Heat of Vaporization
Also: Heat of Condensation
The heat of vaporization is defined as the heat required to vaporize one mole of a substance at its standard boiling point. The heat of vaporization is expressed in kJ/mol. The use of kJ/kg is also possible, but less customary. When the change is from the vapor state to the liquid state (i.e. condensation) this heat is released.
When heat is added to a liquid, the kinetic energy of the molecules in the liquid increases and the increased motion of the molecules is reflected by a rise in the temperature of the liquid. When the boiling point of the substance is reached, however, the temperature remains constant as the change of phase occurs. The continued addition of heat at that temperature is used to break down the intermolecular attractive forces, and also must provide the energy necessary to expand the gas. The heat required to change a liquid to a solid with no change in temperature is called the latent heat of vaporization.
The heat of vaporization of water is about 40.6 kJ/mol (2260 kJ/kg). This is quite a lot: five times the energy needed for heating the water from 0 to 100 degrees Celsius.
See also "Heat of Fusion"
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