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Solubility of Solids in Water
Solubility of Solids in Water is normal for many inorganic salts. This is the case for all nitrates, and most chlorides and sulphides, notable exceptions being the chlorides of silver, lead, and monovalent mercury, and the sulphates of lead, calcium, strontium, and barium. Even these substances are slightly soluble in water; indeed, it is doubtful if any substances are absolutely insoluble, so that the terms soluble and insoluble must be regarded as relative. Basic salts are generally insoluble; acid salts, on the other hand, are usually soluble. The solubility of a salt in water is influenced by several factors such as temperature, pressure, and the dimensions of the particles constituting the solid phase.
The Influence of Temperature
Continuous curvesThese may be roughly classified into the following types:
The curve exhibits sharp breaksTwo possible causes, namely, a change of polymorphic form or a change of hydration, will give rise to a sudden break in the curve. The former case is illustrated by ammonium nitrate, which is capable of existing in no fewer than four crystalline forms. Of these the β-rhombic passes into the α-rhombic variety at about 32° C. At this temperature a break occurs in the solubility curve.
If, starting at the point B, heat be added to the system, ice will melt, and more of the dodecahydrate will dissolve in accordance with the equilibrium curve BCH, which is the solubility curve of this hydrate in water. At 37° C. the dodecahydrate melts, and if anhydrous ferric chloride be added to the system, the temperature at which the dodecahydrate remains in equilibrium with the solution is lowered until the eutectie point C is reached at 27.4° C. At this point the whole solidifies to a solid mixture of the dodecahydrate and heptahydrate. The curve has been followed in the direction of the broken line CH to + 8° C., the solution being supersaturated with respect to the dodecahydrate. Similarly, the curve ED has been continued backwards until it intersects CH at H at 15° C. This is a metastable triple point or eutectic, and is capable of realisation experimentally on account of the fact that the heptahydrate is not so readily formed. Curves EF and FG represent the solubilities of the tetrahydrate and the anhydrous salt respectively. The Influence of Pressure on Solubility of Solids in Water
Sorby concluded that a rise of pressure increases the solubility of those substances which dissolve in a liquid with contraction of volume, but that it decreases the solubility of such substances as dissolve in water with an increase in volume. It was first indicated by Braun that if the change of volume on solution and the thermal effect are known, the quantitative effect of alteration in pressure on the solubility may be calculated. This is in harmony with the Theorem of Le Chatelier. The following data are in harmony with this:
The Effect of Pressure on solubility
Further data for sodium chloride have been published which are in close harmony with those given above, but refer to 25° C.
Physical Condition of the Solid Phase
As long ago as 1870 Stas observed that the solubility of silver chloride varies with its method of preparation, the following results being obtained:
Clearly the smaller the particles of the salt the greater the solubility. This is further supported by Hulett, who investigated the solubilities of calcium and barium sulphates at 25° C. and found them to be as in table. Clearly, therefore, before the absolute solubility of a salt in water at any stated temperature, and under, say, atmospheric pressure, can be given, the size of the particles of the solid phase must be known. This has an intimate connection with the phenomenon of supersaturation, for it is clear that a saturated solution of barium sulphate prepared in contact with particles of diameter 0.1 μ is supersaturated with respect to particles of diameter 1.8 and, upon introduction of such particles, the excess would be precipitated out. Influence of size of particle upon solubility
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