CHAPTER 13 THE PHASE RULE AND PHASE DIAGRAMS
13.2 PHASEDIAGRAMS: BINARYSYSTEMS 429
010203040500 0:2 0:4 0:6 0:8 1:0
A zB Bt=ÆCliquidA(s)+1
l+AB(s)A(s)+AB(s) AB(s)+B(s)l+B(s)
AB(s)+1Figure 13.3 Temperature–composition phase diagram for the binary system of -
naphthylamine (A) and phenol (B) at 1 bar (Ref. [ 128 ]).binary liquid mixture is cooled. An example of this behavior is shown in Fig.13.3, in which
the solid compound contains equal amounts of the two components -naphthylamine and
phenol. The possible solid phases are pure A, pure B, and the solid compound AB. Only one
or two of these solids can be present simultaneously in an equilibrium state. The vertical line
in the figure atzBD0:5represents the solid compound. The temperature at the upper end
of this line is the melting point of the solid compound, 29 C. The solid meltscongruently
to give a liquid of the same composition. A melting process with this behavior is called a
dystectic reaction. The cooling curve for liquid of this composition would display a halt at
the melting point.
The phase diagram in Fig.13.3has two eutectic points. It resembles two simple phase
diagrams like Fig.13.1placed side by side. There is one important difference: the slope of
the freezing-point curve (liquidus curve) is nonzero at the composition of a pure component,
but is zero at the composition of a solid compound that is completely dissociated in the
liquid (as derived theoretically on page 388 ). Thus, the curve in Fig.13.3has a relative
maximum at the composition of the solid compound (zB D 0:5) and is rounded there,
instead of having a cusp—like a Romanesque arch rather than a Gothic arch.
An example of a solid compound that does not melt congruently is shown in Fig.13.4
on the next page. The solid hydrate NaCl2H 2 O is 61.9% NaCl by mass. It decomposes at
0 C to form an aqueous solution of composition 26.3% NaCl by mass and a solid phase of
anhydrous NaCl. These three phases can coexist at equilibrium at 0 C. A phase transition
like this, in which a solid compound changes into a liquid and a different solid, is called
incongruentorperitecticmelting, and the point on the phase diagram at this temperature at
the composition of the liquid is aperitectic point.
Figure13.4shows there are two other temperatures at which three phases can be present
simultaneously: 21 C, where the phases are ice, the solution at its eutectic point, and the
solid hydrate; and 109 C, where the phases are gaseous H 2 O, a solution of composition
28.3% NaCl by mass, and solid NaCl. Note that both segments of the right-hand boundary
of the one-phase solution area have positive slopes, meaning that the solubilities of the solid