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Change in State
The addition of heat to a substance usually increases the temperature, and the removal of heat usually lowers the temperature. This is not always the case. If heat is added to water in an open pan on the stove, the temperature of that water will increase until it reaches 212°F (100°C). At that temperature, water changes state from a liquid to a gas, and water vapor (steam) is produced.
As more heat is added, more of the liquid water is changed to water vapor. But there is no increase in the temperature of either the liquid or the vapor. The temperature will remain constant until all the liquid is changed to vapor. The added heat is absorbed in the process of changing from a liquid to a gaseous state.
This heat is called the latent heat of vaporization. For water, this comprises 970 BTUs per pound. When the water vapor condenses back into the liquid state, an equivalent amount of heat is given off. When water is cooled, the temperature drops until it reaches 32°F (0°C), at which point it freezes or changes into a solid state. As more heat is extracted, more water changes to ice. But there is no decrease in temperature until all the water is changed to ice.
The heat given off in the change of state from liquid to solid is called the latent heat of fusion. For water, this comprises 144 BTUs per pound. When ice changes back into liquid water, an equivalent amount of heat is absorbed.
