What Is a Phase?

A phase is a state of aggregation of matter that has a distinctive structure. Phases may be solid, liquid, or gaseous. A phase may be a pure material or a solution of several components. A solid phase is either amorphous or has a characteristic crystal structure and definite composition range. A physical system may contain more than one solid phase with different crystal structures, or different ranges of possible compositions – one or more mutually insoluble liquid phases (for example oil and water) and a gas phase. All gasses are mutually soluble, and therefore only one gaseous phase is possible.

Single-phase materials include brass (a solid solution of zinc in copper with zinc atoms occupying lattice sites), sodium chloride crystals, glass, and polyethylene. Most plain carbon steels are two-phase materials, consisting of an iron-rich solid solution and iron carbide. A glass of ice water consists of two phases: liquid water and ice. In a glass of tonic water there may be three phases: liquid, ice, and gas bubbles.

The composition of a single phase may vary from place to place, but composition change is always gradual and without abrupt changes. In multiple-phase systems, however, there are discontinuities of composition and structure at phase boundaries. The compositions on each side of the boundary are usually in equilibrium. For example, the oxygen concentration changes abruptly between copper with some oxygen dissolved in it and copper oxide.