When you blow on your cup of coffee
evaporation takes place. In the evaporation the water molecules
leaves the water surface and comes in the air. In this process the
escaping water molecule has to overcome the force of attraction
applied by the other water molecules of the surface. As we know that
the force of attraction between the molecules is of electrostatic
type. So escaping molecule must have enough kinetic energy to
overcome the potential energy of this force of attraction. Since
leaving molecules possess more energy than the ordinary ones. The
left ones are of less energy than the average. Hence the random
motion of molecules decreases. As you may know higher the random
motion of molecules higher the temperature of the substance . So
temperature of liquid decreases.
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