From what has been said above, it will be seen that at any temperature below the critical solution temperature, two conjugate solutions containing water and phenol in different concentration can exist together, one containing excess of water, the other excess of phenol. The following table gives the composition of the two layers, and the values are represented graphically in Fig. 22.[[170]]
Phenol and Water.
C1 is the percentage amount of phenol in the first layer.
C2 ,, ,, ,, second layer.
| Temperature. | C1. | C2. |
| 20° | 8.5 | 72.2 |
| 30° | 8.7 | 69.9 |
| 40° | 9.7 | 66.8 |
| 50° | 12.0 | 62.7 |
| 55° | 14.2 | 60.0 |
| 60° | 17.5 | 56.2 |
| 65° | 22.7 | 49.7 |
| 68.4° | 36.1 | 36.1 |
The critical solution temperature for phenol and water is 68.4°, the critical concentration 36.1 per cent. of phenol. At all temperatures above 68.4°, only homogeneous solutions of phenol and water can be obtained; water and phenol are then miscible in all proportions.
At the critical solution point the system exists in only two phases—liquid and vapour. It ought, therefore, to possess two degrees of freedom. The restriction is, however, imposed that the composition of the two liquid phases, coexisting at a point infinitely near to the critical point, becomes the same, and this disposes of one of the degrees of freedom. The system is therefore univariant; and at a given temperature the pressure will have a definite value. Conversely, if the pressure is fixed (as is the case when the system is under the pressure of its own vapour), then the temperature will also be fixed; that is, the critical solution temperature has a definite value depending only on the substances. If the vapour phase is omitted, the temperature will alter with the pressure; in this case, however, as in the case of other condensed systems, the effect of pressure is slight.
From Fig. 22 it is easy to predict the effect of bringing together water and phenol in any given quantities at any temperature. Start with a solution of phenol and water having the composition represented by the point x. If to this solution phenol is added at constant temperature, it will dissolve, and the composition of the solution will gradually change, as shown by the dotted line xy. When, however, the concentration has reached the value represented by the point y, two liquid layers will be formed, the one solution having the composition represented by y, the other that represented by y′. The system is now univariant, and on further addition of phenol, the composition of the two liquid phases will remain unchanged, but their relative amounts will alter. The phase richer in phenol will increase in amount; that richer in water will decrease, and ultimately disappear, and there will remain the solution y′. Continued addition of phenol will then lead to the point x′, there being now only one liquid phase present.
Since the critical solution point represents the highest temperature at which two liquid phases consisting of phenol and
water can exist together, these two substances can be brought together in any amount whatever at temperatures higher than 68.4°, without the formation of two layers. It will therefore be possible to pass from a system represented by x to one represented by x′, without at any time two liquid phases appearing. Starting with x, the temperature is first raised above the critical solution temperature; phenol is then added until the concentration reaches the point x2. On allowing the temperature to fall, the system will then pass into the condition represented by x′.