reached. At this point phases a and L (liquid) are in equilibrium. Upon raising the temperature infinitesimally above T,a, the liquid phase appears having the
tive amounts of the two phases a and L will then be inversely proportional to the distance between the corresponding end of the tie-line and the fulcrum. Upon reaching T,o, the solid phase is exhausted, the last solid to exist having the composition XI. This is the end of the melting point range, but the temperature is nevertheless lower than the melting point of the pure substance. The second possibility is that too large an amount of impurity is present to dissolve in A, and two solid solutions exist in equilibrium with each other. This case is the same as the first, except that melting always starts a t the eutectic temperature. Considering composition Nz, no change of phase occurs until line aob is reached. At this point a three-phase equilibrium exists between a, p, and L. Upon raising the temperature infinitesimally above aob, the liquid phase
composition xs. Since the liquid phase is richer in B than the original composition, the solid phase will be impoverished in B. But as can be seen by curve TAa, decreasing the concentration of B will raise the melting point. At the resulting higher temperature, the process repeats itself. Thus, the solid phase will change along line TAa and the liquid phase along line TAo, the two phases in equilibrium being connected by horizontal lines or "tie-lines." To find the relative
amounts of the two phases present a t any temperature, draw the tie-line and consider i t as a lever, with the composition vertical NI as the fulcrum. The rela-
appears having the composition x,, and the p phase disappears. The a phase has the composition x2: the rest of the transformation is analogous to case 1. Not all organic solid binary mixtures belong to the eutectic type. Several cases are known in which the melting and freezing curves lie entirely between the melting points of the pure constituents (similar to Figure 5). The d-carvoxime-1-carvoxime diagram shows a maximum in the curves. This type of diagram is rare. An interesting pair of substances is the system composed of the optically active oximes of camphor. These two substances not only form perfect solutions but also have the same melting point and are completely intersoluble. They are therefore thermodynamically indistinguishable, and the S-L transformation curve is a single horizontal line.
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