When a liquid is allowed to evaporate, it absorbs heat from the surroundings and evaporation is accompanied by increase in enthalpy. For example: 10.5 k cals is the increase in enthalpy when one mole of water is allowed to evaporate at 25oC. When the vapors are allowed to condense to liquid state, the heat is evolved and condensation of vapor is accompanied by decrease in enthalpy.
The Evaporation and condensation can be represented as,
Thus the change in enthalpy when a liquid changes into vapor state or when vapor changes into liquid state is called heat of vaporization.
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