Part 9 (2/2)
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In the above table the positive sign indicates evolution of heat, the negative sign, absorption of heat; and the values of the heat effect are expressed in centuple calories. Judging from the heat effect produced on dissolving cupric chloride in a large bulk of water, we should predict that the solubility of that salt would diminish with rise of temperature; as a matter of fact, it increases. This is in accordance with the fact that {111} the last heat of solution is _negative_ (as expressed above), _i.e._ solution of the salt in the almost saturated solution is accompanied by absorption of heat. We are led to expect this from the fact that the heat of solution changes sign from positive to negative as the concentration increases; experiment also showed it to be the case.
Despite its many forms, it should be particularly noted that the solubility curve of any substance is _continuous_, so long as the solid phase, or solid substance in contact with the solution, remains unchanged. If any ”break” or discontinuous change in the direction of the curve occurs, it is a sign that the _solid phase has undergone alteration_. Conversely, if it is known that a change takes place in the solid phase, a break in the solubility curve can be predicted. We shall presently meet with examples of this.[186]
A.--ANHYDROUS SALT AND WATER.
The Solubility Curve.--In studying the equilibria in those systems of two components in which the liquid phase is a solution or phase of varying composition, we shall in the present chapter limit the discussion to those cases where no compounds are formed, but where the components crystallise out in the pure state. Since some of the best-known examples of such systems are yielded by the solutions of anhydrous salts in water, we shall first of all briefly consider some of the results which have been obtained with them.
For the most part the solubility curves have been studied only at temperatures lying between 0 and 100, the solid phase in contact with the solution being the anhydrous salt. For the representation of these equilibria, the concentration-temperature {112} diagram is employed, the concentration being expressed as the number of grams of the salt dissolved in 100 grams of water, or as the number of gram-molecules of salt in 100 gram-molecules of water. The curves thus obtained exhibit the different forms to which reference has already been made. So long as the salt remains unchanged the curve will be continuous, but if the salt alters its form, then the solubility curve will show a break.
[Ill.u.s.tration: FIG. 27.]
Now, we have already seen in Chapter III. that certain substances are capable of existing in various crystalline forms, and these forms are so related to one another that at a given temperature the relative stability of each pair of polymorphic forms undergoes change. Since each crystalline variety of a substance must have its own solubility, there must be a break in the solubility curve at the temperature of transition of the two enantiotropic forms. At this point the two solubility curves must cut, for since the two forms are in equilibrium with respect to their vapour, they must also be in equilibrium with respect to their solutions. From the table on p. 63 it is seen that pota.s.sium nitrate, ammonium nitrate, silver nitrate, thallium nitrate, thallium picrate, are capable of existing in two or more different enantiotropic crystalline forms, the range of stability of these forms being limited by definite temperatures (transition temperature). Since the transition point is not altered by a solvent (provided the latter is not absorbed by the solid phase), we should find on studying the solubility of these substances in water that the solubility curve would exhibit a change in direction at the temperature of transition.
As a matter of fact this has been verified, more especially in the case of ammonium nitrate[187] {113} and thallium picrate.[188] The following table contains the values of the solubility of ammonium nitrate obtained by Muller and Kaufmann, the solubility being expressed in gram-molecules NH_{4}NO_{3} in 100 gram-molecules of water. In Fig. 27 these results are represented graphically. The equilibrium point was approached both from the side of unsaturation and of supersaturation, and the condition of equilibrium was controlled by determinations of the density of the solution.
SOLUBILITY OF AMMONIUM NITRATE.
------------------------------------------------------------ Temperature.
Solubility.
Temperature.
Solubility.
--------------+-------------+--------------+---------------- 12.2
34.50
32.7
57.90 20.2
43.30
34.0
58.89 25.05
48.19
35.0
59.80 28.0
51.86
36.0
61.00 30.0
54.40
37.5
62.90 30.2
54.61
38.0
63.60 31.9
57.20
39.0
65.09 32.1
57.60
40.0
66.80 ------------------------------------------------------------
From the graphic representation of the solubility given in Fig. 27, there is seen to be a distinct change in the direction of the curve at a temperature of 32; and this break in the curve corresponds to the transition of the [beta]-rhombic into the [alpha]-rhombic form of ammonium nitrate (p. 63).
Suspended Transformation and Supersaturation.--As has already been learned, the transformation of the one crystalline form into the other does not necessarily take place immediately the transition point has been pa.s.sed; and it has therefore been found possible in a number of cases to follow the solubility curve of a given crystalline form beyond the point at which it ceases to be the most stable modification. Now, it will be readily seen from Fig. 27 that if the two solubility curves be prolonged beyond the point of intersection, the solubility of the less stable form is greater than that of the more stable. A solution, therefore, which is saturated with respect to the less stable form, _i.e._ which is in equilibrium with that form, is _supersaturated with respect to the more stable modification_. If, {114} therefore, a small quant.i.ty of the more stable form is introduced into the solution, the latter must deposit such an amount of the more stable form that the concentration of the solution corresponds to the solubility of the stable form at the particular temperature. Since, however, the solution is now _unsaturated_ with respect to the less stable variety, the latter, if present, must pa.s.s into solution; and the two processes, deposition of the stable and solution of the metastable form, must go on until the latter form has entirely disappeared and a saturated solution of the stable form is obtained. There will thus be a conversion, through the medium of the solvent, of the less stable into the more stable modification. This behaviour is of practical importance in the determination of transition points (_v._ Appendix).
From the above discussion it will be seen how important is the statement of the solid phase for the definition of saturation and supersaturation.[189]
Solubility Curve at Higher Temperatures.--On pa.s.sing to the consideration of the solubility curves at higher temperatures, two chief cases must be distinguished.
(1) The two components in the fused state can mix in all proportions.
(2) The two components in the fused state cannot mix in all proportions.
1. _Complete Miscibility of the Fused Components._
[Ill.u.s.tration: FIG. 28.]
The best example of this which has been studied, so far as anhydrous salts and water are concerned, is that of silver nitrate and water. The solubility of this salt at temperatures {115} above 100 has been studied chiefly by Etard[190] and by Tilden and Shenstone.[191] The values obtained by Etard are given in the following table, and represented graphically in Fig. 28.
SOLUBILITY OF SILVER NITRATE.
--------------------------------------------------- Temperature.
Parts of dry salt in 100 parts
of solution.
--------------------+------------------------------ -7
46.2 -1
52.1 +5
56.3 10
61.2 20
67.8 40.5
76.8 73
84.0 135
92.8 182
96.9 ---------------------------------------------------
In this figure the composition of the solution is expressed in parts of silver nitrate in 100 parts by weight of the solution, so that 100 per cent. represents pure silver nitrate. As can be seen, the solubility increases with the temperature. At a temperature of about 160 there should be a break in the curve due to change of crystalline form (p. 63). Such a change in the direction of the solubility curve, however, does not in any way alter the essential nature of the relations.h.i.+ps discussed here, and may for the present be left out of account. On following the solubility curve of silver nitrate to higher temperatures, therefore, the concentration of silver nitrate in the solution gradually increases, until at last, at a temperature of 208,[192] the melting point of pure silver nitrate is reached, and the concentration of the water has become zero. The curve throughout its whole extent represents the equilibrium between silver nitrate, solution, and vapour. Conversely, starting with pure silver nitrate in contact with the fused salt, addition of water will lower the melting point, _i.e._ will lower the temperature at which the solid salt can exist in contact with the liquid; {116} and the depression will be all the greater the larger the amount of water added. As the concentration of the water in the liquid phase is increased, therefore, the system will pa.s.s back along the curve from higher to lower temperatures, and from greater to smaller concentrations of silver nitrate in the liquid phase. The curve in Fig. 28 may, therefore, be regarded either as the solubility curve of silver nitrate in water, or as the freezing point curve for silver nitrate in contact with a solution consisting of that salt and water.
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