SOLUTIONS. In chemistry and physics a solution is usually defined as a homogeneous liquid in which two or more chemically dis tinct substances are simultaneously present. Many authorities, however, restrict the term to such homogeneous liquid mixtures as exhibit a definite osmotic pressure (see below). Others extend it so as to make it include gaseous mix tures, and even solid mixtures such as alloys. In the present article we shall adopt the former course, using the term to signify a homogeneous liquid mixture, which is capable of exerting a definite osmotic pressure.
When a solid substance is dissolved in a liquid, the liquid is called the °solvent," and the solid is called the °solute When a solution is prepared by mixing two liquids, either one of the liquids may be regarded as the °solvent," the other then being called the "solute ; but it is customary to consider the one which is present in largest amount as the °solvent." By way of illustrating these terms, let us suppose that approximately equal masses of water and of anhydrous liquid carbolic acid are shaken up together. The two will not mix freely at ordinary temperatures, but the water will take up a small quantity of the acid, and the acid will similarly take up a small quantity of the water; the mixture subsiding, when the agita tion ceases, into two distinct layers, carbolic acid preponderating in the lower one, and water in the upper one. The upper lay'er may then be regarded as a solution of carbolic acid in water, and the lower as a solution of water in carbolic acid; carbolic acid being the "solute° in the upper layer, and the "solvent" in the lower one. This distinction is evidently an arbitrary one, however, and many cases could be cited in which it would be impossible to assign any rea son for regarding either constituent as the "solvent," in preference to the other one. For example, carbolic acid and water dissolve in each other more and more freely as the tem perature rises, until, at temperatures above 183° F., they mix readily in all proportions. If equal masses of these substances were mixed at 185° F., therefore, we could regard either as the solvent, and either as the solute.
When a solid substance is dissolved in a liquid, it sometimes happens that there is no marked evidence of chemical change. In other cases, however, there is a very obvious chemical change. When ordinary cane sugar is dis solved in water, a syrupy lipid results, from which the sugar may be again obtained in the same chemical condition as at first, by allowing the water to evaporate at a sufficiently low tem perature. If sodium hydroxide, on the other hand, is added to a given mass of hydrochloric acid until the acid is just neutralized, it is impossible to obtain either the acid or the sodium hydroxide by mere evaporation of the solution. Nothing but water passes away in the vapor, and the solid substance that remains behind when the water is all gone is found to be common salt. If we choose to do so, we may
regard the mixture of sodium hydroxide and hydrochloric acid as a solution of the hydroxide in the acid; but since we may prepare iden tically the same ultimate mixture by adding common salt to pure water, it is perhaps more convenient to regard it as a solution of common salt in water. For the present we shall take the latter view, confining our attention solely to those cases of solution in which a solid dis solves in a liquid without any obvious chemical change.
When a solid is added to a liquid for which it has no chemical affinity, it may remain en tirely unaffected. In this case, the solid is said to be "insoluble" in the liquid, and no solution is obtained. If 'the solid is not insoluble in the given liquid, it gradually disappears when the two are brought together, the part which ceases to be visible passing into the liquid in the dis solved state. In some cases the change is rapid and in others it is slow. It is always accelerated by agitating the solvent. Some solids appear to mix with certain solvents in all proportions; but in general it will be found that if the supply of the solid is sufficient there will come a time when, under the conditions of the experiment, no more of the solid will dissolve. The solu tion is then said to be °saturated." The quan tity of a given solute that will dissolve in a given solvent depends upon several circum stances, but most notably upon the temperature of the solvent, nearly all substances being more soluble at high temperatures than at lower ones. If a saturated solution of a substance that is more soluble at high temperatures than at lower ones is warmed, it becomes capable of taking up more of the solute; and if such a saturated solution is cooled, it usually deposits some of the solute, until its degree of concentration be comes reduced to that corresponding to satura tion at the lower temperature. It is sometimes possible, however, to cause a solution to retain more of the dissolved solid than corresponds to saturation at a given temperature; though this °supersaturated° condition is quite unstable. Glauber's salt (that is, sulphate of sodium crystallized with 10 molecules of water), ex hibits supersaturation in a marked manner. If a saturated solution of this salt be prepared at a certain definite temperature, and then re moved from contact with any particle of the free, undissolved salt, its temperature may be lowered by a considerable amount before crystallization sets in. If the smallest fragment of the solid salt is placed in such a supersatu rated solution, crystallization at once begins, proceeding rapidly until the concentration is reduced to that corresponding to normal satu ration at the temperature at which the experi ment is performed. Pressure appears to in fluence the solubility of a substance to a measur able extent, but the effect of a change of tem perature is, in general, far more marked.