Change from liquid to solid state, as when water becomes ice. For a given substance, freezing occurs at a definite temperature, known as the freezing point, that is invariable under similar conditions of pressure, and the temperature remains at this point until all the liquid is frozen. The amount of heat per unit mass that has to be removed to freeze a substance is a constant for any given substance, and is known as the latent heat of fusion.
Ice is less dense than water since water expands just before its freezing point is reached. If pressure is applied, expansion is retarded and the freezing point will be lowered. The presence of dissolved substances in a liquid also lowers the freezing point (depression of freezing point), the amount of lowering being proportional to the molecular concentration of the solution. Antifreeze mixtures for automobile radiators and the use of salt to melt ice on roads are common applications of this principle. Animals in arctic conditions, for example insects or fish, cope with the extreme cold either by manufacturing natural “antifreeze” and staying active, or by allowing themselves to freeze in a controlled fashion, that is, they manufacture proteins to act as nuclei for the formation of ice crystals in areas that will not produce cellular damage, and so enable themselves to thaw back to life again.