LATENT HEAT

characteristic amount of energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature. The latent heat associated with melting a solid or freezing a liquid is called the heat of fusion; that associated with vaporizing a liquid or a solid or condensing a vapour is called the heat of vaporization. For example, when a pot of water is kept boiling, the temperature remains at 100 C (212 F) until the last drop evaporates, because all the heat being added to the liquid is absorbed as latent heat of vaporization and carried away by the escaping vapour molecules. Similarly, while ice melts it remains at 0 C (32 F), and the liquid water that is formed with the latent heat of fusion is also at 0 C. The structure of a crystalline solid is maintained by forces of attraction between the individual molecules or ions, which oscillate slightly about their mean positions in the array. When heat is absorbed, these motions increase until at the melting point the attractive forces can no longer preserve the orderly arrangement, and the solid changes into a liquid, in which the individual particles move about independently, attracted to each other only by forces much weaker and less specifically directed in space. When a substance is heated sufficiently, even the weak forces that hold the particles together in the liquid state are overcome, and at the boiling point the liquid transforms into vapour. Latent heat is associated with processes other than changes between solid, liquid, and vapour phases of a single substance. Many solids exist in different crystalline modifications, and the transitions between these are generally attended by absorption or evolution of latent heat. The process of dissolving one substance in another often involves heat; if the solution process is a strictly physical change, the heat is a latent heat. Sometimes, however, the process is accompanied by a chemical change, and part of the heat is that associated with the chemical reaction. See also thermal fusion.