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 sea level. At that temperature, known as water's boiling point, water changes state from a liquid to a gas state 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 is 970 Btu per pound. This change of state is one of the reasons you feel so cold stepping out of a shower. As the water on your skin evaporates, it takes with it 970 Btu per pound of water evaporated. Evaporation has a cooling effect and is therefore used to cool rooftops with water roof spray and cool outside areas in hot arid climates with systems that spray a fine mist on people in the area.

When the water vapor condenses back into the liquid state an equivalent amount of heat is given off ... 970 Btu for every pound of water.

As water is cooled, the temperature drops until it reaches 32°F (0°C), at which point it freezes or changes state 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. Once the water is completely frozen (converted to ice), the temperature will drop once again as heat is extracted at the rate of one degree per Btu per pound. The heat given off in the change of state from liquid to solid is called the "latent heat of fusion". For water, this is 144 Btu per pound. When ice changes back into liquid water, an equivalent amount of heat is absorbed ...144 Btu per pound.

That's why it feels so good to put an ice cube on your skin when you are hot. The ice takes away 144 Btu per pound of water it melts, or changes state.