SOLUTION. When the force of ADHESION is exerted between a solid and a liquid with sufficient energy to overcome the cohesion of the former, the substance is said to be soluble, or to undergo solution, or to be dissolved. The solution of sugar in water, of resin in spirits of wine, of silver in mercury, is now regarded as a form of adhesion.
Certain liquids are also soluble in other liquids, gases in other gases, and gases in liquids, as noticed under DIFFUSION. For the solubility of gases in liquids, we refer to GAS. In the present article we may give a few details respecting the solution of solids in liquids.
The liquid which effects the solution is usually termed the solvent, but sometimes the menstruum. Particular solutions have also special names, such as syrup, which is applied to the solution of sugar in water, while tincture refers to a solution of a solid in alcohoL When a liquid can no longer dissolve further portions of the solid, it is said to be saturated ; that is, the force of cohesion balances that of adhesion.
Generally, however, an elevation of temperature, by diminishing cohe sion, will increase the solvent powers of the liquid. But there are cases in which cold appears to favour solution : thus lime and some of its salts dissolve more readily in water just above the freezing-point than when boiling. Crystallised sulphate of soda requires about ten times its weight of ice-cold water for solution, and its solubility increases rapidly with the temperature up to 91° Fahr., from which point up to the boiling-point of the solution the solubility decreases ; and the liquid, saturated at 91", deposits a portion of the salt by increasing the temperature. Seleniate of soda and sulphate of iron afford similar results, which are due probably to the fact that heat diminishes adhesion as well as cohesion, but the former force decreases more rapidly than the latter. If a liquid added to a solution have a stronger adhesion to the solvent than to the substance dissolved, the latter will often bo thrown down in a pulverulent state: thus, the addition of water to camphorated spirit will throw down camphor.
Solution is favoured by increasing the extent of surface in the solid, as by reducing it to powder. In general, the first portions of a solid disappear rapidly, and the after portions more and more slowly until saturation is reached. Solids present innumerable degrees of solubility;
for while some bodies, such as sulphate of baryta, are almost insoluble, and sulphate of lime only soluble in about the proportion of 1 part in 700 parts of water, I part of sulphate of potash will dissolve in 16 parts of water, and 2 parts of sulphate of magnesia will dissolve in only 3 of water. It is remarkable that water saturated with one salt will dissolve others. As aqueous solutions of solids are heavier than water, the degree of solubility of a solid may be judged of by suspending it in a glass of water, and watching the current as it descends. " If it fall rapidly, and in dense atria:, it will indicate rapid solubility, and the formation of a dense solution • if it fall in a very narrow stream, it will indicate only moderate or solubility ; and by its descending rapidly or in a slow broad stream, or by resting about the substance, a judgment may be mado of the comparative density of the solution produced. If no descending current appear, nor any fluid round the substance of a refractive power or colour different to that of the water, then the body must be very nearly, if not quite, insoluble at common temperatures." (Faraday, 'Chemical Manipulation.') The taste will also frequently give an indication of the solubility of a solid.
In general, a solution due to adhesion partakes of the properties both of tho solvent and of the substance dissolved. Where chemical change intervenes, we have the properties of a third body. Hence, in eases of simple solution, the solvent and the body dissolved have, to some extent, properties in common, as when mercury dissolves many of the metals, and oils dissolve fatty bodies and each other; but in cases of chemical action, the affinities are strongest between bodies the most dissimilar, as when the acids dissolve metals or their oxides, oils the alkalies, on.
The uses of solution are numerous. It allows a body to be purified by filtration or crystallisation, so that one substance may be separated from either by crystallisation or by the use of such fluids in succession as have a solvent power over one or more of the substances present. By means of solution, substances are prepared for the exertion of chemical action, and all obstructions due to the attraction of aggre gation removed.