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Hydrogen chloride is a colourless gas with a most irritating acid smell; it does not support the combustion of a taper. The gas fumes strongly in moist air. It has a higher refractive index than most gases, but those of hydrogen bromide and iodide are still higher: the following values refer to sodium light and S.T.P.
| Oxygen | 1.00028 | | Nitrogen | 1.00030 | | Hydrogen | 1.00014 | | Hydrogen chloride | 1.000438 | | Hydrogen bromide | 1.000573 | | Hydrogen iodide | 1.000911 |
The normal density of the gas is 1.6392 gm. per litre. When very strongly heated the gas is slightly dissociated into its elements: 2HCl <=> H2 + Cl2; the thermal dissociation of hydrogen chloride, in percentages, is given below:
| t°C | 427 | 727 | 1537 | 1727 | | % | 1.1x10-5 | 1.34x10-3 | 0.274 | 0.41 |
The gas is also decomposed to some extent by radium emanation and by ultraviolet light.
Burning sodium introduced into a jar of the gas burns with a bright yellow flame, producing solid sodium chloride and liberating hydrogen:
2Na + 2HCl = 2NaCl + H2.
When hydrochloric acid gas is passed through a U-tube cooled in liquid air, it condenses to a snow-white crystalline solid, which sometimes exhibits pink patches; this melts at -111.4° to a colourless liquid, of density 1.184 at the boiling point, -85.0°. The perfectly dry liquid is without action on zinc, iron, magnesium, quicklime, and some carbonates, all of which are readily dissolved by the aqueous acid, but it readily dissolves aluminium with evolution of hydrogen: 2Al + 6HCl = 2AlCl3 + 3H2. The liquid expands on heating, between -80° and +30°, more rapidly than a gas. The critical temperature of hydrogen chloride is 51.45°; the critical pressure is 81.55 atm.
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