Page:The New International Encyclopædia 1st ed. v. 18.djvu/388

* SOLUTION. 334 SOLUTION. well as do dilute solutions in general (see fur- ther below). Passing now to solution of liquids in liquids, we find, first of all, that some liquids (e.g. water and alcohol) are miscible in all propor- tions, that the mutual solubility of others (e.g. water and ether) is limited, and that still others are practically insoluble in each other. There are strong reasons for assuming that the third of these classes is really identical with the sec- ond; only the amounts dissolved are so small that they cannot be detected by the analytical means at our disposal. One of the most im- portant properties of solutions of liquids in liquids is their vapor-tension, which plays an important part in processes of fractional dis- tillation. (!?ee Distillation.) When two liq- uids, A and B, are mixed, the vapor-tension of either undergoes a diminution: A in the solution is less volatile than in the free state, and so is B. The vapor-tension of each in the solution is termed its "partial vapor-tension,' and the total vapor-tension of the solution is equal to the sum of the diminished, partial vapor-tensions of its ingredients. If A and B are mutually soluble to a limited extent, two solutions may be formed (viz. A in B and B in A), of which the partial as well as the total vapor-tensions are respective- ly equal. Take, for instance, water and etiier; if shaken up in sufficient relative quantities and then allowed to .stand undisturbed they will form two distinct liquid layers, the upper a satu- rated solution of water in ether, the lower a saturated solution of ether in water; the partial vapor-tension of the water in the upper equals the partial vapor-tension of the water in the lower .solution; the partial vapor-tension of the ether in the upper equals the partial vapor-ten- sion of the ether in the lower solution; and hence, the total vapor-tension of the upper solu- tion equals the total vapor-tension of the lower. Analogous relations are found in all cases ex- amined. The solubility of solids in liquids is invariably limited. As a rule it increases with the temper- ature, but cases are known (e.g. that of sodium sulphate, with respect to water) in which an elevation of temperature may cause a decrease in solubility. A fact important to remember is that if a solid is capable of existing in two or more different modifications (e.g. in difl'erent allo- tropic forms, in an anhydrous form and one or more forms containing water of crystallization, etc.), each modification has its own solubility, and a solution exactly saturated with the more soluble modification is more or less 'supersatu- rated' Avith the less soluble one. Bearing in mind that the supersaturation of a solution is destroyed, with rapid separation of the excess of dissolved substance, when a trace of the latter is introduced into the solution, the following experiment may serve to illustrate the point under consideration: Let ordinary Glauber's salt, i.e. sodium sulphate containing 10 mole- cules of water of crystallization (NajSC-lOHjO), be heated to boiling with about one-half its weight of water, in a flask whose mouth is loosely closed with a plug of cotton (to keep out particles of Glauber's salt that may be floating in the air). If the solution tlnis obtained be cooled to — 10° C, a sodium sulpliate containing seven molecules of water of crvstallization (KajSOj.THjO) will separate out, and when the separation is complete the mother-liquor will be exactly saturated with respect to tliis salt. Now, Na.SOj.THjO has a greater solubility than Na^SOj.lOHoO. Hence, the saturated mother- liquor of NaoSOj.THjO must evidently he super- saturated with respect to Na^SO^.lOHjO. As a matter of fact, if a trace of the latter be now introduced into our mother-liquor, a new crystal- lization will set in, a mass of Na^SOj.lOHjO sep- arating out and leaving the solution exactly sat- urated with respect also to tliis form of the salt. Such, as well as a host of other phenomena, com- plicate exceedingly the jiroblem of discovering a precise relationsliip Ijctween the solubility of substances in various solvents and their nature. Solid Solutions. The existence of solutions in the solid state has only been recognized within recent years. It was mentioned in the article on diffusion (q.v.) that cases of that phenomenon have now been actually observed in solids. But there is also an increasing number of indirect proofs that many homogeneous solid mixtures are true solutions, i.e. might be formed by the inter-dift'usion of their ingredients, although actually such a process wouhl of course be very slow. Isomorphous crystalline mixtures, while homogeneous, may not be solutions at all; for it is possible that in them free dift'usion cannot take place, the molecules of either of the in- gredients being controlled by the forces that de- termine the crystalline form of the whole; but this is not certain. Among solid solutions con- taining fluids may be mentioned the well-known case of metallic palladium and hydrogen gas. The two were formerly supposed to comljine chemically, forn^iing the compound Pd^H. But the composition of this substance has now been shown to vary with the temperature. Hence it cannot be considered as a chemical compound (see Chemistry), and as it is formed by direct diffusion of hydrogen into palladium, it must be considered as a true solid solution. OssiOTic Pressure. It may be seen from the above that a theory of solutions does not yet exist. Some of the most important questions with regard to solutions remain unanswered and the known facts are mostly uncorrelated : in brief, the .subject is largely not yet rationalized. In one of its phases, however, the subject of solutions has, within recent years, received a development which must be counted among the most brilliant scientific achievements of our time. The achievement in question is based on the most characteristic property of solutions, viz. the capacity of the 'solute' (i.e. the dissolved sub- stance) to difl'use within the solution until the concentration of the latter is the same at all its points. Let an aqueous 'solution of sugar, for in- stance, be placed at the bottom of a vessel, and let some pure water be introduced over it, cau- tiously, so as not to disturb the solution; the result will be that the sugar will gradually dif- fuse upward, and after a certain length of time the liqviid will have a perfectly luiiform com- position throughout. Now, to cause this motion of the sugar upward, against gravity, there must obvfously be some force. An analogous case that readily suggests itself to the mind is that of gases. A gas. too. will flow upward, and. like a substance in solution, will distribiite itself evenly within an available volume. Of course.