It has already been learned that in the case of sodium sulphate and water, the solubility curve of the salt undergoes a sudden change in direction at the transition point, and that this is accompanied by a change in the solid phase in equilibrium with the solution. The same behaviour is also found in the case of double salts. To illustrate this, we shall briefly discuss the solubility relations of a few double salts, beginning with one of the simplest cases, that of the formation of rubidium racemate from rubidium _d_- and _l_-tartrates. The solubilities are represented diagrammatically in Fig. 100, the numerical data being contained in the following table, in which the solubility is expressed as the number of gram-molecules Rb_{2}C_{4}H_{4}O_{6} in 100 gm.-molecules of water.[346] --------------------------------------------------------------- Temperature. | Solubility of tartrate | Solubility of racemate. | mixture. | --------------------------------------------------------------- 25° | 13.03 | 10.91 35° | -- | 12.63 40.4° | -- | 13.48 40.7° | 13.46 | -- 54° | 13.83 | -- --------------------------------------------------------------- In Fig. 100 the curve AB represents the solubility of the racemate, while A'BC represents the solubility of the mixed tartrates. Below the transition point, therefore, the solubility of the racemate is less than that of the mixed tartrates. The solution, saturated with respect to the latter, will be supersaturated with respect to the racemate; and if a nucleus of this is present, racemate will be deposited, and the mixed tartrates, if present in equimolecular amounts, will ultimately {266} entirely disappear, and only racemate will be left as solid phase. The solution will then have the composition represented by a point on the curve AB. Conversely, above the transition point, the saturated solution of the racemate would be supersaturated with respect to the two tartrates, and transformation into the latter would ensue. If, therefore, a solution of equimolecular proportions of rubidium _d_- and _l_-tartrates is allowed to evaporate at a temperature above 40°, a mixture of the two tartrates will be deposited; while at temperatures below 40° the racemate will separate out. [Illustration: FIG. 100.]