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.

Solubility of salts in water

This is well illustrated by the various solubility curves shown in Figs

Continuous curves

These may be roughly classified into the following types:

1. The solubility remains fairly constant at all temperatures - sodium chloride.
2. The solubility rises steadily with the temperature – potassium bromide.
3. The solubility rises rapidly with the temperature – potassium nitrate.
4. The solubility falls steadily - calcium chromate.
5. The solubility rises to a maximum and then falls – calcium sulphate.
6. The solubility decreases to a minimum and then rises, as exemplified by calcium acetate and propionate1 and by anhydrous sodium sulphate, the minimum in the latter case occurring at about 120° C.

The curve exhibits sharp breaks

Two 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.

Solubility of sodium sulphate.

The effect of change of degree of hydration in the case of those substances that can combine with water is shown in the solubility curve of sodium sulphate (fig.). Below 32.8° C. the stable form of this salt crystallises with ten molecules of water, but above this temperature the anhydrous salt is stable. This transition - point is sharply marked by a break in the curve. So also the points of intersection in the solubility curve of ferric chloride (fig. 50) in water.

Solubility curves of ferric chloride

A study of the curves in fig. is particularly interesting from the point of view of the Phase Rule. AB represents the various states of equilibrium between ice and ferric chloride solutions, a minimum temperature being reached at the cryohydric point B, which is -55° C. At this point ice, solution, and the dodecahydrate of ferric chloride are in equilibrium. The number of degrees of freedom is nil - in other words, the system is invariant, and if heat be subtracted the liquid phase will solidify without change of temperature until the whole has become a solid mass of ice and dodecahydrate. Further abstraction of heat merely lowers the temperature of the system as a whole.

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.

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

Salt.		Change of Volume on Solution in Water.		Grams of Salt in 100 Grams of Solution at 18° C., at:
1 Atmos. Pressure.	400 Atmos. Pressure.	500 Atmos. Pressure.
Sodium chloride	contraction	26.4	. . .	27.0
Ammonium chloride	expansion	27.2	. . .	25.8
Alum	contraction	11.5	14.2	. . .

Further data for sodium chloride have been published which are in close harmony with those given above, but refer to 25° C.

Pressure in kilograms per sq. cm	1	250	500	750
Grams of NaCl per 100 grams solution	26.44	26.58	26.72	26.82

As long ago as 1870 Stas observed that the solubility of silver chloride varies with its method of preparation, the following results being obtained:

		Solubility
Flocculent silver chloride	0.0140 gram/litre	20° C.
Powdered silver chloride	0.0060 gram/litre	17° C.
Granular silver chloride	0.001 gram/litre	15° C.

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

Salt.	Diameter of Particles supposed Spherical.	Solubility at 25° C. (Mass per Litre.)
Calcium sulphate	2μ	2.085 grams
	0.3μ	2.476 grams
Barium sulphate	1.8μ	2.29 mg
	0.1μ	4.15 mg