Vapour densities, abnormal

It will be noticed that we make no assumption as to the molecular weight of the solvent in the liquid state. Equation (8) refers to the vapour only. It is to be expected, therefore, that when the solvent does not yield a vapour having the normal density, the value of the molecular lowering will be abnormal. Raoult found that when acetic acid was used as solvent the observed molecular lowering was 0 0163. Acetic acid, however, is known to be polymerised in the state of vapour at the boiling-point the molecular weight as determined by the vapour density is 1 64 times the normal (C2H4O2 = 60). The number of mols per unit volume will be reduced in the same ratio, and hence we must write (3) ... 

In confirmation it was observed that such substances (e.g., acetic acid) gave abnormally high vapour densities. 

Mitscherlich measured vapour densities at higher temperatures, finding abnormally high values for sulphur, phosphorus, and arsenic, and the density of phosphorus pentachloride only half the normal value (see p. 219). He described the toxicological detection of white phosphorus by distillation in steam and the glow seen in the condenser. His researches on benzene and its derivatives (see p. 331), on etherification and contact action (see p. 262) and affinity (see p. 617) are described elsewhere. 

Soon after Deville s first researches on dissociation, Cannizzaro, Kopp, and Kekule suggested that abnormal vapour densities are due to the dissociation of the substances, the products recombining on cooling. This was not accepted by Deville for the abnormally small densities of sal ammoniac and... 

Associating and Dissociating Substances.—In the case of certain substances it is found that the molar weight, calculated from the vapour density, has a value sometimes greater and sometimes less than that corresponding with the formula which, on other grounds, must be assigned to the substance and it is also found that in those cases, the vapour density is not independent of, but alters with, the temperature. These cases of abnormal vapour densities, as they were termed, are accounted for by the assumption of association or of dissociation of the molecules of the substance in the vapour state and from the values of the density obtained one can... 

The normal substances, however, really exhibit small deviations which are all the greater the more complex is the molecule of the substance. The theory of van der Waals, or in fact any hypothesis from which a theorem of corresponding states could be derived, assumes however that the transition from the gaseous to the liquid state, as well as the changes of density in either state, result from alterations in the propinquity of molecules which otherwise remain unaltered. Any association or dissociation of the substance would therefore give rise to abnormalities, and in fact the substances which deviate most from the normal relations (e.g.l water, acetic acid) are those which appear, on other grounds, to be associated in the liquid state. In the case of acetic acid the commencement of polymerisation, even in the state of vapour, is evident from the abnormal densities. 

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