Concentration-Temperature Diagram.--In the case of salts which can form crystalline hydrates, the temperature-concentration diagram, representing the equilibria of the {134} different possible systems, must necessarily be somewhat more complicated than where no such combination of the components occurs. For, as has already been pointed out, each substance has its own solubility curve; and there will therefore be as many solubility curves as there are solid phases possible, _the curve for each particular solid phase being continuous so long as it remains unchanged in contact with the solution_. As an example of the relationships met with in such cases, we shall first of all consider the systems formed of sodium sulphate and water. [Illustration: FIG. 33.] Sodium Sulphate and Water.--At the ordinary temperatures, sodium sulphate crystallises from water with ten molecules of water of crystallisation, forming Glauber's salt. On determining the solubility of this salt in water, it is found that the solubility increases as the temperature rises, the values of the solubility, represented graphically by the curve AC (Fig. 33), being given in the following table.[209] The numbers denote grams of sodium sulphate, calculated as anhydrous salt, dissolved by 100 grams of water. SOLUBILITY OF Na_{2}SO_{4},10H_{2}O. -------------------------- Temperature. | Solubility. -------------------------- 0° | 5.02 10° | 9.00 15° | 13.20 18° | 16.80 20° | 19.40 25° | 28.00 30° | 40.00 33° | 50.76 34° | 55.00 -------------------------- {135} On continuing the investigation at higher temperatures, it was found that the solubility no longer increased, but _decreased with rise of temperature_. At the same time, it was observed that the solid phase was now different from that in contact with the solution at temperatures below 33°; for whereas in the latter case the solid phase was sodium sulphate decahydrate, at temperatures above 33° the solid phase was the anhydrous salt. The course of the solubility curve of anhydrous sodium sulphate is shown by BD, and the values of the solubility are given in the following table:--[210] SOLUBILITY OF ANHYDROUS SODIUM SULPHATE. -------------------------- Temperature. | Solubility. -------------------------- 18° | 53.25 20° | 52.76 25° | 51.53 30° | 50.37 33° | 49.71 34° | 49.53 36° | 49.27 40.15° | 48.78 50.40° | 46.82 -------------------------- As is evident from the figure, the solubility curve which is obtained when anhydrous sodium sulphate is present as the solid phase, cuts the curve representing the solubility of the decahydrate, at a temperature of about 33°.