Dear Anurag,

Its not that van der waals forces are stronger only at critical temperature , there effect is stronger as we go down the temperature, its just that,

above critical temperature kinetic energy of gaseaous molecules is high which can''''t be countered by van der waals forces by appling any pressure, but as a gas reaches its critical temperature it''''s moleculer kinetic energy becomes low hence can be countered by van-der-waals forces at any certain pressure, allowing liquification of gas.

Also a gas below its critical temperature is called vapour.

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