CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 221
Obr.7.14:Dependence of the freezing temperature (the upper curves) and the melting temperature
(the lower curves) on composition for a two-component system; the components are completely miscible
in the liquid phase, the solid phases are partially miscible. Case a) is typical of mixtures with small
differences in the melting temperatures of pure components; case b) is typical of mixtures with great
differences in the melting temperatures of pure components.
7.9.6 Formation of a compound in the solid phase
In Figure7.15a, substances 1 and 2 form a compound of compositionx′ 1. This compound is
stable at its melting temperatureTf′, which is known as thecongruent melting point. In
Figure7.15b the compound of compositionx′ 1 is not stable at its melting temperature and
Tf′is known as theincongruent melting pointin view of the fact that, in contrast to the
congruent melting point, there is a different composition of the liquid and solid phases in this
case.
7.9.7 Three-component systems
A three-component diagram delimits the homogeneous and heterogeneous regions in a three-
component system at constant temperature and pressure (usually atmospheric).
Figure 7.4 shows the solubility of two salts A and B in water (component C). The liquid
phase is denoted (γ) and the solid phases are denoted (α) and (β).
When we want to observe changes in equilibrium composition at different temperatures, we
have to use three-dimensional diagrams.