Phases and first-order transition

by **Algerien1970** Sun 14 Jun - 23:37

Phases

A phase is a [You must be registered and logged in to see this link.], physically distinct and mechanically separable part of a system. Gibbs described a phase as being "uniform throughout, not only in chemical composition, but also in physical state". Although a phase must be homogeneous, it need not be continuous. Thus a single phase may exist as separate crystals or drops. [You must be registered and logged in to see this link.] that consist of more than one phase are said to be heterogeneous with the individual phases separated by phase boundaries. The number of phases present in a system depends upon the chemical nature of the constituents and variables such as composition, pressure or temperature.

Single-component systems may exist as one or more phases. An equilibrium sample of ice, liquid water and water vapour consists of three different phases. The properties and composition of each of the phases are homogenous, but the solid, liquid and gas may be mechanically separated. A mixture of crystals of rhombic and monoclinic sulphur consists of two phases, as the mixture may easily be separated into crystals of the two different [You must be registered and logged in to see this link.].

Multi-component systems may also exist as one or more phases. However, mixtures of gases only ever form a single phase; gases are always [You must be registered and logged in to see this link.] and cannot, therefore be mechanically separated.

Some combinations of liquids are also miscible so that the resulting mixture cannot be mechanically separated into its individual constituents. For example, a mixture of liquid toluene and liquid butanol forms a homogeneous single-phase [You must be registered and logged in to see this link.] at all compositions. Other combinations of liquids are not, however, miscible and instead tend to form separate layers, each of forms a different phase. At room temperature, most mixtures of oil and water separate into two phases.

Mixtures of solids too may be either miscible or immiscible. Brass is a crystalline alloy of copper and zinc. For low concentrations of zinc, the two metals are completely miscible and form a single-phase solid solution. However, if the concentration of zinc is increased above about 40 wt%, the solid solution becomes saturated and crystals of two distinct phases form.

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