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Latent heat
From Wikipedia, the free encyclopedia
In thermochemistry, latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of state.[1][2] A change of "state", i.e. solid, liquid, or gas, is 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 latent heat is generally obsolete, where instead the modern conception is that of enthalpy of transformation.
Two latent heats or enthalpies of transformation are typically described: one is the latent heat of fusion (melting), and the other is the latent heat of vaporization (evaporation). They are so named as to describe the direction of heat flow from one phase to the next:
- solid → liquid → gas.
The energy change is endothermic, i.e. the process absorbs energy, when going from solid to liquid to gas, but exothermic, releases energy, when going in the opposite direction. In the atmosphere, for example, when a molecule of water evaporates from the surface of any body of water, heat 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 turn water into water vapor, water vapor is a way that a body releases energy. If the water vapor is returned to a liquid or solid phase (by condensation or deposition), the stored energy is released as sensible heat onto the surface where condensation (or deposition) has occurred.
| Substance | Latent Heat Fusion J/g |
Melting Temp °C |
Latent Heat Vaporization J/g |
Boiling Temp °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 |
| Toluene | -93 | 351 | 110.6 | |
| Turpentine | 293 | |||
| Water | 334 | 0 | 2272 | 100 |