Solution

Solution. Most liquids have the power of dissolving substances - that is, causing such substances to liquefy - and no liquid has this power so much as water. The dissolved substance may be either a gas or a liquid or a solid. When a gas is absorbed by a liquid, the amount dissolved varies with the temperature of the solution, the pressure to which the solution is subjected, and the nature of the gas itself. It is usual for the amount of gas contained in a solution to decrease with rise of temperature; thus the oxygen and carbon dioxide contained in cold water are given off on boiling. Henry discovered the law connecting the amount of gas dissolved with the pressure, and this law states that the volume of gas absorbed varies directly with the pressure. Thus, if water at ordinary pressure - i.e. under one atmosphere - will dissolve one litre of a gas, it will also dissolve apparently one litre under a pressure of two atmospheres; but one litre of the gas under two atmospheres is equivalent to two litres under one atmosphere, so we see that in the second case twice as much gas has really been absorbed. The amount of any gas dissolved from a mixture is determined by the pressure of that gas alone, and the law of such absorption is known as Dalton's law of partial pressures.

Usually a solid becomes more soluble in a liquid as the temperature rises; but there are a few exceptions to this, notably lime, which is much more soluble in cold than in boiling water. When the liquid will not dissolve any more of the substance, it is said to be saturated. In some cases, when a hot saturated solution is allowed to cool slowly without agitation, the solid is not precipitated, and hence the solution contains at the lower temperature more substances than it would dissolve naturally at that temperature. Such a solution is said to be supersaturated. It is in an unstable state, and generally slight agitation, or the addition of a grain or two of some solid, will cause solidification to occur so rapidly that a rise of temperature is at once observed. Sodium sulphate shows this phenomenon exceedingly well, and it generally occurs best with those salts which contain a large amount of water or crystallisation.

When substances are dissolved in any solvent, the solution exhibits properties different from those of the pure solvent. Thus if a solution of sugar and water be contained in a tube, with what is known as a semipermeable membrane at its base, and this be placed in water, the solution rises in the tube, owing to the entry or water through the mem~ brane, until a constant limit is reached. [OSMOSE.] There is thus a head of solution indicating a pressure, and this is known as the osmotic pressure of the solution. Osmotic pressure might be measured in other ways, and by means of this experimental quantity van't Hoff found he could apply thermodynamics to solutions, and hence originated what is known as the new theory of solutions. For dilute solutions at constant temperature it has been found that the osmotic pressure is proportional to the concentration, and further that the value of the osmotic pressure is the same as that which the substance would exert if it could be gasified and made to occupy (at the same temperature) a volume equal to the volume of the solution. It will thus be seen that in dilute solutions the dissolved substance behaves very much like a gas. It has been found also that the vapour pressure of a solution is lower than that of the pure solvent, and that the amount or lowering depends on the molecular weight of the dissolved substance. This causes the boiling-point to be higher, and the freezing-point to be lower, for a solution than for the solvent, and observations on these alterations of temperature are often used as a means of determining the molecular weight of a dissolved substance.

It has been found that salts, acids, and bases give abnormally large values of the osmotic pressure and related properties; they behave as though more molecules were present than are actually there, and hence it has been suggested that these substances have really dissociated. Since those substances which exhibit these peculiarities are always found to be conductors of electricity, a theory of electrolytic dissociation has been largely accepted. In the case of a solution of hydrochloric acid, for example, Faraday supposed each molecule consisted of two parts oppositely charged with electricity, thus +H -Cl. These parts he called the ions, and the passage of the current was supposed to consist of the movement of the two ions in opposite directions. These ideas of Faraday's have been supported by many workers, and in 1887 Arrhenius published his hypothesis that in such solutions a portion of the molecules exists decomposed into ions even when no current is passing. It is not possible to expound this theory more fully here, but work done on solutions of many different electrolytes shows that, whatever may be the real state of an electrolyte in solution, the ion theory affords a reliable working hypothesis.