Supersaturated Solutions

When a solution of a solid in water, already saturated at a given temperature, is heated up with more of the solid until the whole of the latter has passed into solution, crystallisation or precipitation of the excess of solute does not always take place upon cooling the system to the original temperature. It is clear that the solution must now hold a greater quantity of substance than corresponds to the ordinary solubility and is said to be supersaturated. Such solutions can readily be prepared by heating up sodium thiosulphate, sodium acetate, or sodium sulphate with water, and allowing to cool without agitation.

Supersaturated solutions, however, are always liable to crystallise spontaneously, particularly on exposure to air. Lowell was apparently the first to show, however, in the case of sodium sulphate, that crystallisation was not induced by contact with air that had been previously passed through water, sulphuric acid, caustic alkalies, glass wool, or even through a series of empty flasks. Fifteen years later Violette and Gernez independently threw considerable light upon the subject by showing that the spontaneous crystallisation of supersaturated solutions of sodium sulphate in contact with air is due to the presence of minute crystals of the salt in suspension in the latter which serve as nuclei stimulating crystallisation. Hence by washing or filtering the air Lowell had removed these suspended nuclei, and in consequence retarded crystallisation. Lecoq de Boisbaudran showed, in the following year, that not only could minute crystals of the same substance serve as nuclei, but that crystals of isomorphous bodies yield precisely the same result; and it is now known that this property is shared by many substances that are not strictly isomorphous with the dissolved salt, provided their molecular volumes are closely similar. Ostwald showed that nuclei weighing only 10^-10 to 10^-8 grams were usually quite sufficient to induce crystallisation of supersaturated solutions. Furthermore, it appears from numerous researches that mere mechanical friction is sufficient to induce crystallisation, such crystallisation taking place in the complete absence of crystalline nuclei.

Supersaturated solutions of liquids in liquids have only been realised in a few cases, but supersaturated solutions of gases in liquids are not uncommon.

Tap water saturated with air at 7° C. can be incubated at 18° C. for six days without appreciable loss of oxygen. At this higher temperature it is, of course, supersaturated, but so long as the containing vessel is not shaken, and chemical actions such as fermentation are excluded, no appreciable loss of oxygen occurs.

The addition of any powdered substance to such a solution, however, will break down the supersaturation since the gases in the pores of the powder act as nuclei. Supersaturation of this kind differs from that considered above, inasmuch as the nuclei immediately escape from the liquid, whereby their influence is severely limited.