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When aqueous hydrochloric acid containing 20.24 per cent. of HCl is distilled under 760 mm. pressure, the acid passes over completely without change of composition, as though it were a pure compound. If a weaker acid (e.g., 15 per cent.) is taken, a more dilute acid passes over into the receiver until the residue in the retort contains 20.24 per cent. of HCl, whereas if a stronger acid (e.g., 30 per cent.) is distilled, it loses hydrogen chloride gas with a little moisture until the same 20.24 per cent. acid is left. In both cases the residual acid distils off without change of composition. Since the composition remains constant during distillation, the vapour has the same composition as the liquid, hence the boiling point (110°) remains constant. This is the maximum boiling point for the aqueous acid; both weaker and stronger solutions boil at lower temperatures.
The relative numbers of molecules of HCl and H2O in the liquid of maximum boiling point are 1:10 very nearly. Hence Bineau concluded that the liquid was a chemical compound, HCl,10H2O. The vapour-density was found to be only about 10, showing that the vapour is a mixture of hydrogen chloride and steam. Roscoe and Dittmar (1860), carried out the distillation under various pressures, and found that the concentration of the acid of maximum boiling point decreased with rise of pressure:
| Pressure mm. Hg | 50 | 700 | 760 | 800 | 1800 | | Per cent. HCl in max. b. pt. | 23.2 | 20.4 | 20.24 | 20.2 | 18.7 |
The composition of a compound would be independent of the pressure over a certain range (possibly limited). It is therefore improbable that HCl,10H2O exists in the liquid; the maximum boiling point acid is a solution, and the composition at 760 mm. agrees approximately with a chemical formula only by accident. By passing hydrogen chloride gas into the concentrated aqueous acid at -23°, Pierre and Pouchot obtained a crystalline hydrate, HCl,2H2O (m. pt. -18°), decomposing on warming. Rupert (1907) obtained the hydrate HCl,H2O.
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