In elements, this is known as allotropy, whereas in compounds it is known as polymorphism. Phase transitions can also occur when a solid changes to a different structure without changing its chemical makeup. Metastable states do not appear on usual phase diagrams. This occurs in superheating, supercooling, and supersaturation, for example. ![]() The resulting state is metastable, i.e., less stable than the phase to which the transition would have occurred, but not unstable either. In exception to the usual case, it is sometimes possible to change the state of a system diabatically (as opposed to adiabatically) in such a way that it can be brought past a phase transition point without undergoing a phase transition. A phase transition usually occurs when the pressure or temperature changes and the system crosses from one region to another, like water turning from liquid to solid as soon as the temperature drops below the freezing point. Such a diagram usually depicts states in equilibrium. For a single component, the most stable phase at different temperatures and pressures can be shown on a phase diagram.
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