Influence Of Chemical Relationship

For the first three pairs of substances and the last in Table 6, the changes of volume and of temperature are exceedingly small, but the expansion and fall of temperature are much more noticeable with benzene and toluene, and it may be remarked that the chemical relationship is not quite so close in this case, the benzene molecule being wholly aromatic, the toluene partly aromatic and partly aliphatic.

This pair of liquids has been very carefully studied by Rosanoff,

1 Young and Fortey, " The Vapour Pressures and Boiling Points of Mixed Liquids, Part II.," Trans. Chem. Soc., 1903, 83, 45.

Bacon and Schulze.1 Their results confirm those given in Table 6 and are fully considered in Chapter VI (Composition Of Liquid And Vapour Phases. Theoretical).

With regard to the mean differences P' - P, it may be said that they are very small, the corresponding temperature differences being only 0.17°, 0.18°, 0.20°, 0.12° and 0.02° respectively, and the values of 100(P'-P)/P 0.58, 0.57, 0.60, 0.35 and 0.08 [compare Table 7, p. 34]. The conclusion may be drawn from the results that for mixtures of closely related compounds the relation between vapour pressure and molecular composition is represented by a line which is very nearly, if not quite, straight.

Errors Of Experiment

The greater the difference between the boiling points of the two liquids, the more difficult is it to determine the boiling point of a mixture with accuracy, and the greater also to some extent is the probability of error in the value of P. This may partly account for the irregular results with n-hexane and n-octane.

Comparison of Heat Change, Volume Change, and Vapour Pressure.

- It does not appear that change of volume, or temperature, when the liquids are mixed can be relied on as a safe guide to the behaviour of a mixture as regards its vapour pressure, for one would expect that, as a general rule, the actual pressure should be lower than the calculated when there is contraction and rise of temperature, as, in fact, is observed with the second and third pairs of liquids ; and that, when there is expansion and fall of temperature, the observed pressure should be higher than the calculated, as is the case with the first pair. With benzene and toluene, however, the rule is not followed, for the actual pressure is the lower, although there is expansion and fall of temperature on mixing.

T. W. Price2 has recently found that the vapour pressures of mixtures of acetone and methyl-ethyl ketone agree fairly well with those calculated from the formula P = mPa+(100-m)Pb. These two liquids show no change in volume on admixture ; their critical pressures differ by at least 4.7 atm.

Components Not Closely Related

When we come to consider the behaviour of liquids which are not so closely related,' we find in many cases much greater changes of volume and temperature, and also much larger values for 100(P' - P)/P, as will be seen from Table 7 (p. 34).

It will be seen that contraction is frequently accompanied by absorption of heat, and that P' - P may have a positive value when there is contraction, and even when there is both contraction and evolution of heat on mixing the components. In the case of mixtures of the alcohols with benzene, it is remarkable that, although the value of (P' - P)/P diminish as the molecular weights of the alcohols increase, yet the fall in temperature, from methyl to isobutyl alcohol, becomes

1 " A Method of finding the Partial from the Total Vapour Pressures of Binary Mixtures, and a Theory of Fractional Distillation," J. Amer. Chem. Soc, 1914, 36, 1993.

2 " Vapour Pressures and Densities of Mixtures of Acetone and Methyl-ethyl Ketone," Trans. Chem. Soc, 1919, 115, 1116.

D greater, and the very small volume-changes pass from negative to positive.

Table 7

The pressures are from observations by Zawidski, Konowaloff, Lehfeldt, Kohnstamm, Jackson and Young, Fortey and Young, or have been specially determined.