[33] Thus the metallic chlorides, which are decomposed to a greater or less degree by water, correspond with feeble bases. Such are, for example, MgCl2, AlCl3, SbCl3, BiCl3. The decomposition of magnesium chloride (and also carnallite) by sulphuric acid proceeds at the ordinary temperature; water decomposes MgCl2 to the extent of 50 p.c. when aided by heat, and may be employed as a convenient method for the production of hydrochloric acid. Hydrochloric acid is also produced by the ignition of certain metallic chlorides in a stream of hydrogen, especially of those metals which are easily reduced and difficultly oxidised—for instance, silver chloride. Lead chloride, when heated to redness in a current of steam, gives hydrochloric acid and lead oxide. The multitude of the cases of formation of hydrochloric acid are understood from the fact that it is a substance which is comparatively very stable, resembling water in this respect, and even most probably more stable than water, because, at a high temperature and even under the action of light, chlorine decomposes water, with the formation of hydrochloric acid. The combination of chlorine and hydrogen also proceeds by their direct action, as we shall afterwards describe.
[34] According to Ansdell (1880) the sp. gr. of liquid hydrochloric acid at 0° = 0·908, at 11·67° = 0·854, at 22·7° = 0·808, at 33° = 0·748. Hence it is seen that the expansion of this liquid is greater than that of gases (Chapter II., Note [34]).
[35] According to Roscoe and Dittmar at a pressure of three atmospheres the solution of constant boiling point contains 18 p.c. of hydrogen chloride, and at a pressure of one-tenth atmosphere 23 p.c. The percentage is intermediate at medium pressures.
[36] At 0° 25 p.c., at 100° 20·7 p.c.; Roscoe and Dittmar.
[37] This crystallo-hydrate (obtained by Pierre and Puchot, and investigated by Roozeboom) is analogous to NaCl,2H2O. The crystals HCl,2H2O at -22° have a specific gravity 1·46; the vapour tension (under dissociation) of the solution having a composition HCl,2H2O at -24° = 760, at -19° = 1,010, at -18° = 1,057, at -17° = 1,112 mm. of mercury. In a solid state the crystallo-hydrate at -17·7° has the same tension, whilst at lower temperatures it is much less: at -24° about 150, at -19° about 580 mm. A mixture of fuming hydrochloric acid with snow reduces the temperature to -38°. If another equivalent of water be added to the hydrate HCl,2H2O at -18°, the temperature of solidification falls to -25°, and the hydrate HCl,3H2O is formed (Pickering, 1893).
[38] According to Roscoe at 0° one hundred grams of water at a pressure p (in millimetres of mercury) dissolves—
| p | = | 100 | 200 | 300 | 500 | 700 | 1000 |
| Grams HCl | 65·7 | 70·7 | 73·8 | 78·2 | 81·7 | 85·6 |
At a pressure of 760 millimetres and temperature t, one hundred grams of water dissolves
| t | = | 0 | 8° | 16° | 24° | 40° | 60° |
| Grams HCl | 82·5 | 78·3 | 74·2 | 70·0 | 63·3 | 56·1 |
Roozeboom (1886) showed that at t° solutions containing c grams of hydrogen chloride per 100 grams of water may (with the variation of the pressure p) be formed together with the crystallo-hydrate HCl,2H2O: