In the physical sciences, a phase is a part of a system that is uniform in chemical composition and physical properties (eg. density, crystal structure, index of refraction, viscosity, etc.) and is separated from other homogeneous parts of the system by boundary surfaces.
Gibbs phase rule describes the number of phases that can be present at equilibrium for a given system at various conditions. The phase rule indicates that for a single component system at most three phases (usually gas, liquid and solid) can co-exist in equilibrium. The three phases can all co-exist only at a single specific temperature and pressure, characteristic of the material, called the Triple point. The phase rule also indicates that two phases can only co-exist at equilibrium for specific combinations of temperature and pressure. For example for a liquid-gas sytem if the vapor pressure is lower than that corresponding to the temperature, the system will not be at equilibrium, rather the liquid will tend to evaporate until the vapor pressure reaches the appropriate level or all of the liquid is consumed. Likewise, if the vapor pressure is too great for the given temperature condensation will occur.
For the case of multi-component systems the phase rule indicates that additional phases are possible. A common example of this occurs in mixtures of mutually insoluble substances such as water and oil. When mixed, water and oil tend to separate into two distinct phases, one consisting primarily of water and the other consisting primarily of oil. In this case the water containing phase is said to be hydrophilic (which literally means, water-loving). The oil containing phase is hydrophobic (afraid of or hating water).
Many substances can exist in a variety of solid phases each corresponding to a unique crystal structure. These varying crystal phases of the same substance are called polymorphs. Diamond and graphite are examples of polymorphs of carbon. Graphite is composed of layers of hexagonally arranged carbon atoms, in which each carbon atom is strongly bound to three neighboring atoms in the same layer and is weakly bound to atoms in the neighboring layers. By contrast in diamond each carbon atom is strongly bound to four neighboring carbon atoms in a cubic array. The unique crystal structures of graphite and diamond are responsible for the vastly different properties of these two materials.
Each polymorph of a given substance is usually only thermodynamically stable over a specific range of conditions. For example, diamond is only stable at extremely high pressures. Graphite is the stable form of carbon at normal atmospheric pressures. Although diamond is not stable at atmospheric pressures and should transform to graphite, we know that diamonds exist at these pressures. This is because at normal temperatures the transformation from diamond to graphite is extremely slow. If we were to heat the diamond, the rate of transformation would increase and the diamond would become graphite. However, at normal temperatures the diamond can persist for a very long time. Non-equilibrium phases like diamond that exist for long periods of time are said to be Metastable.
Another important example of metastable polymorphs occurs during the processing of steel. Steels are often subjected to a variety of thermal treatments designed to produce various combinations of stable and metastable iron phases. In this way the steel properties, such as hardness and strength can be adjusted by controlling the relative amounts and crystal sizes of the various phases that form.