Physical Chemistry Third Edition

6.6 Phase Diagrams of Nonideal Mixtures 289

AP does not exist in the liquid state. When it melts an equimolar liquid solution of

aniline and phenol results. This is calledcongruent melting.

The phase diagram resembles two phase diagrams set side by side. The right half of

the diagram is the phase diagram for the two substances AP and A. The left half of the

diagram is the phase diagram for the two substancesPand AP. The tie-line area at the

lower right represents the coexistence of nearly pure A with nearly pure AP, and the

corresponding area on the left represents the coexistence of nearly pure P with nearly

pure AP. Solid aniline and phenol cannot coexist with each other at equilibrium because

of the occurrence of the compound.

Figure 6.22 shows the temperature–composition phase diagram of copper and

lanthanum. There are four different compounds: LaCu 6 , LaCu 4 , LaCu 2 , and LaCu.

There are only two maxima in the diagram, corresponding to congruent melting of

the compounds LaCu 2 and LaCu 6. The two compounds LaCu and LaCu 4 do not melt

to form a single liquid. The compound LaCu melts at 551◦C to form two phases: a

liquid solution with lanthanum mole fraction equal to 0.57 and solid LaCu 2. This phe-

nomenon is calledincongruent meltingbecause the liquid phase does not have the

same composition as the solid phase from which it formed, and another solid phase is

formed. Solid LaCu 2 and solid LaCu can equilibrate with the liquid solution as indicated

by the tie line connecting the points representing the three phases. The point rep-

resenting the composition of the liquid is called aperitectic point. Just as with a

10838 C

9138 C

8408 C 7358 C

7258 C

5518 C

4688 C

8348 C 8128 C

Liquid

L 1 La

L 1 LaCu

LaCu 1 La

LaCu 1

LaCu 2

Cu 1

LaCu 6

600

800

1000

400

0 0.5 0.1

L 1 LaCu 2

LaCu 6 LaCu 4 LaCu 2 LaCu La

LaCu

(^16)
LaCu
4
LaCu 21
LaCu 4
Cu
xLa
tC
/^8 C
Figure 6.22 solid–liquid temperature–composition Phase Diagram of Copper and
Lanthanum.From R. E. Dickerson,Molecular Thermodynamics, W. A. Benjamin, Inc.,
New York, 1969, p. 379.