PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 213

Note: In the case of a heterogeneous azeotrope, the composition of the vapour phase is

not identical with the composition of the equilibrium liquid phases.

• Ideal vapour phase, both components totally immiscible in the liquid phase
  The overall pressure above a heterogeneous mixture is

p=ps 1 +ps 2 (7.56)

and the composition of the vapour phase is

y 1 =

ps 1

ps 1 +ps 2

. (7.57)

Note:The consequence of relation (7.56) is that a heterogeneous mixture of two very little

miscible substances boils at a temperature which is lower than the boiling point of any of

its components.

7.8.4 Three-component system containing two liquid phases.

Figure7.10shows a binodal line EC 1 F and a tie line EDF. The system of a global composition
given by point D split into two liquid phases whose composition is given [see7.4.4] by points
E and F on the binodal line. The amount of substance ratio of both liquid phases is given by
the lever rule [see7.4.5]. The compositions of both coexisting phases merge at point C 1 (the
critical point).

• Thedistribution coefficientis defined as the ratio of the molar fractions or concen-
  trations of theithcomponent in one and the other liquid phases.

Kx,i=

x(i`^2 )

x(i`^1 )

or Kc,i=

c(i`^2 )

c(i`^1 )

, i= 1, 2 , 3. (7.58)

From (7.55) we also have

Kx,i=

γi(`^1 )

γi(`^2 )

. (7.59)