When a substance changes phase from liquid to gas, heat energy is required. The heat energy that is used to transform a liquid into a gas, or vapor, is called the heat of vaporization. As thermal energy is applied to a volume of a given liquid at a constant given pressure, the temperature of the liquid rises. With the continued application of heat, the temperature steadily increases until, at some characteristic point (called the boiling point) the liquid begins to shift from liquid to gas. While boiling, the temperature does change even though heat energy is being added. The heat, instead of causing a rise in temperature, is absorbed by the molecules as they are liberated into gaseous form. The heat that is required in this process is the heat of vaporization for the liquid. However, the amount of heat required is always dependent on the external pressure exerted on the liquid and the actual composition of the liquid itself.

Because heat energy is consumed without a concomitant increase in temperature, it is also known as latent heat of transformation. Heat of vaporization for substances is dependent on a number of different factors, including pressure and external temperature. For example, the human body exploits the vaporization of water to cool itself through sweat--and it does so at temperatures far below water's boiling point. Substances like rubbing alcohol have vaporization points that are even lower than their boiling points. Heat of vaporization is often expressed as the amount of heat (in Joules) that is required to change 1 gram of liquid into gas. For example, the heat of vaporization of water is 2,260 Joules of heat per gram. At sea level, this translates into about 540 calories per gram of water vapor.

This relatively high value is an important characteristic of water because it allows water to absorb large amounts of heat as it changes phase. This quality explains how perspiration cools body temperature as it evaporates from skin, and why burn injuries from steam are more serious than from water at the same (boiling) temperature.