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Latent heat describes the amount of energy in the form of heat that is required for a material to undergo a change of phase (also known as "change of state"). This concept was introduced around 1750 by Joseph Black. Two latent heats 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 when going from solid to liquid to gas, but exothermic when going in the opposite direction. For example, in the atmosphere 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 for a body to release 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 sublimation) has occurred.
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