General Conclusions

The following conclusions may be drawn from the foregoing data: 1. When the two components are chemically very closely related, the changes of volume and of temperature on mixing the liquids are, as a rule at any rate, in considerable.

2. When not only is the chemical relationship very close, but the critical pressures are equal and

(Van der Waals), the vapour pressure of the mixture is accurately represented by the formula

3. When the components are very closely related, but the critical pressures are not equal, the percentage difference between the observed and calculated pressures - 100(P' - P)/P - is very small, even, in some cases, when there is molecular association in the liquid state, as with methyl and ethyl alcohol.

4. When the two liquids are not closely related, even if there is no molecular association in the case of either of them, the percentage differences are, as a rule, much greater.

5. When the components are not very closely related and the molecules of one or both of them are associated in the liquid state, the values of 100(P' - P)/P are usually very large indeed.

Importance of Chemical Relationship and Molecular

Association

In considering, then, the probable behaviour, as regards vapour pressure, of two liquids, it would appear that chemical relationship is the chief point to be considered, and that, if the two liquids are not closely related, the question whether the molecules of either or both of them are associated in the liquid state is also of great importance.

Alcohols - Water - Paraffins

The influence of chemical relationship is well seen by comparing the properties of mixtures of the monhydric aliphatic alcohols with water on the one hand and with the corresponding paraffins on the other.1

These alcohols may be regarded as being formed from water by the replacement of one atom of hydrogen by an alkyl group, thus (CnH2n+1) - O - H, or as hydroxyl derivatives of the paraffins, thus CnH2n+1(OH).

The alcohols are thus related to, and their properties are intermediate between those of water and of the paraffins ; and it is found that, in the homologous series, as the magnitude of the alkyl group increases, the properties of the alcohols recede from those of water and approach those of the corresponding paraffins. This is well seen in Table 8.

Table 8

Thus, methyl alcohol boils only 35.3° lower than water, but 2294° higher than methane, while cetyl alcohol boils 244° higher than water, but only 56.5° higher than the corresponding paraffin.

Water And Alcohols - Action Of Dehydrating Agents

Again, most dehydrating agents, which react or combine with water, behave in a somewhat similar manner towards the alcohols, though to a smaller degree, and to a diminishing extent as the molecular weight increases, and this fact accounts for the unsatisfactory results obtained with them. Thus, phosphoric anhydride gives phosphoric acid with water and a mixture of ethyl hydrogen phosphates with ethyl alcohol; with barium oxide, water forms barium hydrate, while ethyl alcohol forms, according to Forcrand, a compound 3BaO,4C2H60 ; sodium acts in precisely the same way on the alcohols as on water, but the intensity of the action diminishes rapidly as the complexity of the alkyl group increases; calcium chloride forms a crystalline hexahydrate with water and a crystalline tetra-alcoholate with methyl or ethyl alcohol; anhydrous copper sulphate dissolves rapidly in water, and, on evaporation, crystals of CuS04,5H20 are deposited; in methyl alcohol the sulphate dissolves slowly, but to a considerable extent, giving a blue solution from which, according to Forcrand, greenish-blue crystals of CuS04,Ch40 may be obtained ; anhydrous copper sulphate is, however, quite insoluble in ethyl alcohol, and will extract some water from strong spirit, but it is not a sufficiently powerful dehydrating agent to remove the whole.

1 Young and Fortey, "The Properties of Mixtures of the Lower Alcohols (1) with Water, (2) with Benzene, and with Benzene and Water," Trans. Chem. Soc, 1902, 81, 717 and 739.

Fig. 19. - Vapour pressures of mixtures of alcohols with water.