CHAPTER 13 THE PHASE RULE AND PHASE DIAGRAMS
13.2 PHASEDIAGRAMS: BINARYSYSTEMS 436
(^45)
t=
ÆC
25:0 (^70)
p
=kPa
(^6) 9:5
benzene^0
zA
methanol^1
Figure 13.9 Liquidus and vaporus surfaces for the binary system of methanol (A)
and benzene.aCross-sections are hatched in the direction of the tie lines. The dashed
curve is the azeotrope vapor-pressure curve.
aRef. [ 157 ].
univariant. At a given temperature, the azeotrope can exist at only one pressure and have
only one composition. AsT changes, so dopandzAalong anazeotrope vapor-pressure
curveas illustrated by the dashed curve in Fig.13.9.
Figure13.10on the next page summarizes the general appearance of some relatively
simple temperature–composition phase diagrams of binary systems. If the system does not
form an azeotrope (zeotropicbehavior), the equilibrated gas phase is richer in one compo-
nent than the liquid phase at all liquid compositions, and the liquid mixture can be separated
into its two components by fractional distillation. The gas in equilibrium with an azeotropic
mixture, however, is not enriched in either component. Fractional distillation of a system
with an azeotrope leads to separation into one pure component and the azeotropic mixture.
More complicated behavior is shown in the phase diagrams of Fig.13.11. These are
binary systems with partially-miscible liquids in which the boiling point is reached before
an upper consolute temperature can be observed.
13.2.6 Solid–gas systems
As an example of a two-component system with equilibrated solid and gas phases, con-
sider the components CuSO 4 and H 2 O, denoted A and B respectively. In the pressure–
composition phase diagram shown in Fig.13.12on page 438 , the composition variablezB
is as usual the mole fraction of component B in the system as a whole.
The anhydrous salt and its hydrates (solid compounds) form the series of solids CuSO 4 ,
CuSO 4 H 2 O, CuSO 4 3H 2 O, and CuSO 4 5H 2 O. In the phase diagram these formulas are