Hydrochloric acid may also be formed by transmitting moist chlorine through a red-hot porcelain tube; oxygen being at the same time liberated.
Prep. Hydrochloric acid, save for the purposes of illustrative experiment, is never obtained by any of the above processes. An easy mode of procuring it, when required for laboratory use, is to heat the ordinary aqueous solution of the acid in a flask, and to collect the gas, which is given off by displacement. It may also be readily got by introducing pieces of common salt (which should have been previously fused in a crucible at a red-heat and allowed to cool) into a glass retort, and pouring over them about twice their weight of oil of vitriol. The hydrochloric acid, which escapes very abundantly, must be collected either by displacement or over mercury.
Prop. Hydrochloric acid is a colourless gas, very acid to the taste, and irritating to the eyes; and induces coughing even if breathed in small quantities, or when largely diluted. It is very destructive to vegetation, on which account the soda manufacturer is compelled by law to condense and thus prevent the escape of its fumes. It has a specific gravity of 1·261. When subjected to a pressure of 40 atmospheres at 50° F., it becomes a colourless fluid capable of dissolving bitumen, and having a specific gravity of 1·27. It has never been frozen. Hydrochloric acid neither burns, nor supports combustion. The white fumes which it forms when exposed to the air, are due to its condensing the atmospheric moisture, and thus giving rise to a body less volatile than water. This gas is greedily and instantly absorbed by water. A fragment of ice placed in a jar of the gas absorbs it, and becomes immediately dissolved.
Hydrochloric Acid, Solution of. The hydrochloric acid of commerce is a solution of the above gas in water. When exposed to the air it emits grey fumes. Water at 40° F. absorbs about 480 times its bulk of hydrochloric acid, increasing in volume about one third in doing so, acquiring a density of 1·2109, and then containing nearly forty-three per cent. of the acid.
Strength of Solution of Hydrochloric Acid, 77° Fahr. (E. Davy.)
| Sp. Gravity. | Hydrochloric acid in 100 parts. | Sp. Gravity. | Hydrochloric acid in 100 parts. |
| 1·21 | 42·43 | 1·10 | 20·20 |
| 1·20 | 40·40 | 1·09 | 18·18 |
| 1·19 | 38·38 | 1·08 | 16·16 |
| 1·18 | 36·36 | 1·07 | 14·14 |
| 1·17 | 34·34 | 1·06 | 12·12 |
| 1·16 | 32·32 | 1·05 | 10·10 |
| 1·15 | 30·30 | 1·04 | 8·08 |
| 1·14 | 28·28 | 1·03 | 6·06 |
| 1·13 | 26·26 | 1·02 | 4·04 |
| 1·12 | 24·24 | 1·01 | 2·02 |
| 1·11 | 22·22 |
In the laboratory, solution of hydrochloric acid is in constant use. It may be easily prepared from chloride of sodium and sulphuric acid. The retort should be connected with a couple of Woulfe’s bottles; into the first of which a small quantity of water should be poured, to detain any impurities mechanically carried over with the gas;
the second bottle should contain four parts of water, and should be placed in a vessel of cold water, as the gas in becoming condensed, disengages a large amount of heat. The gas comes off and is absorbed readily by the water upon applying a gentle heat to the retort.
It is by this last method that solution of hydrochloric acid is obtained in such enormous quantities[347] for the various purposes in which it is used, in the arts and manufactures.
[347] In South Lancashire alone, more than 1000 tons of hydrochloric acid in solution are made weekly.