Thermodynamics and Chemistry

CHAPTER 13 THE PHASE RULE AND PHASE DIAGRAMS

13.2 PHASEDIAGRAMS: BINARYSYSTEMS 430

 40

 20

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90 100

H 2 O mass percent NaCl NaCl

t=

ÆC

H 2 O(g)CNaCl(s)

H 2 O(g)

Csln

sln slnCNaCl(s)

H 2 O(s)

Csln

slnCNaCl
 2 H 2 O(s)

H 2 O(s)CNaCl
 2 H 2 O(s)

NaCl
 2 H 2 O(s)

CNaCl(s)

peritectic

point

Figure 13.4 Temperature–composition phase diagram for the binary system of H 2 O

and NaCl at 1 bar. (Data from Refs. [ 37 ] and [ 167 ].)

hydrate and the anhydrous salt both increase with increasing temperature.

13.2.3 Partially-miscible liquids

When two liquids that are partially miscible are combined in certain proportions, phase
separation occurs (Sec.11.1.6). Two liquid phases in equilibrium with one another are
calledconjugate phases. Obviously the two phases must have different compositions or
they would be identical; the difference is called amiscibility gap. A binary system with
two phases has two degrees of freedom, so that at a given temperature and pressure each
conjugate phase has a fixed composition.
The typical dependence of a miscibility gap on temperature is shown in Fig.13.5on
the next page. The miscibility gap (the difference in compositions at the left and right
boundaries of the two-phase area) decreases as the temperature increases until at theupper
consolute temperature, also called theupper critical solution temperature, the gap vanishes.
The point at the maximum of the boundary curve of the two-phase area, where the tempera-
ture is the upper consolute temperature, is theconsolute pointorcritical point. At this point,
the two liquid phases become identical, just as the liquid and gas phases become identical
at the critical point of a pure substance. Critical opalescence (page 205 ) is observed in the
vicinity of this point, caused by large local composition fluctuations. At temperatures at
and above the critical point, the system is a single binary liquid mixture.
Suppose we combine6:0mol of component A (methyl acetate) and4:0mol of compo-
nent B (carbon disulfide) in a cylindrical vessel and adjust the temperature to 200 K. The