# 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.[1][2]

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:

[itex]Q = mL \,[/itex]

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.

## Table of latent heats

Latent heats and change of phase temps of common fluids and gases
Substance Latent Heat
Fusion
J/g
Melting
Point
°C
Latent Heat
Vaporization
J/g
Boiling
Point
°C
Alcohol, ethyl 108 -114 855 78.3
Ammonia 339 -75 1369 -33.34
Carbon dioxide 184 -57 574 -78
Helium     21 -268.93
Hydrogen 58 -259 455 -253
Nitrogen 25.7 -210 200 -196
Oxygen 13.9 -219 213 -183
R-134A -101 215.9 -26.6
Toluene   -93 351 110.6
Turpentine     293
Water 334 0 2260 100

## References

1. Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
2. Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8.
3. Textbook: Young and Geller College Physics, 8e, Pearson Education