The Indifferent Point.—We have already seen that at 30.2° calcium chloride hexahydrate melts congruently, and that, provided the pressure is maintained constant, addition or withdrawal of heat will cause the complete liquefaction or solidification, without the temperature of the system undergoing change. This behaviour, therefore, is similar to, but is not quite the same as the fusion of a simple substance such as ice; and the difference is due to the fact that in the case of the hexahydrate the emission of vapour by the liquid phase causes an alteration in the composition of the latter, owing to the non-volatility of the calcium chloride; whereas in the case of ice this is, of course, not so.
Consider, however, for the present that the vapour phase is absent, and that we are dealing with the two-phase system solid—solution. Then, since there are two components, the system is bivariant. For any given value of the pressure, therefore, we should expect that the system could exist at different temperatures; which, indeed, is the case. It has, however, already been noted that when the composition of the liquid phase becomes the same as that of the solid, the system then behaves as a univariant system; for, at a given pressure, the system solid—solution can exist only at one temperature, change of temperature producing complete transformation in
one or other direction. The variability of the system has therefore been diminished.
This behaviour will perhaps be more clearly understood when one reflects that since the composition of the two phases is the same, the system may be regarded as being formed of one component, just as the system NH4Cl
A point such as has just been referred to, which represents the special behaviour of a system of two (or more) components, in which the composition of two phases becomes identical, is known as an indifferent point,[[229]] and it has been shown[[230]] that at a given pressure the temperature in the indifferent point is the maximum or minimum temperature possible at the particular pressure[[231]] (cf. critical solution temperature). At such a point a system loses one degree of freedom, or behaves like a system of the next lower order.
The Hydrates of Ferric Chloride.—A better illustration of the formation of compounds possessing a definite melting point, and of the existence of retroflex solubility curves, is afforded by the hydrates of ferric chloride, which not only possess definite points of fusion, but these melting points are stable. A very brief description of the relations met with will suffice.[[232]]
Ferric chloride can form no less than four stable hydrates, viz. Fe2Cl6,12H2O, Fe2Cl6,7H2O, Fe2Cl6,5H2O, and Fe2Cl6,4H2O, and each of these hydrates possesses a definite, stable melting point. On analogy with the behaviour of calcium chloride, therefore, we shall expect that the solubility curves of these different hydrates will exhibit a series of temperature maxima; the points of maximum temperature representing systems in which the composition of the solid and liquid phases is the same. A graphical representation of the solubility relations is given in Fig. 39, and the composition of the different saturated solutions which can be formed is given in the following tables, the composition being expressed in molecules of Fe2Cl6 to 100 molecules of water. The figures printed in thick type refer to transition and melting points.
