Hydrochloric Acid (sym. HCl; equiv. 36.5) is one of the most important compounds in inorganic chemistry. If the two gases which enter into its composition (hydrogen and chlorine) be mixed in equal volumes they will remain without action upon each other if kept in the dark; but as soon as they are brought into direct sunlight they unite with a loud explosion, and hydrochloric acid gas is the result. The principal characters of this gas are that it is colourless, intensely acid, irrespirable, and even when largely diluted is very irritating to the lungs and eyes, and very injurious to vegetation; that it is heavier than air (its specific gravity being 1.2474, air being taken at 1.000); that it can be condensed into a colourless liquid; that it is very soluble in water, and that it is neither combustible nor a supporter of combustion. When allowed to escape into the air it produces white fumes by condensing the atmospheric moisture. If the air be previously dried no such fumes are apparent.
The solutions of this gas in water form the acid which was first known as Spirit of Salt, then as Muriatic Acid, and which is now termed Hydrochloric or Chlorhydric Acid. A saturated watery solution of this gas at 40° (4.4° C.) has a specific gravity of 1.21, and consists of 1 equivalent of the gas dissolved in 3 equivalents of water. It forms a colourless, fuming liquid, which acts as a caustic. On heating it the gas is evolved abundantly until the temperature reaches 230° (110° C.), when there distils over a diluted solution, having a specific gravity of 1.1, and consisting of 1 equivalent of the gas and 8 equivalents of water. It is to these solutions of hydrochloric acid that the term hydrochloric acid is far more commonly applied than to the gas itself. They possess the ordinary characters of an energetic acid, and neutralise the strongest bases. The neutralisation is, however, not in consequence of the acid combining with the oxide, but is due to the simultaneous decomposition of the acid and of the oxide, water and a metallic chloride being formed. If M represents the metal the reaction is expressed by the equation . All metals which at a red heat decompose water also decompose this acid, and cause an evolution of hydrogen, the reaction being expressed as follows: . Hydrochloric acid gas is a common gaseous volcanic product. Free hydrochloric acid in a very dilute form is also a constituent of the gastric juice of man and animals, and plays an essential part in the digestive process.
Commercial muriatic acid—to use the name employed by manufacturing chemists—is made by heating in iron cylinders common salt (chloride of sodium) and oil of vitriol (hydrated sulphuric acid), and condensing the evolved gas in water contained in a series of stoneware Wolffian Bottles (q.v.), the reaction being explained by the equation:
This commercial acid may contain various impurities—as, for example, iron (which gives it a bright deep yellow colour), the chlorides of sodium and arsenic—the latter being derived from the oil of vitriol—sulphuric and sulphurous acids, chlorine, &c., from which it can be purified to a great extent by dilution and redistillation. 'If pure,' says Miller, 'the acid should leave no residue when evaporated; on saturating it with ammonia it should give no precipitate of oxide of iron; sulphuretted hydrogen should produce no turbidity in it, which would be the case if arsenic, free chlorine, or sulphurous acid were present; and on dilution with three or four times its bulk of water no white cloud of sulphate of baryta should be produced by the addition of chloride of barium.' The presence of hydrochloric acid, or of the soluble chlorides in solution, may be detected by the addition of a few drops of a solution of nitrate of silver, which occasions the formation of a white curdy precipitate of chloride of silver, which is insoluble in nitric acid, but dissolves in a solution of ammonia.
Liquid hydrochloric acid (under the name of spirit of salt) was known to the alchemists. Hydrochloric acid gas was discovered by Priestley in 1772; and Davy in 1810 ascertained that it was composed of chlorine and hydrogen. In many respects hydrobromic, hydrofluoric, and hydriodic acids resemble hydrochloric acid.