PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 219

whereTis the melting temperature of a dilute solution,Tf, 1 is the melting temperature

of a pure solvent,m 2 denotes the molality of solute^2 ,m 1 orm 2 denote the mass of the

respective component, andKCdenotes the cryoscopic constant which may be determined

from the properties of a pure solvent

KC=

RTf^2 , 1 M 1

∆fusH 1

. (7.68)

7.9.4 Two-component systems with completely miscible

components in both the liquid and solid phases

In Figure7.12, the liquidus is indicated by the thicker line and the solidus by the thinner line.
Both curves may be calculated by solving two equations (7.62) fori= 1 and 2, provided that we
know the dependence of all activity coefficients on composition. Sometimes we may encounter
the type shown in Figure7.12b, where the liquidus and solidus meet. It is an analogy to the
azeotropic point in the liquid-vapour equilibrium [see7.6.6].

7.9.5 Two-component systems with partially miscible components

in either the liquid or the solid phase

phase. 7.9.3 Two-component systems with totally immiscible components in the solid

phase. Figure7.13is an extension of Figure7.11with the region of two liquids added.

Points C, D, E represent the coexistence of three phases, with the number of degrees of

freedom decreasing, according to the Gibbs phase law [see7.3], to zero.

the liquid and solid phases 7.9.4 Two-component systems with completely miscible components in both

phase. In the event of large differences in the melting points, the case shown in Figure

7.14atransfers to that shown in Figure7.14b. Point E is the eutectic point and point

P is the peritectic point [see7.9.1].

(^2) If the solute dissociates,m 2 has to be replaced byναm 2 , whereνis the number of particles formed by the
dissociation of the molecule, andαis the degree of dissociation.