C_{1} is the percentage amount of phenol in the first layer. C_{2} " " " second layer. -------------+--------+-------- Temperature. | C_{1}.| C_{2}. -------------+--------+-------- 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 -------------+--------+-------- {99} 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.