Latent heat is the quantity of heat required to bring about a change of state of a unit mass of a substance from solid to liquid (latent heat of fusion) or from liquid to gas (latent heat of vaporization or of condensation) or from solid to gas directly (latent heat of vaporization) without change of temperature (i.e., isothermally).
At the freezing point and boiling point of a substance, adding heat produces no rise in temperature until the change of state is complete. The energy required to effect the change of state is absorbed in the form of latent heat, and an equal amount of heat is liberated in the reverse process.
In the case of ice changing to water at 0 °C, for example, the heat energy absorbed during melting is used exclusively to overcome the intermolecular forces in the order ice structure; only when this is accomplished, and further heat is added does the kinetic energy of the water molecules and, hence, the temperature of the water begin to rise above 0 °C.
Latent heat of fusion
The latent heat of fusion of a substance (or enthalpy of fusion), is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid, at constant pressure.
Latent heat of condensation
The latent heat of condensation (or enthalpy of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (the substance absorbs heat), whereas enthalpy changes of condensation are always negative (the substance releases heat).
Latent heat of sublimation
The latent heat of sublimation (or enthalpy of sublimation) is the heat required to change one mole of a substance from solid state to gaseous state at a given combination of temperature and pressure, usually standard temperature and pressure (STP). The heat of sublimation is usually expressed in kJ/mol, although the less customary kJ/kg is also encountered.
Latent heat of vaporization
The latent heat of vaporization (or enthalpy of vaporization) is the amount of energy needed to change a liquid into vapor once it has reached its boiling point. Together with heat capacity, it is an essential property in determining how effectively a solvent can regulate the internal temperature of an organism.
The fact that water has both a high heat of vaporization and a high heat capacity makes it ideal in this respect and is one of the reasons it is so essential to life as we know it.