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 \[\mathbf{(}{{\mathbf{Z}}_{\mathbf{c}}}\mathbf{)}\] 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.