The amount of heat energy absorbed or lost by a substance when changing phases is called “latent heat.” When steam is condensed towater, the temperature doesn’t change, but heat must be extracted from
the steam as it goes through a phase change to water. To change water to steam, heat must be added. When a substance changes from a solid to a liquid or from a liquid to a vapor, the heat absorbed is in the form of latent heat. This heat energy is referred to as latent heat because it cannot be sensed by measuring the temperature.
The latent heat of vaporization for hydrocarbon compounds is given in Table 2-9. The latent heat of vaporization of water is given by hfg in the steam table (Table 2-6).