The standard enthalpy change of vaporization is a physical property of substances. It is defined as the amount of heat (or energy) required per unit mass of a substance to completely vaporize the substance at its boiling point. The heat of vaporization is expressed in kJ/mol, or calories/gram. The use of kJ/kg is also possible, but less customary. Other units still in use in certain places include Btu/lb.
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 a unit mass of a substance is completely condensed at its boiling point.
The standard enthalpy change of vaporization of water is about 2260 kJ/kg which is equal to 40.8 kJ/mol. This is quite a lot: it is five times the energy needed for heating the water from 0 °C to 100 °C.
Heats of vaporization of the elements
Other common substances
See also
Reference
Sears, Zemansky et. al., University Physics, Addison-Wessley Publishing Company, Sixth ed., 1982, ISBN 0-201-07199-1
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