Latent heat

In thermochemistry, latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase (i.e. solid, liquid, or gas), – also called a phase transition.

The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden. The term is now dated, replaced by "enthalpy of transformation".

Two latent heats (or enthalpies) are typically described: latent heat of fusion (melting), and latent heat of vaporization (boiling). The names describe the direction of heat flow from one phase to the next: solid → liquid → gas.

The change is endothermic, i.e. the system absorbs energy, when the change is from solid to liquid to gas. It is exothermic (the process releases energy) when it is in the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains more water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

Latent heat equation
The equation for latent heat is:


 * $$Q = mL \,$$

where:


 * Q is the amount of energy required to change the phase of the substance (in joules),
 * m is the mass of the substance,
 * L is the specific latent heat for a particular substance.

In other words, specific latent heat is found when energy is divided by mass.