Elements of Chemistry, / In a New Systematic Order, Containing all the Modern Discoveries

by Antoine Laurent Lavoisier

SECT. I.

Of the Solution of Salts.

In chemical language, the terms of solution and dissolution have long been confounded, and have very improperly been indiscriminately employed for expressing both the division of the particles of a salt in a fluid, such as water, and the division of a metal in an acid. A few reflections upon the effects of these two operations will suffice to show that they ought not to be confounded together. In the solution of salts, the saline particles are only separated from each other, whilst neither the salt nor the water are at all decomposed; we are able to recover both the one and the other in the same quantity as before the operation. The same thing takes place in the solution of resins in alkohol. During metallic dissolutions, on the contrary, a decomposition, either of the acid, or of the water which dilutes it, always takes place; the metal combines with oxygen, and is changed into an oxyd, and a gasseous substance is disengaged; so that in reality none of the substances employed remain, after the operation, in the same state they were in before. This article is entirely confined to the consideration of solution.

To understand properly what takes place during the solution of salts, it is necessary to know, that, in most of these operations, two distinct effects are complicated together, viz. solution by water, and solution by caloric; and, as the explanation of most of the phenomena of solution depends upon the distinction of these two circumstances, I shall enlarge a little upon their nature.

Nitrat of potash, usually called nitre or saltpetre, contains very little water of cristallization, perhaps even none at all; yet this salt liquifies in a degree of heat very little superior to that of boiling water. This liquifaction cannot therefore be produced by means of the water of cristallization, but in consequence of the salt being very fusible in its nature, and from its passing from the solid to the liquid state of aggregation, when but a little raised above the temperature of boiling water. All salts are in this manner susceptible of being liquified by caloric, but in higher or lower degrees of temperature. Some of these, as the acetites of potash and soda, liquify with a very moderate heat, whilst others, as sulphat of potash, lime, &c. require the strongest fires we are capable of producing. This liquifaction of salts by caloric produces exactly the same phenomena with the melting of ice; it is accomplished in each salt by a determinate degree of heat, which remains invariably the same during the whole time of the liquifaction. Caloric is employed, and becomes fixed during the melting of the salt, and is, on the contrary, disengaged when the salt coagulates. These are general phenomena which universally occur during the passage of every species of substance from the solid to the fluid state of aggregation, and from fluid to solid.

These phenomena arising from solution by caloric are always less or more conjoined with those which take place during solutions in water. We cannot pour water upon a salt, on purpose to dissolve it, without employing a compound solvent, both water and caloric; hence we may distinguish several different cases of solution, according to the nature and mode of existence of each salt. If, for instance, a salt be difficultly soluble in water, and readily so by caloric, it evidently follows, that this salt will be difficultly soluble in cold water, and considerably in hot water; such is nitrat of potash, and more especially oxygenated muriat of potash. If another salt be little soluble both in water and caloric, the difference of its solubility in cold and warm water will be very inconsiderable; sulphat of lime is of this kind. From these considerations, it follows, that there is a necessary relation between the following circumstances; the solubility of a salt in cold water, its solubility in boiling water, and the degree of temperature at which the same salt liquifies by caloric, unassisted by water; and that the difference of solubility in hot and cold water is so much greater in proportion to its ready solution in caloric, or in proportion to its susceptibility of liquifying in a low degree of temperature.

The above is a general view of solution; but, for want of particular facts, and sufficiently exact experiments, it is still nothing more than an approximation towards a particular theory. The means of compleating this part of chemical science is extremely simple; we have only to ascertain how much of each salt is dissolved by a certain quantity of water at different degrees of temperature; and as, by the experiments published by Mr de la Place and me, the quantity of caloric contained in a pound of water at each degree of the thermometer is accurately known, it will be very easy to determine, by simple experiments, the proportion of water and caloric required for solution by each salt, what quantity of caloric is absorbed by each at the moment of liquifaction, and how much is disengaged at the moment of cristallization. Hence the reason why salts are more rapidly soluble in hot than in cold water is perfectly evident. In all solutions of salts caloric is employed; when that is furnished intermediately from the surrounding bodies, it can only arrive slowly to the salt; whereas this is greatly accelerated when the requisite caloric exists ready combined with the water of solution.

In general, the specific gravity of water is augmented by holding salts in solution; but there are some exceptions to the rule. Some time hence, the quantities of radical, of oxygen, and of base, which constitute each neutral salt, the quantity of water and caloric necessary for solution, the increased specific gravity communicated to water, and the figure of the elementary particles of the cristals, will all be accurately known. From these all the circumstances and phenomena of cristallization will be explained, and by these means this part of chemistry will be compleated. Mr Seguin has formed the plan of a thorough investigation of this kind, which he is extremely capable of executing.

The solution of salts in water requires no particular apparatus; small glass phials of different sizes, Pl. II. Fig. 16. and 17. pans of earthern ware, A, Fig. 1. and 2. long-necked matrasses, Fig. 14. and pans or basons of copper or of silver, Fig. 13. and 15. answer very well for these operations.