Part 8 (2/2)
--------------+-----------------+----------------- Temperature.
C_{1} per cent.
C_{2} per cent.
--------------+-----------------+----------------- -10
34.5
89.7 +10
26.1
90.0 30
21.9
89.9 50
17.5
89.0 70
16.2
85.7 90
16.1
84.8 110
17.7
80.0 130
21.8
71.9 140
26.0
64.0 151.8
44.2
44.2 --------------+-----------------+-----------------
{101}
These numbers and Fig. 23 show clearly the occurrence of a minimum in the solubility of the ketone in water, and also a minimum (at about 10) in the solubility of water in methylethylketone. Minima of solubility have also been found in other cases.
[Ill.u.s.tration: FIG. 24.]
Triethylamine and Water.--Although in most of the cases studied the solubility of one liquid in another increases with rise of temperature, this is not so in all cases. Thus, at temperatures below 18, triethylamine and water mix together in all proportions; but, on raising the temperature, the h.o.m.ogeneous solution becomes turbid and separates into two layers. In this case, therefore, the critical solution temperature is found in the direction of lower temperature, not in the direction of higher.[172] This behaviour is clearly shown by the graphic representation in Fig. 24, and also by the numbers in the following table:--
TRIETHYLAMINE AND WATER.
-------------+-----------------+---------------- Temperature.
C_{1} per cent.
C_{2} per cent.
-------------+-----------------+---------------- 70
1.6
-- 50
2.9
-- 30
5.6
96 25
7.3
95.5 20
15.5
73 18.5
30
30 -------------+-----------------+----------------
General Form of Concentration-Temperature Curve.--From the preceding figures it will be seen that the general {102} form of the solubility curve is somewhat parabolic in shape; in the case of triethylamine and water, the closed end of the curve is very flat. Since for all liquids there is a point (critical point) at which the liquid and gaseous states become identical, and since all gases are miscible in all proportions, it follows that there must be some temperature at which the liquids become perfectly miscible. In the case of triethylamine and water, which has just been considered, there must therefore be an upper critical solution temperature, so that the complete solubility relations would be represented by a closed curve of an ellipsoidal aspect. An example of such a curve is furnished by nicotine and water. At temperatures below 60 and above 210, nicotine and water mix in all proportions.[173] Although it is possible that this is the general form of the curve for all pairs of liquids, there are as yet insufficient data to prove it.
With regard to the closed end of the curve it may be said that it is continuous; the critical solution point is not the intersection of two curves, for such a break in the continuity of the curve could occur only if there were some discontinuity in one of the phases. No such discontinuity exists. The curve is, therefore, not to be considered as two solubility curves cutting at a point; it is a curve of equilibrium between two components, and so long as the phases undergo continuous change, the curve representing the equilibrium must also be continuous. As has already been emphasized, a distinction between solvent and solute is merely conventional (p. 93).
Pressure-Concentration Diagram.--In considering the pressure-concentration diagram of a system of two liquid components, a distinction must be drawn between the total pressure of the system and the partial pressures of the components. On studying the total pressure of a system, it is found that two cases can be obtained.[174]
So long as there is only one liquid phase, the system is bivariant. The pressure therefore can change with the concentration and the temperature.
If the temperature is maintained {103} constant, the pressure will vary only with the concentration, and this variation can therefore be represented by a curve. If, however, two liquid phases are formed, the system becomes univariant: and if one of the variables, say the temperature, is arbitrarily fixed, the system no longer possesses any degree of freedom. _When two liquid phases are formed, therefore, the concentrations and the vapour pressure have definite values, which are maintained so long as the two liquid phases are present_; the temperature being supposed constant.
In Fig. 25 is given a diagrammatic representation of the two kinds of pressure-concentration curves which have so far been obtained. In the one case, the vapour pressure of the invariant system (at constant temperature) lies higher than the vapour pressure of either of the pure components; a phenomenon which is very generally found in the case of partially miscible liquids, _e.g._ ether and water.[175] Accordingly, by the addition of water to ether, or of ether to water, there is an increase in the _total_ vapour pressure of the system.
[Ill.u.s.tration: FIG. 25.]
With regard to the second type, the vapour pressure of the systems with two liquid phases lies between that of the two single components. An example of this is found in sulphur dioxide and water.[176] On adding sulphur dioxide to water there is an increase of the total vapour pressure; but on adding water to liquid sulphur dioxide, the total vapour pressure is diminished.
The case that the vapour pressure of the system with two {104} liquid phases is _less_ than that of each of the components is not possible.
<script>