Solubility congruent melting temperatures

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 crystallisation 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. i molecule of CaCl2 to 6 molecules of water, this temperature must be the congruent melting-point of the hexahydrate. At this point, called an indifferent point (p. no), the hydrate will fuse or the solution will solidify without change of temperature and without change of composition, when the pressure is maintained constant. 

Figure 12.5 Effect of temperature on the solubility curves and the co-crystal domain in the ternary phase diagram of (a) a congruently melting cocrystal (c/.. Figure 12.1(a)) and (b) an incongruently melting co-crystal cf, Figure 12.1(b)). Ideal miscibility of all components in the liquid phase is assumed. The eutectic grooves are indicated by continuous lines, the peritectic groove by a dashed line. The two-phase regions in the 75 °C isothermal cross section are shaded.

Pure hydrates are crystalline ice-like solids usually prepared from water or ice and an appropriate guest species. Hydrates of water-soluble guests have well-defined phase diagrams characteristic of two-phase systems with compound formation."." If the hydrate melts congruently, it is simply a matter of freezing a solution of the correct composition to obtain the hydrate. For systems that melt incongruently, the aqueous solution must be quenched below the peritectic temperature and conditioned so that the crystalline hydrate can form on... 

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