PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 216

7.9 Liquid-solid equilibrium in mixtures.

This section deals with relations between the temperature and composition of phases during
the freezing and melting of mixtures. The effect of pressure on melting and freezing is small
and usually it is not studied.

7.9.1 Basic terms

Note:Since most experimental data on liquid-solid phase equilibrium relate to atmospheric

pressure, the value of equilibrium pressure in these cases is often not stated.

• A set of equilibrium freezing temperatures [see7.1.10] changing in a given system with
  the composition of the liquid phase (the upper line in the binary system in Figure7.12)
  is known as theliquidus.


• A set of equilibrium melting temperatures [see7.1.8] changing in a given system with
  the composition of the solid phase (the lower line in the binary system in Figure7.12)
  is known as thesolidus.


• Theeutectic point(denoted E in Figures7.11,7.13,7.14a,7.15) in a two-component
  system is defined as the point of intersection of both curves of the liquidus whose slopes
  have opposite signs in the vicinity of this point. At this point there are three phases in
  equilibrium, and according to the Gibbs phase law [see7.3], the system has no degree
  of freedom at this point at constant pressure.


• Theperitectic point(denoted P in Figures7.14b,7.15b) is the point in the vicinity
  of which the slopes of both curves of freezing have the same sign. At this point there are
  three phases in equilibrium, and according to the Gibbs phase law [see7.3], the system
  has no degree of freedom at this point at constant pressure

7.9.2 General condition of equilibrium.

From the intensive criterion of equilibrium (7.2)μ(l)i =μ(s)i we may derive

ln

γi(l)x(l)i

γi(s)x(s)i

=

∫T

Tfi

∆fusHi

RT^2

dT. (7.62)