It is a familiar fact that pure substances tend to exist in one of three distinct states: solid, liquid, and gas. Take water, for example. As ice is heated at atmospheric pressure, it suddenly melts into liquid at a specific temperature. As the liquid continues to be heated, it eventually reaches a temperature at which it spontaneously vaporizes into a gas. These transitions are discontinuous; they occur at specific state conditions or particular combinations of T and P. At exactly those conditions, the system can exist in more than one form such that two (or more) phases are in equilibrium with each other.

Although we are typically familiar with phase behavior at atmospheric pressure, most substances experience a diverse set of phases over a broad range of pressures. Pure substances often have more than one crystalline phase, depending on the pressure. Figure 10.1 shows a schematic representation of a P–T phase diagram of water that illustrates the kind of complex behavior that can exist. In the case of mixtures, there are even more possibilities for phase equilibrium: for example, one can have equilibrium between two liquids of different compositions, or among multiple solid and liquid phases.