Editing Phase rule (section)

===Two-component systems===
For binary mixtures of two chemically independent components,  {{nowrap|1=''C'' = 2}} so that {{nowrap|1=''F'' = 4 − ''P''}}. In addition to temperature and pressure, the other degree of freedom is the composition of each phase, often expressed as [[mole fraction]] or mass fraction of one component.<ref name="Sonntag"/>

[[Image:Binary Boiling Point Diagram new.svg|thumb|320px|right|Boiling Point Diagram]]
As an example, consider the system of two completely miscible liquids such as [[toluene]] and [[benzene]], in equilibrium with their vapours. This system may be described by a [[Vapor–liquid equilibrium#Boiling-point diagrams|boiling-point diagram]] which shows the composition (mole fraction) of the two phases in equilibrium as functions of temperature (at a fixed pressure).

Four thermodynamic variables which may describe the system include temperature (''T''), pressure (''p''), mole fraction of component 1 (toluene) in the liquid phase (''x''<sub>1L</sub>), and mole fraction of component 1 in the vapour phase (''x''<sub>1V</sub>). However, since two phases are present ({{nowrap|1=''P'' = 2}}) in equilibrium, only two of these variables can be independent ({{nowrap|1=''F'' = 2}}). This is because the four variables are constrained by two relations: the equality of the chemical potentials of liquid toluene and toluene vapour, and the corresponding equality for benzene.

For given ''T'' and ''p'', there will be two phases at equilibrium when the overall composition of the system ('''system point''') lies in between the two curves. A horizontal line ([[Contour line#Temperature and related subjects|isotherm]] or tie line) can be drawn through any such system point, and intersects the curve for each phase at its equilibrium composition. The quantity of each phase is given by the [[lever rule]] (expressed in the variable corresponding to the ''x''-axis, here mole fraction).

For the analysis of [[fractional distillation]], the two independent variables are instead considered to be liquid-phase composition (x<sub>1L</sub>) and pressure. In that case the phase rule implies that the equilibrium temperature ([[boiling point]]) and vapour-phase composition are determined.

Liquid–vapour [[phase diagram]]s for other systems may have [[azeotrope]]s (maxima or minima) in the composition curves, but the application of the phase rule is unchanged. The only difference is that the compositions of the two phases are equal exactly at the azeotropic composition.