The solubility of this salt has been determined from the cryohydric point, which lies at about -55°, up to the melting point of the salt.[225] The solubility increases with rise of temperature, as is shown by the figures in the following table, and by the (diagrammatic) curve AB in Fig. 37. In the table, the numbers under the heading "solubility" denote the number of grams of CaCl_{2} dissolved in 100 grams {146} of water; those under the heading "composition," the number of gram-molecules of water in the solution to one gram-molecule of CaCl_{2}. SOLUBILITY OF CALCIUM CHLORIDE HEXAHYDRATE. ----------------------------------------- Temperature. | Solubility. | Composition. ----------------------------------------- -55° | 42.5 | 14.5 -25° | 50.0 | 12.3 -10° | 55.0 | 11.2 0° | 59.5 | 10.37 10° | 65.0 | 9.49 20° | 74.5 | 8.28 25° | 82.0 | 7.52 28.5° | 90.5 | 6.81 29.5° | 95.5 | 6.46 30.2° | 102.7 | 6.00 29.6° | 109.0 | 5.70 29.2° | 112.8 | 5.41 ----------------------------------------- So far as the first portion of the curve is concerned, it resembles the most general type of solubility curve. In the present case the solubility is so great and increases so rapidly with rise of temperature, that a point is reached at which the water of crystallization of the salt is sufficient for its complete solution. This temperature is 30.2°; and since the composition of the solution is the same as that of the solid salt, viz. 1 mol. of CaCl_{2} to 6 mols. of water, this temperature must be the melting point of the hexahydrate. At this point the hydrate will fuse or the solution will solidify without change of temperature and without change of composition. Such a melting point is called a _congruent_ melting point. But the solubility curve of calcium chloride hexahydrate differs markedly from the other solubility curves hitherto considered in that it possesses a _retroflex portion_, represented in the figure by BC. As is evident from the figure, therefore, calcium chloride hexahydrate exhibits the peculiar and, as it was at first thought, impossible behaviour that it can be in equilibrium at one and the same temperature with two different solutions, one of which contains more, the other less, water than the solid hydrate; for it must be remembered that {147} throughout the whole course of the curve ABC the solid phase present in equilibrium with the solution is the hexahydrate.