A.—Anhydrous Salt and Water.
The Solubility Curve.—In studying the equilibria in those systems of two components in which the liquid phase is a solution or phase of varying composition, we shall in the present chapter limit the discussion to those cases where no compounds are formed, but where the components crystallise out in the pure state. Since some of the best-known examples of such systems are yielded by the solutions of anhydrous salts in water, we shall first of all briefly consider some of the results which have been obtained with them.
For the most part the solubility curves have been studied only at temperatures lying between 0° and 100°, the solid phase in contact with the solution being the anhydrous salt. For the representation of these equilibria, the concentration-temperature
diagram is employed, the concentration being expressed as the number of grams of the salt dissolved in 100 grams of water, or as the number of gram-molecules of salt in 100 gram-molecules of water. The curves thus obtained exhibit the different forms to which reference has already been made. So long as the salt remains unchanged the curve will be continuous, but if the salt alters its form, then the solubility curve will show a break.
Now, we have already seen in Chapter III. that certain substances are capable of existing in various crystalline forms, and these forms are so related to one another that at a given temperature the relative stability of each pair of polymorphic forms undergoes change. Since each crystalline variety of a substance must have its own solubility, there must be a break in the solubility curve at the temperature of transition of the two enantiotropic forms. At this point the two solubility curves must cut, for since the two forms are in equilibrium with respect to their vapour, they must also be in equilibrium with respect to their solutions. From the table on p. [63] it is seen that potassium nitrate, ammonium nitrate, silver nitrate, thallium nitrate, thallium picrate, are capable of existing in two or more different enantiotropic crystalline forms, the range of stability of these forms being limited by definite temperatures (transition temperature). Since the transition point is not altered by a solvent (provided the latter is not absorbed by the solid phase), we should find on studying the solubility of these substances in water that the solubility curve would exhibit a change in direction at the temperature of transition. As a matter of fact this has been verified, more especially in the case of ammonium nitrate[[187]]
and thallium picrate.[[188]] The following table contains the values of the solubility of ammonium nitrate obtained by Müller and Kaufmann, the solubility being expressed in gram-molecules NH4NO3 in 100 gram-molecules of water. In Fig. 27 these results are represented graphically. The equilibrium point was approached both from the side of unsaturation and of supersaturation, and the condition of equilibrium was controlled by determinations of the density of the solution.
Solubility of Ammonium Nitrate.
| Temperature. | Solubility. | Temperature. | Solubility. |
| 12.2° | 34.50 | 32.7° | 57.90 |
| 20.2° | 43.30 | 34.0° | 58.89 |
| 25.05° | 48.19 | 35.0° | 59.80 |
| 28.0° | 51.86 | 36.0° | 61.00 |
| 30.0° | 54.40 | 37.5° | 62.90 |
| 30.2° | 54.61 | 38.0° | 63.60 |
| 31.9° | 57.20 | 39.0° | 65.09 |
| 32.1° | 57.60 | 40.0° | 66.80 |
From the graphic representation of the solubility given in Fig. 27, there is seen to be a distinct change in the direction of the curve at a temperature of 32°; and this break in the curve corresponds to the transition of the β-rhombic into the α-rhombic form of ammonium nitrate (p. [63]).