Statical and Dynamical Methods of Determination.It has been stated that the boiling point of a pure liquid under a given pressure may be determined by either the statical or the dynamical method, the curve which shows the relation between temperature and pressure representing not only the vapour pressures of the liquid at different temperatures, but also its boiling points under different pressures; this statement applies equally to the boiling points of any given mixture under different pressures.

As regards the vapour pressures of mixtures, it has been shown that there are two simple cases: (a) That of non-miscible liquids, for which the vapour pressure of the two (or more) liquids together is equal to the sum of the vapour pressures of the components at the same temperature;

(b) That of closely related compounds, which are miscible in all proportions, and of a few other pairs of infinitely miscible liquids, for which the formula holds good. It is in these two cases only that the boiling points can be calculated from the vapour pressures of the components.

Non-miscible Liquids

Liebig had observed in 1832 that when ethylene dichloride and water were distilled together, the boiling point was considerably lower than that of either pure component, and Gay Lussac in the same year pointed out that, since the two liquids are non-miscible, the total pressure must be equal to the sum of the vapour pressures of the pure components at the observed boiling point.

Dalton's law of partial pressures is, in fact, applicable to the case of non-miscible liquids, each vapour behaving as an indifferent gas to the others, and the boiling point of each liquid depends on the partial pressure of its own vapour. The temperature is necessarily the same for all the liquids present, and the total pressure, if the distillation is carried out in the ordinary way, is equal to that of the atmosphere. The boiling point is therefore that temperature at which the sum of the vapour pressures of the components is equal to the atmospheric pressure.

Calculation From Vapour Pressures (Chlorobenzene And Water)

In Table 9 the vapour pressures of chlorobenzene and of water,1 two liquids which are practically non-miscible, are given for each degree from 89° to 93°, and also the sum of the vapour pressures at the same temperatures.

Table 9. Vapour Pressures in mm

Thus, when chlorobenzene and water are heated together, say in a barometer tube, the observed vapour pressure at 90° should be 733.8 mm., 761.6 mm. at 91°, and so on. Conversely, when chloro-benzene and water are distilled together in an ordinary distillation bulb, so that the vapours are unmixed with air, the observed boning point, when the atmospheric pressure is 761-6 mm., should be 91°.

Experimental Verification

In an actual experiment 100 c.c. of chlorobenzene and 80 c.c. of water were distilled together when the barometric pressure was 740.2 mm., and it was found that the temperature varied only between 90.25° and 90.35°, until there was scarcely any chlorobenzene visible in the residual liquid, when it rose rapidly to nearly 100°. The theoretical boiling point is calculated as follows :- The increase of pressure for 1° rise of temperature from 90° to 91° is 761.6-733-8 = 27.8 mm. It may be assumed without sensible error that the value of dp/dt is constant over this small range of temperature and the boiling point should therefore be which is very close to that actually observed.

Liquids Miscible Within Limits

Before considering the behaviour of those infinitely miscible liquids for which the relation between vapour pressure and molecular composition is represented by a straight line, it will be well to take the case of partially miscible liquids.

The boiling point of a pair of such substances is higher than that calculated as above for non-miscible liquids, but if the miscibility is slight, the "difference between the observed and calculated temperatures is not serious, and the observed boiling point will be decidedly lower than that of either component.

Aniline And Water

Water dissolves only about 3 per cent of aniline, and aniline about 5 per cent of water at 12°, though the solubility in each case is considerably greater at 100°. Fifty c.c. of aniline and 200 c.c. of water were distilled together under a pressure of 746.4 mm., and the boiling point was found to remain nearly constant at 98.75° for some time but afterwards rose slowly to 99.65° while there was still a moderate amount of water visible with the residual aniline. The distillation was then stopped and a large amount of water was added to the residue ; the temperature, when the distillation was recommenced, rose to 98.9°. It would therefore appear that the composition of the liquid in the still may alter considerably without producing any great change in the boiling point, though the temperature does not remain so constant as it does with chlorobenzene and water.

1 Young, "Distillation," Thorpe's Dictionary of Applied Chemistry.

The vapour pressures of aniline and water at 97° to 99° are as follows: Table 10

Vapour Pressures in mm.

The calculated boiling point under a pressure of 746.4 mm. would therefore be which is 0.85° lower than the temperature, which remained nearly constant for some time, and 1.75° lower than that which was observed when there was still a moderate amount of water present.