The heat of vaporization is a physical property of substances. It is defined as the heat required to vaporize one mole of a substance at its boiling point under standard pressure (101325 Pa). The heat of vaporization is expressed in kJ/mol. The use of kJ/kg is also possible, but less customary.
Because vaporization is the opposite process of condensation, the term heat of condensation is also used. The latter is defined as the heat released when one mole of the substance condenses at its boiling point under standard pressure.
The heat of vaporization of water is about 2260 kJ/kg which is equal to 40.8 kJ/mol. This is quite a lot: five times the energy needed for heating the water from 0 to 100 degrees Celsius.
Heats of vaporization of the elements