Part 12 (2/2)

8.28 25

82.0

7.52 28.5

90.5

6.81 29.5

95.5

6.46 30.2

102.7

6.00 29.6

109.0

5.70 29.2

112.8

5.41 -----------------------------------------

So far as the first portion of the curve is concerned, it resembles the most general type of solubility curve. In the present case the solubility is so great and increases so rapidly with rise of temperature, that a point is reached at which the water of crystallization of the salt is sufficient for its complete solution. This temperature is 30.2; and since the composition of the solution is the same as that of the solid salt, viz. 1 mol. of CaCl_{2} to 6 mols. of water, this temperature must be the melting point of the hexahydrate. At this point the hydrate will fuse or the solution will solidify without change of temperature and without change of composition. Such a melting point is called a _congruent_ melting point.

But the solubility curve of calcium chloride hexahydrate differs markedly from the other solubility curves. .h.i.therto considered in that it possesses a _retroflex portion_, represented in the figure by BC. As is evident from the figure, therefore, calcium chloride hexahydrate exhibits the peculiar and, as it was at first thought, impossible behaviour that it can be in equilibrium at one and the same temperature with two different solutions, one of which contains more, the other less, water than the solid hydrate; for it must be remembered that {147} throughout the whole course of the curve ABC the solid phase present in equilibrium with the solution is the hexahydrate.

Such a behaviour, however, on the part of calcium chloride hexahydrate will appear less strange if one reflects that the melting point of the hydrate will, like the melting point of other substances, be lowered by the addition of a second substance. If, therefore, water is added to the hydrate at its melting point, the temperature at which the solid hydrate will be in equilibrium with the liquid phase (solution) will be lowered; or if, on the other hand, anhydrous calcium chloride is added to the hydrate at its melting point (or what is the same thing, if water is removed from the solution), the temperature at which the hydrate will be in equilibrium with the liquid will also be lowered; _i.e._ the hydrate will melt at a lower temperature. In the former case we have the hydrate in equilibrium with a solution containing more water, in the latter case with a solution containing less water than is contained in the hydrate itself.

It has already been stated (p. 109) that the solubility curve (in general, the equilibrium curve) is continuous so long as the solid phase remains unchanged; and we shall therefore expect that the curve ABC will be continuous. Formerly, however, it was considered by some that the curve was not continuous, but that the melting point is the point of intersection of two curves, a solubility curve and a fusion curve. Although the earlier solubility determinations were insufficient to decide this point conclusively, more recent investigation has proved beyond doubt that the curve is continuous and exhibits no break.[226]

{148}

Although in taking up the discussion of the equilibria between calcium chloride and water, it was desired especially to call attention to the form of the solubility curve in the case of salt hydrates possessing a definite melting point, nevertheless, for the sake of completeness, brief mention may be made of the other systems which these two components can form.

[Ill.u.s.tration: FIG. 38.]

Besides the hexahydrate, the solubility curve of which has already been described, calcium chloride can also crystallize in two different forms, each of which contains four molecules {149} of water of crystallization; these are distinguished as [alpha]-tetrahydrate, and [beta]-tetrahydrate.

Two other hydrates are also known, viz. a dihydrate and a monohydrate. The solubility curves of these different hydrates are given in Fig. 38.

On following the solubility curve of the hexahydrate from the ordinary temperature upwards, it is seen that at a temperature of 29.8 represented by the point H, it cuts the solubility curve of the [alpha]-tetrahydrate.

This point is therefore a quadruple point at which the four phases hexahydrate, [alpha]-tetrahydrate, solution, and vapour can coexist. It is also the transition point for these two hydrates. Since, at temperatures above 29.8, the [alpha]-tetrahydrate is the stable form, it is evident from the data given before (p. 146), as also from Fig. 38, that the portion of the solubility curve of the hexahydrate lying above this temperature represents _metastable_ equilibria. The realization of the metastable melting point of the hexahydrate is, therefore, due to suspended transformation. At the transition point, 29.8, the solubility of the hexahydrate and [alpha]-tetrahydrate is 100.6 parts of CaCl_{2} in 100 parts of water.

The retroflex portion of the solubility curve of the hexahydrate extends to only 1 below the melting point of the hydrate. At 29.2 crystals of a new hydrate, [beta]-tetrahydrate, separate out, and the solution, which now contains 112.8 parts of CaCl_{2} to 100 parts of water, is saturated with respect to the two hydrates. Throughout its whole extent the solubility curve EDF of the [beta]-tetrahydrate represents metastable equilibria. The upper limit of the solubility curve of [beta]-tetrahydrate is reached at 38.4 (F), the point of intersection with the curve for the dihydrate.

Above 29.8 the stable hydrate is the [alpha]-tetrahydrate; and its solubility curve extends to 45.3 (K), at which temperature it cuts the solubility curve of the dihydrate. The curve of the latter hydrate extends to 175.5 (L), and is then succeeded by the curve for the monohydrate. The solubility curve of the anhydrous salt does not begin until a temperature of about 260. The whole diagram, therefore, shows a succession of stable hydrates, a metastable hydrate, a metastable melting point and retroflex solubility curve. {150}

Pressure-Temperature Diagram.--The complete study of the equilibria between the two components calcium chloride and water would require the discussion of the vapour pressure of the different systems, and its variation with the temperature. For our present purpose, however, such a discussion would not be of great value, and will therefore be omitted here; in general, the same relations.h.i.+ps would be found as in the case of sodium sulphate (p. 138), except that the rounded portion of the solubility curve of the hexahydrate would be represented by a similar rounded portion in the pressure curve.[227] As in the case of sodium sulphate, the transition points of the different hydrates would be indicated by breaks in the curve of pressures.

Finally, mention may again be made of the difference of the pressure of dissociation of the hexahydrate according as it becomes dehydrated to the [alpha]- or the [beta]-tetrahydrate (p. 88).

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 {151} 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 NH_{4}Cl <--> NH_{3} + HCl was regarded as being composed of one component when the vapour had the same total composition as the solid (p. 13). One component in two phases, however, const.i.tutes a univariant system, and we can therefore see that calcium chloride hexahydrate in contact with solution of the same composition will const.i.tute a univariant system. The temperature of equilibrium will, however, vary with the pressure;[228] if the latter is constant, the temperature will also be constant.

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 ill.u.s.tration 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]

{152}

Ferric chloride can form no less than four stable hydrates, viz.

Fe_{2}Cl_{6},12H_{2}O, Fe_{2}Cl_{6},7H_{2}O, Fe_{2}Cl_{6},5H_{2}O, and Fe_{2}Cl_{6},4H_{2}O, and each of these hydrates possesses a definite, stable melting point. On a.n.a.logy 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 Fe_{2}Cl_{6} to 100 molecules of water. The figures printed in thick type refer to transition and melting points.

[Ill.u.s.tration: FIG. 39.]

{153}

COMPOSITION OF THE SATURATED SOLUTIONS OF FERRIC CHLORIDE AND ITS HYDRATES.

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