So far, taste and solvent power were considered to be the characteristic feature of acids. In the time of Robert Boyle (1627-1691), they were further distinguished from other substances by the change which they produced in the colour of certain vegetable extracts. Tincture of red cabbage was first used, but, as this liquid rapidly deteriorates on keeping, it was soon replaced by a solution of litmus, a colouring matter obtained from Roccella tinctoria and other lichens. It imparts to water a purple colour, which is changed to red by the addition of acids.
Alkalis. Wood ashes were valued in very early times because they were found to be good for removing dirt from the skin. Mixed with vegetable oil or animal fat, they formed a very primitive kind of soap, which was afterwards much improved by using the aqueous extract instead of the ashes themselves, and also by the addition of a little caustic lime.
When plant ashes are treated with water, about 10 per cent. dissolves. If the insoluble matter is then allowed to settle down and the clear liquid evaporated to dryness, a whitish residue is obtained. The soluble matter thus extracted from the ashes of plants which grow in or near the sea is mainly soda; that from land plants, mainly potash. Formerly no distinction was made, and the general term “alkali” was applied to both.
In order to bring the properties of alkalis into contrast with those of acids, we cannot do better than make a few simple experiments with a weak solution of washing soda. Its taste is very different from that of an acid; it is generally described as caustic. If a little is rubbed between the fingers, it feels smooth, almost like very thin oil. It does not dissolve metals or limestone. Its action on vegetable colouring matter is just as striking as that of acids. Tincture of red cabbage becomes green; the purple of litmus is changed to a light blue. This colour change is characteristic of alkalis.
Neutralization. When the colour of litmus solution has been changed to red by the addition of an acid, the original colour can be restored by adding an alkali. The change can be repeated as often as desired by adding acid and alkali alternately. From this we get a distinct impression of antithesis between the two. In popular language, an alkali “kills” an acid; in Chemistry, the same idea is expressed by the term “neutralization.”
Salts. Both “neutralization” and “killing the acid” are modes of expression which describe the phenomenon fairly well. When an acid is neutralized, its characteristic taste, its solvent power, and its action on litmus, are all changed; in fact, the acid as an acid ceases to exist, and so does the alkali. When the neutral solution is evaporated to dryness, a residue is found which on examination proves to be neither the acid nor the alkali, but a compound formed from the two. This substance is called a salt.
To most people, salt is the name for that particular substance which is taken as a condiment with food. Its use in this connection dates from time immemorial. It is distinctly unfortunate that another and very much wider usage of the term has been introduced into Chemistry. When the early chemists recognized that other substances, which they vaguely designated as “saline bodies,” were similar to common salt in composition, they took the name of the individual and applied it to the whole class.
OTHER METHODS OF SALT FORMATION
Solution of Metals in Acids. Alkalis are not the only substances which neutralize acids. Speaking in a broad and general sense, we may say that an acid is neutralized when a metal is dissolved in it, because, when the point is reached at which no more metal will dissolve, all the characteristic properties of the acid are destroyed. A salt is formed in this case also.
An example will now be given to illustrate this method of salt formation. Before two pieces of metal can be united by soldering, it is necessary to clean the surfaces of the metal and the soldering iron. The liquid used for this purpose is made by adding scraps of zinc to muriatic acid (hydrochloric acid). The zinc dissolves with effervescence, which is caused by the escape of hydrogen gas. When effervescence ceases and no more zinc will dissolve, the liquid is ready for use. The acid has been “killed” or neutralized by the metal. A salt called zinc chloride has been formed. This salt can be recovered from the liquid by evaporation.