The heat required to change the state of a substance without changing its temperature is called its latent heat, or hidden heat. Theoretically, any substance can be a gas, liquid, or solid, depending on its temperature and pressure. It takes heat to change a substance from a solid to a liquid or, from a liquid to a gas.
For example, it takes 144 Btu of latent heat to change 1 lb of ice at 32ºF to 1 lb of water at 32ºF. It takes 180 Btu of sensible heat to raise the temperature of 1 lb of water 180ºF from 32ºF to 212ºF. It takes 970 Btu of latent heat to change 1 lb of water to steam at 212ºF. When the opposite change is effected, equal amounts of heat are taken out or given up by the substance.
This exchange of heat, or the capability of a medium, such as water to take and give up heat, is the basis for most of the heating and air-conditioning industry. Most of the functions of the industry are concerned with adding or removing heat at a central point and distributing the heated or cooled medium throughout a structure to warm or cool the space.