This curve exhibits the greatest degree of contrast to the freezing-point curve which is obtained when the pure components crystallize out. For, since the curve passes through a maximum, it is evident that the freezing point of each of the components must be raised by the addition of the other component.
Example.—Very few cases belonging to this type are known. The best example is found in the freezing-point curve of mixtures of d- and l-carvoxime[[272]] (C10H14N.OH). The freezing points and melting points of the different mixtures of d- and l-carvoxime are given in the following table, and represented graphically in Fig. 51:—
| Per cent. of d-carvoxime. | Per cent. of l-carvoxime. | Freezing point. | Melting point. |
| 100 | 0 | 72.0° | 72.0° |
| 99 | 1 | 72.4° | — |
| 98 | 2 | 73.0° | — |
| 95 | 5 | 75.4° | 73.0° |
| 90 | 10 | 79.0° | 75.0° |
| 80 | 20 | 84.6° | 80.0° |
| 70 | 30 | 88.2° | 85.0° |
| 60 | 40 | 90.4° | — |
| 50 | 50 | 91.4° | 91.4° |
| 25 | 75 | 86.4° | 82.0° |
| 8 | 92 | 77.4° | — |
| 1 | 99 | 72.4° | — |
| 0 | 100 | 72.0° | 72.0° |
In this figure, the melting-point curve, i.e. the temperature-concentration curve for the mixed crystals, is represented by the lower curve. Since the addition of the lævo-form to the dextro-form raises the melting point of the latter, the concentration of the lævo-form (on the right-hand branch of the curve) must, in accordance with the rule given, be greater in the solid phase than in the liquid. Similarly, since addition of the dextro-form raises the melting point of the lævo-form, the solid phase (on the left-hand branch of the curve) must be richer in dextro- than in lævo-carvoxime. At the maximum point, the melting-point and freezing-point curves touch; at this point, therefore, the composition of the solid and liquid phases must be identical. It is evident, therefore, that at the maximum point the liquid will solidify, or the solid will liquefy completely without change of temperature; and, accordingly, mixed crystals of the composition represented by the maximum point will exhibit a definite melting point, and will in this respect behave like a simple substance.
(c) The freezing-point curve passes through a minimum (Curve III., Fig. 49).
In this case, as in the case of those systems where the pure components are deposited, a minimum freezing point is obtained. In the latter case, however, there are two freezing-point curves which intersect at a eutectic point; in the case where mixed crystals are formed there is only one continuous curve. On one side of the minimum point the liquid phase contains relatively more, on the other side relatively less, of the one component than does the solid phase; while at the minimum point the composition of the two phases is the same. At this point, therefore, complete solidification or complete liquefaction will occur without change of temperature, and the mixed crystals will accordingly exhibit a definite melting point.
Example.—As an example of this there may be taken the mixed crystals of mercuric bromide and iodide.[[273]] Mercuric bromide melts at 236.5°, and mercuric iodide at 255.4°. The mixed crystal of definite constant melting point (minimum point) contains 59 mols. per cent. of mercuric bromide, the melting point being 216.1°.