For r = 3, this equation yields the pair of solutions xBI = 0.07, xBII = 0.93. The exact correspondence of these phase compositions with those obtained by the analytical method is expected because both methods are based on the same model.

Problem 8.3 offers an additional exercise in analyzing phase separation in a binary regular solution.

Real systems exhibiting phase separation

The symmetry of the two-phase boundary in Fig. 8.4 arises from the use of regular solution theory to account for nonideality. Deviations from this model are common. Figure 8.6 shows the phase diagram of the binary liquid system consisting of n-hexane and nitrobenzene. The critical solution temperature of 295 K is achieved at a mole fraction of nitrobenzene of 0.6. The symmetry of the curve in Fig. 8.4 is lost because hex cannot be represented by the single-parameter regular solution formula given by Eq (8.15).

Fig. 8.6 Phase diagram for nitrobenzene and n-hexane

Real systems may exhibit the characteristic features of both ideal (or near-ideal) melting (Fig. 8.3) and phase separation (Fig. 8.4) in the same phase diagram. Figure 8.7 shows the phase diagram of the ZrO2 - ThO2 system. Although technically three-component systems, oxides can generally be represented as pseudo-binary systems consisting of the two very stable compounds.

Fig. 8.7 The thoria-zirconia phase diagram
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