HYDROCHLORIC ACID, or HYDRO GEN CHLORIDE, an important compound of hydrogen and chlorine, which under the names of salt° and amuriatic acid," has been known in aqueous solution from very early times. It has the chemical formula HO, and may be formed by exposing a mixture of equal volumes of hydrogen and chlorine to diffuse daylight, the combination taking place quietly under these circumstances, but explosively un der the influence of direct sunlight. A more convenient and usual method of preparing hy drochloric acid is by treating common salt (aso dium chloride,a NaCI) with strong sulphuric acid (HtSO4). The reaction is as follows: NaCI H2S404-- HNaSO4 + HO. Pure hy drochloric acid is a colorless gas, 1,259 times as heavy as an equal volume of air at the same temperature and pressure, and with the molecu lar weight 36.457. By weight hydrochloric acid consists of 2.74 per cent of hydrogen and 9726 per cent of chlorine. At a temperature of 50° F. it condenses, under a pressure of 40 atmospheres, to a colorless liquid, which boils, under ordinary atmospheric pressure, at 171° F. below zero, and solidifies at a temperature about 6° below the boiling point. The specific heat of the gas at constant pressure (compared with water) is about 0.19; and the ratio of its specific heat at constant pressure to its specific heat at constant volume, at ordinary temperatures, is 1.389. Hydrochloric acid gas was first prepared, in an approximately pure state, by Priestley, in 1774; but it was believed to be an oxide of a new element (provisionally called amuriuma) until Davy, in 1810, showed that it is a com pound of hydrogen and chlorine.
Hydrochloric acid gas is exceedingly soluble in water. If a dry glass flask, which is com pletely filled with the dry gaseous acid, be brought mouth downward under water, and the stopper is then removed, solution takes place with such extreme rapidity that the water is often drawn into the flask suddenly enough to break it. By passing a stream of the gaseous
acid into water, an aqueous solution may be prepared which has a specific gravity, when saturated, of 1.2257 at 32° F., and contains 45 per cent by weight of the acid. The commer cial acid is commonly known, to the present day, as amuriatic acid.° Large quantities of it are obtained as a by-product in the manufacture of sodium carbonate by the action of sulphuric acid upon common salt. The hydrochloric acid gas liberated in this process is passed up through stone towers packed with hard coke over which trickles water, which greedily absorbs the gas. The acid solution which collects at the bottom of the tower, if too weak for concentration, is taken to the top and again allowed to seep down and gather a larger content of gas. The acid thus obtained is not pure but contains sulphuric acid, ferric chloride, arsenic and free chlorine and sulphurous acid gases. These impurities give it a yellowish tinge. The pure acid (in aqueous solution) is obtained by re moving the arsenic with stannous chloride and then distilling; or by digesting the diluted acid solution with bright copper strips and distilling with ferrous chloride. Aqueous hydrochloric acid is largely used in the laboratory and also in the arts, in the manufacture of a great variety of substances.
Hydrochloric acid combines with metallic bases to form salts which are known as "chlo rides" Common salt (chloride of sodium, NaCI) is the most familiar and the most abundant example, in nature, of this class of substances. The chlorides of silver, gold, mercury, barium, aluminum, platinum and ammonia are also im portant and are described under MUCURY. ALUMINUM, etc. The largest use of hydro chloric acid is in the, manufacture of chlorine (q.v.) and chlorides of the metals. It is also used extensively in the arts, as well as in the manufacture of agricultural fertilizers, coal tar colors, etc.