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INTRODUCTION
When a liquid is placed in an open vessel, it slowly escapes into
gas phase, eventually leaving the vessel empty. This phenomenon
is known as evaporation. Evaporation of liquids can be explained
in terms of kinetic molecular model. Although there are strong inter-molecular
attractive forces which hold molecules of a liquid together, the
molecules having sufficient kinetic energy can escape into gas phase
if such molecules happen to come near the surface. In a sample of
liquid all the molecules do not have same kinetic energy. There
is a small fraction of molecules which have enough kinetic energy
to overcome the attractive forces and escape into gas phase.
Evaporation causes cooling. This is due to the reason that the
molecules, which undergo evaporation, are high-energy molecules;
therefore the kinetic energy of molecules which are left behind
is less. Since the remaining molecules have lower average kinetic
energy therefore, temperature must be lower. If the temperature
is kept constant the remaining liquid will have the same distribution
of molecular kinetic energies and the high-energy molecule will
keep on escaping from the liquid into the gas phase. If the liquid
is taken in an open vessel, evaporation will continue until whole
of the liquid evaporates.
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