It will be readily understood that hydrochloric acid may be obtained from all other metallic chlorides.[33] It is frequently formed in other reactions, many of which we shall meet with in the further course of this work. It is, for instance, formed by the action of water on sulphur chloride, phosphorus chloride, antimony chloride, &c.

Hydrochloric acid is a colourless gas having a pungent suffocating odour and an acid taste. This gas fumes in air and attracts moisture, because it forms vapour containing a compound of hydrochloric acid and water. Hydrochloric acid is liquefied by cold, and under a pressure of 40 atmospheres, into a colourless liquid of sp. gr. 0·908 at 0°,[34] boiling point -35° and absolute boiling point +52°. We have already seen (Chapter [I].) that hydrochloric acid combines very energetically with water, and in so doing evolves a considerable amount of heat. The solution saturated in the cold attains a density 1·23. On heating such a solution containing about 45 parts of acid per 100 parts, the hydrochloric acid gas is expelled with only a slight admixture of aqueous vapour. But it is impossible to entirely separate the whole of the hydrochloric acid from the water by this means, as could be done in the case of an ammoniacal solution. The temperature required for the evolution of the gas rises and reaches 110°–111°, and after this remains constant—that is, a solution having a constant boiling point is obtained (as with HNO3), which, however, does not (Roscoe and Dittmar) present a constant composition under different pressures, because the hydrate is decomposed in distillation, as is seen from the determinations of its vapour density (Bineau). Judging from the facts (1) that with decrease of the pressure under which the distillation proceeds the solution of constant boiling point approaches to a composition of 25 p.c. of hydrochloric acid,[35] (2) that by passing a stream of dry air through a solution of hydrochloric acid there is obtained in the residue a solution which also approaches to 25 p.c. of acid, and more nearly as the temperature falls,[36] (3) that many of the properties of solutions of hydrochloric acid vary distinctly according as they contain more or less than 25 p.c. of hydrochloric acid (for instance, antimonious sulphide gives hydrogen sulphide with a stronger acid, but is not acted on by a weaker solution, also a stronger solution fumes in the air, &c.), and (4) that the composition HCl,6H2O corresponds with 25·26 p.c. HCl—judging from all these data, and also from the loss of tension which occurs in the combination of hydrochloric acid with water, it may be said that they form a definite hydrate of the composition HCl,6H2O. Besides this hydrate there exists also a crystallo-hydrate, HCl,2H2O,[37] which is formed by the absorption of hydrochloric acid by a saturated solution at a temperature of -23°. It crystallises and melts at -18°.[38]

The mean specific gravities at 15°, taking water at its maximum density (4°) as 10,000, for solutions containing p per cent. of hydrogen chloride are—

pSpS
510,2422511,266
1010,4903011,522
1510,7443511,773
2011,0014011,997

The formula S = 9,991·6 + 49·43p + 0·0571p2, up to p = 25·26, which answers to the hydrate HCl,6H2O mentioned above, gives the specific gravity. Above this percentage S = 9,785·1 + 65·10p - 0·240p2. The rise of specific gravity with an increase of percentage (or the differential ds/dp) reaches a maximum at about 25 p.c.[39] The intermediate solution, HCl,6H2O, is further distinguished by the fact that the variation of the specific gravity with the variation of temperature is a constant quantity, so that the specific gravity of this solution is equal to 11,352·7(1 - 0·000447t), where 0·000447 is the coefficient of expansion of the solution.[40] In the case of more dilute solutions, as with water, the specific gravity per 1° (or the differential ds / dt ) rises with a rise of temperature.[41]

p =05101520
S0–S15 = 7·223385264
S15–S130 = 34·142505967

Whilst for solutions which contain a greater proportion of hydrogen chloride than HCl,6H2O, these coefficients decrease with a rise of temperature; for instance, for 30 p.c. of hydrogen chloride S0 - S15 = 88 and S15 - S30 = 87 (according to Marignac's data). In the case of HCl,6H2O these differences are constant, and equal 76.

Thus the formation of two definite hydrates, HCl,2H2O and HCl,6H2O, between hydrochloric acid and water may be accepted upon the basis of many facts. But both of them, if they occur in a liquid state, dissociate with great facility into hydrogen chloride and water, and are completely decomposed when distilled.

All solutions of hydrochloric acid present the properties of an energetic acid. They not only transform blue vegetable colouring matter into red, and disengage carbonic acid gas from carbonates, &c., but they also entirely saturate bases, even such energetic ones as potash, lime, &c. In a dry state, however, hydrochloric acid does not alter vegetable dyes, and does not effect many double decompositions which easily take place in the presence of water. This is explained by the fact that the gaso-elastic state of the hydrochloric acid prevents its entering into reaction. However, incandescent iron, zinc, sodium, &c., act on gaseous hydrochloric acid, displacing the hydrogen and leaving half a volume of hydrogen for each volume of hydrochloric acid gas; this reaction may serve for determining the composition of hydrochloric acid. Combined with water hydrochloric acid acts as an acid much resembling nitric acid[42] in its energy and in many of its reactions; however, the latter contains oxygen, which is disengaged with great ease, and so very frequently acts as an oxidiser, which hydrochloric acid is not capable of doing. The majority of metals (even those which do not displace the H from H2SO4, but which, like copper, decompose it to the limit of SO2) displace the hydrogen from hydrochloric acid. Thus hydrogen is disengaged by the action of zinc, and even of copper and tin.[42 bis] Only a few metals withstand its action; for example, gold and platinum. Lead in compact masses is only acted on feebly, because the lead chloride formed is insoluble and prevents the further action of the acid on the metal. The same is to be remarked with respect to the feeble action of hydrochloric acid on mercury and silver, because the compounds of these metals, AgCl and HgCl, are insoluble in water. Metallic chlorides are not only formed by the action of hydrochloric acid on the metals, but also by many other methods; for instance, by the action of hydrochloric acid on the carbonates, oxides, and hydroxides, and also by the action of chlorine on metals and certain of their compounds. Metallic chlorides have a composition MCl; for example, NaCl, KCl, AgCl, HgCl, if the metal replaces hydrogen equivalent for equivalent, or, as it is said, if it be monatomic or univalent. In the case of bivalent metals, they have a composition MCl2; for example, CaCl2, CuCl2, PbCl2, HgCl2, FeCl2, MnCl2. The composition of the haloid salts of other metals presents a further variation; for example, AlCl3, PtCl4, &c. Many metals, for instance Fe, give several degrees of combination with chlorine (FeCl2, FeCl3) as with hydrogen. In their composition the metallic chlorides differ from the corresponding oxides, in that the O is replaced by Cl2, as should follow from the law of substitution, because oxygen gives OH2, and is consequently bivalent, whilst chlorine forms HCl, and is therefore univalent. So, for instance, ferrous oxide, FeO, corresponds with ferrous chloride, FeCl2, and the oxide Fe2O3 with ferric chloride, which is also seen from the origin of these compounds, for FeCl2 is obtained by the action of hydrochloric acid on ferrous oxide or carbonate and FeCl3 by its action on ferric oxide. In a word, all the typical properties of acids are shown by hydrochloric acid, and all the typical properties of salts in the metallic chlorides derived from it. Acids and salts composed like HCl and MnCl2m without any oxygen bear the name of haloid salts; for instance, HCl is a haloid acid, NaCl a haloid salt, chlorine a halogen. The capacity of hydrochloric acid to give, by its action on bases, MO, a metallic chloride, MCl2, and water, is limited at high temperatures by the reverse reaction MCl2 + H2O = MO + 2HCl, and the more pronounced are the basic properties of MO the feebler is the reverse action, while for feebler bases such as Al2O3, MgO, &c., this reverse reaction proceeds with ease. Metallic chlorides corresponding with the peroxides either do not exist, or are easily decomposed with the disengagement of chlorine. Thus there is no compound BaCl4 corresponding with the peroxide BaO2. Metallic chlorides having the general aspect of salts, like their representative sodium chloride, are, as a rule, easily fusible, more so than the oxides (for instance, CaO is infusible at a furnace heat, whilst CaCl2 is easily fused) and many other salts. Under the action of heat many chlorides are more stable than the oxides, some can even be converted into vapour; thus corrosive sublimate, HgCl2, is particularly volatile, whilst the oxide HgO decomposes at a red heat. Silver chloride, AgCl, is fusible and is decomposed with difficulty, whilst Ag2O is easily decomposed. The majority of the metallic chlorides are soluble in water, but silver chloride, cuprous chloride, mercurous chloride, and lead chloride are sparingly soluble in water, and are therefore easily obtained as precipitates when a solution of the salts of these metals is mixed with a solution of any chloride or even with hydrochloric acid. The metal contained in a haloid salt may often be replaced by another metal, or even by hydrogen, just as is the case with a metal in an oxide. Thus copper displaces mercury from a solution of mercuric chloride, HgCl2 + Cu = CuCl2 + Hg, and hydrogen at a red heat displaces silver from silver chloride, 2AgCl + H2 = Ag2 + 2HCl. These, and a whole series of similar reactions, form the typical methods of double saline decompositions. The measure of decomposition and the conditions under which reactions of double saline decompositions proceed in one or in the other direction are determined by the properties of the compounds which take part in the reaction, and of those capable of formation at the temperature, &c., as was shown in the preceding portions of this chapter, and as will be frequently found hereafter.

If hydrochloric acid enters into double decomposition with basic oxides and their hydrates, this is only due to its acid properties; and for the same reason it rarely enters into double decomposition with acids and acid anhydrides. Sometimes, however, it combines with the latter, as, for instance, with the anhydride of sulphuric acid, forming the compound SO3HCl; and in other cases it acts on acids, giving up its hydrogen to their oxygen and forming chlorine, as will be seen in the [following chapter].