SOLUTIONS OF LIQUIDS IN LIQUIDS
We have already seen (p. [95]) that when two liquids are brought together, they may mix in all proportions and form one homogeneous liquid phase; or, only partial miscibility may occur, and two phases be formed consisting of two mutually saturated solutions. In the latter case, the concentration of the components in either phase and also the vapour pressure of the system had, at a given temperature, perfectly definite values. In the case of three liquid components, a similar behaviour may be found, although complete miscibility of three components with the formation of only one liquid phase is of much rarer occurrence than in the case of two components. When only partial miscibility occurs, various cases are met with according as the three components form one, two, or three pairs of partially miscible liquids. Further, when two of the components are only partially miscible, the addition of the third may cause either an increase or a diminution in the mutual solubility of these. An increase in the mutual solubility is generally found when the third component dissolves readily in each of the other two; but when the third component dissolves only sparingly in the other two, its addition diminishes the mutual solubility of the latter.
We shall consider here only a few examples illustrating the three chief cases which can occur, viz. (1) A and B, and also B and C are miscible in all proportions, while A and C are only partially miscible. (2) A and B are miscible in all proportions, but A and C and B and C are only partially miscible. (3) A and B, B and C, and A and C are only partially miscible. A, B, and C here represent the three components.
1.—The three components form only one pair of partially miscible liquids.
An example of this is found in the three substances: chloroform, water, and acetic acid.[[320]] Chloroform and acetic acid, and water and acetic acid, are miscible with one another in all proportions, but chloroform and water are only partially miscible with one another. If, therefore, chloroform is shaken with a larger quantity of water than it can dissolve, two layers will be formed consisting one of a saturated solution of water in chloroform, the other of a saturated solution of chloroform in water. The composition of these two solutions at a temperature of about 18°, will be represented by the points a and b in Fig. 84; a representing a solution of the composition: chloroform, 99 per cent.; water, 1 per cent.; and b a solution of the composition: chloroform, 0.8 per cent.; water, 99.2 per cent. When acetic acid is added, it distributes itself between the two liquid layers, and two conjugate ternary solutions, consisting of chloroform, water, and acetic acid are thereby produced which are in equilibrium with one another, and the composition of which will be represented by two points inside the triangle. In this way a series of pairs of ternary solutions will be obtained by the addition of acetic acid to the mixture of chloroform and water. By this addition, also, not only do the two liquid phases become increasingly rich in acetic acid, but the mutual solubility of the chloroform and water increases; so that the layer a becomes relatively richer in water, and layer b relatively richer in chloroform. This is seen from the following table, which gives the percentage composition of different conjugate ternary solutions at 18°.
| Heavier layer. | Lighter layer. | ||||
| Chloroform. | Water. | Acetic acid. | Chloroform. | Water. | Acetic acid. |
| 99.01 | 0.99 | 0 | 0.84 | 99.16 | 0 |
| 91.85 | 1.38 | 6.77 | 1.21 | 73.69 | 25.10 |
| 80.00 | 2.28 | 17.72 | 7.30 | 48.58 | 44.12 |
| 70.13 | 4.12 | 25.75 | 15.11 | 34.71 | 50.18 |
| 67.15 | 5.20 | 27.65 | 18.33 | 31.11 | 50.56 |
| 59.99 | 7.93 | 32.08 | 25.20 | 25.39 | 49.41 |
| 55.81 | 9.58 | 34.61 | 28.85 | 23.28 | 47.87 |
By the continued addition of acetic acid, the composition of the successive conjugate solutions in equilibrium with one another becomes, as the table shows, more nearly the same, and a point is at length reached at which the two solutions become identical. This will therefore be a critical point (p. [98]). Increased addition of acetic acid beyond this point will lead to a single homogeneous solution.
These relationships are represented graphically by the curve aKb, Fig. 84. The points on the branch aK represent the composition of the solutions relatively rich in chloroform (heavier layer), those on the curve bK the composition of solutions relatively rich in water (lighter layer); and the points on these two branches representing conjugate solutions are joined together by "tie-lines." Thus, the points a′b′ represent conjugate solutions, and the line a′b′ is a tie-line.