--------------------------------------------------------------- |Change of | Solubility (at 18°) (grams salt |volume by | in 1 gram of solution). |dissolving 1 gm.|--------------------------- Salt. |of salt in the | | |saturated | Pressure | Pressure |solution. | = 1 atm. | = 500 atm. ------------------+----------------+----------+---------------- Sodium chloride | -0.07 | 0.264 | 0.270 Ammonium chloride | +0.10 | 0.272 | 0.258 Alum | -0.067 | 0.115 | 0.142 | | |(_p_ = 400 atm.) ------------------------------------------------------------- As can be seen, a large increase of the pressure brings about a no more than appreciable alteration of the solubility; a result which is due, as in the case of the alteration of the fusion point with the pressure, to the small change in volume accompanying solution or increase of pressure. For all practical purposes, therefore, the solubility as determined under atmospheric pressure may be taken as equal to the true {108} solubility, that is, the solubility when the system is under the pressure of its own vapour. The Saturated Solution.--From what has been said above, it will be seen that the condition of saturation of a solution can be defined only with respect to a certain solid phase; if no solid is present, the system is undefined, for it then consists of only two phases, and is therefore bivariant. Under such circumstances not only can there be at one given temperature solutions of different concentration, all containing less of one of the components than when that component is present in the solid form, but there can also exist solutions containing more of that component than corresponds to the equilibrium when the solid is present. In the former case the solutions are _unsaturated_, in the latter case they are _supersaturated with respect to a certain solid phase_; in themselves, the solutions are stable, and are neither unsaturated nor supersaturated. Further, if the solid substance can exist in different allotropic modifications, the particular form of the substance which is in equilibrium with the solution must be known, in order that the statement of the solubility may be definite; for each form has its own solubility, and, as we shall see presently, the less stable form has the greater solubility (cf. p. 47). In all determinations of the solubility, therefore, not only must the concentration of the components in the solution be determined, but equal importance should be attached to the characterisation of the solid phase present.