Concise Physical Chemistry

c09 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

COMPOUND PHASE DIAGRAMS 137

other of the components to a miscible solution until the coexistence curve is crossed.

The transition from a clear miscible system to an opaque two-phase emulsion can

be quite dramatic in some cases, making it look as though water has turned into

milk. Like Type I diagrams, the number of degrees of freedom is 3. We usually hold

p=const at 1 atm so that the rest of the phase behavior can be represented in two

dimensions. On the dome-shaped coexistence curve, there is 1 degree of freedom.

The horizontal under the dome is called atie line. The two intersections of the tie

line with the coexistence curve give the composition of the two coexisting phases.

The dome need not be symmetrical. Quite a variety of shapes are possible, including

one that is closed at the topandat the bottom, forming a closed irregular oval that is

essentially an island of immiscibility in a sea of miscibility. The coexistence curve

can approach the verticals representing pure A and pure B quite closely, giving rise

to the folk saying “oil and water don’t mix.” Actually, they do mix but one phase

is overwhelmingly oil-rich and the other is overwhelmingly water-rich. That is why

one does not wish to drink water that has come into contact with oil or gasoline.

9.6.3 Type III

Type III solid–liquid phase diagrams are familiar as having aeutectic point. The locus

of melting points of mixtures of A and B vs. XBfollows the two curves in Fig. 9.10.

In general, mixtures of the two components have a lower melting point than either

component, A or B, alone. Ordinary electrical solder is a eutectic mixture of lead and

tin having a melting point that is lower than either pure Pb or pure Sn. We rely on

the fact that the melting temperature of apurecompound is higher than that of an

impure (mixed) sample of the same compound according to Fig. 9.10. Generations

of pre-medical students have been judged partly on the basis of the melting point

behavior of the compounds they prepare in the organic chemistry laboratory.

9.7 COMPOUND PHASE DIAGRAMS

Many complicated phase diagrams can be read quite easily as two or more simple-

phase diagrams stuck together. The principle will be illustrated using a compound

XB

T, K

A B

FIGURE 9.10 A Type III phase diagram. The low melting mixture is called aeutectic

mixture.