Salts constitute an extremely important class of substances in chemistry, of which common or sea salt (sodium chloride) may be mentioned as the most familiar example. Epsom salt (magnesium sulphate), Glauber's salt (sodium sulphate), saltpetre (potassium nitrate), Rochelle salt (sodium potassii tartrate) are other well-known salts. Common salt appears to have been known from the earliest times, and the fact that the same Greek word hals is used in the feminine, signifying the ocean, and in the masculine, signifying salt, would seem to indicate the sea as the source from which common salt was obtained in the first instance. The application of the name salt to substances other than common salt would follow from the more or less general similarity of these to common salt in appearance, and in the more easily observed properties, such as solubility, taste, &c.
By the term salt chemists now ordinarily understand a substance which may be looked upon as derived from an acid by the replacement of part or the whole of the hydrogen of the acid by means of a metal or of a Radical (q.v.) capable of playing the part of a metal; such as, for instance, the radical , which is called ammonium. The acids themselves are even looked upon as constituting the hydrogen terms in the various series of salts, and are sometimes called hydrogen salts.
There are several general modes of formation of salts. One of the most important of these depends upon the mutual action upon each other of an acid and a Base (q.v.), when the typical characters of each of these substances disappear and a salt is produced, usually with the simultaneous production of water. For example, when nitric acid and the basic oxide of lead act upon each other, lead nitrate and water are produced, thus: . Salts are also frequently produced when metals are dissolved in acids, the hydrogen of the acid being displaced by the metal. Thus, iron dissolves in dilute hydrochloric or sulphuric acid with the evolution of hydrogen and the formation of a ferrous salt: . Strictly analogous to the displacement of hydrogen from sulphuric acid by means of iron is the displacement of copper from cupric sulphate by the same metal—this action also giving rise to ferrous sulphate while copper is precipitated: . In the process called double decomposition (see CHEMISTRY, Vol. III. p. 152) two new salts are frequently produced when the solutions of two salts are mixed together, as, for instance, when sodium chloride and silver nitrate act mutually upon each other, yielding silver chloride and sodium nitrate: . There are other modes of salt formation which are of minor importance.
Salts are frequently considered as consisting of metal and salt radical, the latter comprising all that portion of a salt which is not metal, as explained in the article RADICAL. In the cases of the so-called haloid salts (fluorides, chlorides, bromides, iodides) the salt radical consists of one element only, while in other salts the salt radical contains two or more different elements. The names given to certain classes of salts may be shortly explained. Normal salts are those resulting from the displacement of the whole of the displacable hydrogen of an acid by means of a metal. Neutral salts are such as do not exhibit either the acid or the alkaline reaction when dissolved. Basic salts and acid salts, as contrasted with the normal salts, are respectively intermediate in composition between the normal salt and the base and between the normal salt and the acid. These salts still possess the respective characters of a base and of an acid. Double salts may contain two or more metals in combination with the same salt radical, or two or more salt radicals in combination with the same metal, or they may contain more than one metal and more than one salt radical.
SMELLING-SALTS are a preparation of carbonate of ammonia with some of the sweet-scented volatile oils, used as a restorative by persons suffering from faintness. The pungency of the ammonia is all that is useful, and the oils are added to make it more agreeable. Oils of lavender, lemon, cloves, and bergamot are those chiefly used. The celebrated Preston smelling-salts are scented with oils of cloves and pimento. The manufacture of ornamental bottles to contain this preparation is an important branch of the glass and silversmith's trades.