Miscibility

The gradual change in properties of the alcohols is also well shown by the fact that the lower alcohols are miscible with water in all proportions, the intermediate ones within limits, while the higher alcohols are practically insoluble in water.

Volume And Heat Changes

Lastly, the contraction and heat evolution that take place on mixing the alcohols with water diminish with rise of molecular weight; there is, in fact, increasing heat absorption in the case of the higher alcohols.

Vapour Pressures

The lowest member of the series, methyl alcohol, bears the closest resemblance to water, and, as Konowaloff has shown, the vapour pressures of mixtures of these substances are in all cases intermediate between those of the pure components, and the curve representing the relation between vapour pressure and molecular composition does not deviate very greatly from a straight line, as will be seen from Fig. 19, in which the vapour pressures of mixtures of four alcohols with water are given, at the boiling points of the alcohols under a pressure of 400 mm.

With ethyl alcohol and water the deviation is considerable, and more recent and accurate observations have shown that a particular mixture exerts a maximum vapour pressure, but the experiments of Konowaloff are not sufficiently numerous to bring out this point.

When, however, we come to mixtures of n-propyl alcohol and water, we find that there is a very well defined maximum pressure, although the liquids are miscible in all proportions, and the curve shows considerable resemblance to that representing the behaviour of the partially miscible liquids, isobutyl alcohol and water.

Mixtures Of Maximum Vapour Pressure

The question whether a mixture of maximum vapour pressure will be formed in any given case depends partly on the deviation of the vapour pressure-molecular composition curve from straightness, partly on the difference between the vapour pressures of the two components. Thus, in the case of ethyl alcohol and water, the difference between the vapour pressures at 63° is 229 mm., and there is a maximum vapour pressure, very slightly higher than that of pure ethyl alcohol, for a mixture containing 894 molecules per cent of alcohol (b, Fig. 20). If the difference between the vapour pressures of the pure substances at the same temperature were 350 mm., and the deviation of the curve from straightness were the same, there would be no maximum vapour pressure (a, Fig. 20) ; while, if the vapour pressures of alcohol and water were equal and the deviation the same, the maximum vapour pressure would be far more obvious, and the molecular percentage of alcohol in the mixture that exerted it would be something like 20, instead of 89.4 (c, Fig.. 20).

The gradual divergence of the properties of the alcohols from those of water, as the molecular weight increases, is indicated by the increasing curvature of the pressure-molecular composition lines

Fig. 20.

(Fig. 19). The maximum differences between the pressures represented by the actual curves and the theoretical straight lines are roughly as follows: -

Mixtures Of N-Hexane With Alcohols

We cannot well study the miscibility of the alcohols with the corresponding paraffins, because the first four of these hydrocarbons are gaseous at the ordinary temperature, and most of them are very difficult to prepare in a pure state. Normal hexane, however, may be conveniently prepared by the action of sodium on propyl iodide, and is easily purified. This paraffin is only partially miscible with methyl alcohol at the ordinary temperature, but mixes in all proportions with those of the higher alcohols which have been investigated - ethyl, propyl, isobutyl and isoamyl alcohols. Again, normal hexane forms mixtures of maximum vapour pressure with the lower alcohols, but not with isoamyl alcohol or any other of higher boiling point. Lastly, the fall of temperature, on mixing hexane with the alcohols in equimolecular proportions, diminishes slightly as the molecular weights of the alcohol rise. [Ethyl alcohol -2.55°, n-Propyl alcohol -2.40°, Isobutyl alcohol -2.35°, Isoamyl alcohol -1.85°.] In all these respects, therefore, it may be said that the properties of the alcohols approach those of normal hexane, but recede from those of water, as their molecular weights increase.

Mixtures Of Benzene With Alcohols

Benzene is much more easily obtained in quantity than hexane, and behaves in a somewhat similar manner. The lower alcohols are miscible with benzene in all proportions; but while methyl, ethyl, isopropyl, n-propyl, tertiary butyl and isobutyl alcohols form mixtures of maximum vapour pressure with that hydrocarbon, isoamyl alcohol does not, and it is practically certain that no alcohol of higher boiling point would form such a mixture.

It will be seen from Table 7 (p. 34) that while for mixtures of the alcohols with water, the values of 100(P' - P)/P show a steady rise as the molecular weights of the alcohols increase, the corresponding values for mixtures with benzene fall regularly. It may also be mentioned that the solubility of the alcohols in benzene, relatively to that in water, becomes greater as the molecular weights of the alcohols increase, for while methyl alcohol can be separated from its benzene solution with the greatest ease, and ethyl alcohol without difficulty by treatment with water, the extraction of propyl alcohol is more troublesome, and that of isobutyl or amyl alcohol by this process is exceedingly tedious. On the other hand, the variations in the temperature and volume changes which occur on admixture are, generally, in the same direction for benzene as for water, though they are much smaller. It should, however, be noted that the relationship of the alcohols to the aromatic hydrocarbon, benzene, is by no means so close as to a paraffin, such as normal hexane; indeed, there is expansion and absorption of heat when benzene and hexane are mixed together and the two substances, in all probability, form a mixture of maximum vapour pressure.

A full account and discussion of all possible types of vapour pressure curves of binary mixtures has been given by A. Marshall.1

1 "The Vapour Pressures of Binary Mixtures," Trans. Chem. Soc, 1906, 89, 1350.