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A number of physical changes are associated with the change of temperature of a substance. Almost all substances expand in volume when heated and contract when cooled. The behavior of water between 0 and 4 C (32 and 39 F) constitutes an important exception to this rule. The phase of a substance refers to its occurrence as either a solid, liquid, or gas, and phase changes in pure substances occur at definite temperatures and pressures. The process of changing from solid to gas is referred to as sublimation, from solid to liquid as melting, and from liquid to vapor as vaporization. If the pressure is constant, these processes occur at constant temperature. The amount of heat required to produce a change of phase is called latent heat, and hence, latent heats of sublimation, melting, and vaporization exist (see Distillation; Evaporation). If water is boiled in an open vessel at a pressure of 1 atm, the temperature does not rise above 100 C (212 F), no matter how much heat is added. The heat that is absorbed without changing the temperature of the water is the latent heat; it is not lost but is expended in changing the water to steam and is then stored as energy in the steam; it is again released when the steam is condensed to form water (see Condensation). Similarly, if a mixture of water and ice in a glass is heated, its temperature will not change until all the ice is melted. The latent heat absorbed is used up in overcoming the forces holding the particles of ice together and is stored as energy in the water. To melt 1 g of ice, 79.7 cal are needed, and to convert 1 g of water to steam at 100 C, 541 cal are needed.