CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 182
7.4 Phase diagrams
7.4.1 General terms
- Phase diagramis a diagram allowing us to determine in which phase or phases a system
exists under a given temperature, pressure, and the global (also total) composition of
the system. - Phase diagram of a pure substanceshows the dependence of equilibrium pressure
on equilibrium temperature. The lines in the diagram represent the curves of the boiling,
melting, sublimation and crystalline transformation points. - Phase diagrams of two-component systemsusually show the dependence of equi-
librium pressures on composition at a fixed temperature (isothermal diagrams), or the
dependence of equilibrium temperatures on composition at a fixed pressure (isobaric
diagrams). - Phase diagrams of three-component systemsdelimit the homogeneous and het-
erogeneous regions in dependence on the mixture composition at constant temperature
and pressure. They are drawn as triangular diagrams. - Binodalline is a set of points (a curve) separating the homogeneous and heterogeneous
regions from one another. - Conodeor the tie-line is a straight line connecting two points on the binodal line of a
phase diagram which represent the composition of the coexisting (conjugated) phases.
7.4.2 Phase diagram of a one-component system
The phase diagram of a one-component system delimits the regions in which its individual
phases exist at various temperatures and pressures (Figure7.1).
- The areas denoted (g), (l), (sα), (sβ) are one-phase regions.
- The lines in the diagram indicate the regions in which two phases coexist; the number
of degrees of freedom [see (7.7)] drops to 1. - The curves intersections indicatetriple points(in Figure7.1there are two triple points
denotedT 1 andT 2 ). They are points with zero degrees of freedom.