130 SMITH'S INTERMEDIATE CHEMISTRY 



molecular weight of the substance. As we have seen (p. 74) ; 

 it is made up of 1.008 g. of hydrogen combined with 35.46 g. of 

 chlorine. 



Is the gas heavier or lighter than air? This question is an- 

 swered at once if we recall the fact that the 22.4-liter cube-full of 

 air weighs 28.95 g. (p. 85). The gas is one-fourth heavier. It 

 may therefore be collected by upward displacement (Fig. 26a, p. 52) . 



The gas can be liquefied by pressure alone at any temperature 

 below 52 (its critical temperature). One atmosphere pressure 

 will liquefy it at 84, which is therefore the boiling-point of 

 liquefied hydrogen chloride. 



/ When the concentrated aqueous solution is heated, it is the hydro- 

 gen chloride and not the water which is vaporized, for the most part. 

 When the concentration has been reduced to 20.2 per cent, the 

 rest of the mixture distils unchanged at 110. This occurs because, 

 at this concentration, the hydrogen chloride is carried off in the 

 bubbles of steam in the same proportion in which it is present 

 in the liquid. If a dilute solution is used, water is the chief prod- 

 uct of distillation (about 100), but gradually the boiling-point 

 rises and, when the concentration has reached 20.2 per cent once 

 more, the same hydrochloric acid of constant boiling-point (110 

 at 760 mm.), as it is called, forms the residue. ) 



Chemical Properties of Hydrogen Chloride. In the case 

 of a compound, the chemical property in regard to which we first 

 enquire is its stability (p. 27) . Is it easy or difficult to decompose 

 by heating? Hydrogen chloride must be heated above 1500 

 before even a trace of it is dissociated into hydrogen and chlorine. 

 Pure hydrogen chloride is therefore a yg^jgjg&fe and, from a 

 chemical point of view, rather an inactive substance. It has no 

 action on non-metals, such as phosphorus, carbon, sulphur, etc. 

 However, many of the more active metals (see p. 54), such as 

 potassium, sodium, and magnesium, decompose it. Hydrogen 

 is set free, and the chloride of the metal is formed. 

 2K + 2HC1 -> 2KC1 + H, | 



