| Molecules of bromo- cinnamic aldehyde in 100 mols. of mixture. | Freezing point. | Deviation from straight line. |
| 0.00 | 31.22° | — |
| 10.48 | 37.28° | 2.04° |
| 21.91 | 43.12° | 3.50° |
| 30.07 | 46.80° | 4.05° |
| 45.04 | 52.94° | 4.45° |
| 62.16 | 58.82° | 3.77° |
| 82.98 | 65.07° | 2.03° |
| 93.50 | 67.91° | 0.84° |
| 100.00 | 69.56° | — |
Melting-point Curve.—This curve, like the freezing-point curve, must also be continuous, and the melting points of the different solid solutions will lie between the melting points of the pure components. This is represented by the dotted line in Fig. 49, I. The relative position of the two curves, which can be deduced with the help of thermodynamics and also by experimental determination, is found in all cases to be in accordance with the following rule: At any given temperature, the concentration of that component by the addition of which the freezing point is depressed, is greater in the liquid than in the solid phase; or, conversely, the concentration of that component by the addition of which the freezing point is raised, is greater in the solid than in the liquid phase. An illustration of this rule is afforded by the two substances chloro- and bromo-cinnamic aldehyde already mentioned. As can be seen from the above table, the addition of chlorocinnamic aldehyde lowers the melting point of the bromo-compound. In accordance with the rule, therefore, the concentration of the chloro-compound in the liquid phase must be greater than in the solid phase; and this was found experimentally. At a temperature of 49.44°, the liquid contained 58.52 per cent., the solid only 52.57 per cent. of the chlorocinnamic aldehyde.
From this it will also be clear that on cooling a fused mixture of two substances capable of forming mixed crystals,
the temperature of solidification will not remain constant during the separation of the solid; nor, on the other hand, will the temperature of liquefaction of the solid solution be constant. Thus, for example, if a liquid solution of two components, A and B, having the composition represented by the point x (Fig. 50), is allowed to cool, the system will pass along the line xx′. At the temperature of the point a, mixed crystals will be deposited, the composition of which will be that represented by b. As the temperature continues to fall, more and more solid will be deposited; and since the solid phase is relatively rich in the component B, the liquid will become relatively poorer in this. The composition of the liquid solution will therefore pass along the curve ad, the composition of the solid solution at the same time passing along the curve bc; at the point c the liquid will solidify completely.[[271]]
Conversely, if mixed crystals of the composition and at the temperature x′ are heated, liquefaction will begin at the temperature c, yielding a liquid of the composition d. On continuing to add heat, the temperature of the mass will rise, more of the solid will melt, and the composition of the two phases will change as represented by the curves da and cb. When the temperature has risen to a, complete liquefaction will have occurred. The process of solidification or of liquefaction is therefore extended over a temperature interval ac.
Even when the freezing-point curve is a straight line joining
the melting points of the pure components, the melting-point curve will not necessarily coincide with the freezing-point curve, although it may approach very near to it; complete coincidence can take place only when the melting points of the two components are identical. An example of this will be given later (Chap. XII.).
(b) The freezing-point curve passes through a maximum (Curve II., Fig. 49).
