A gram molecular weight of calcium chloride in dissolving in an excess of water evolves 18,723 calories, and in dissolving in alcohol 17,555 units of heat, according to Pickering.
Roozeboom made detailed researches on the crystallo-hydrates of calcium chloride (1889), and found that CaCl2,6H2O melts at 30·2°, and is formed at low temperatures from solutions containing not more than 103 parts of calcium chloride per 100 parts of water; if the amount of salt (always to 100 parts of water) reaches 120 parts, then tabular crystals of CaCl2,4H2Oβ are formed, which at temperatures above 38·4° are converted into the crystallo-hydrates CaCl2,2H2O, whilst at temperatures below 18° the β variety passes into the more stable CaCl2,4H2Oα, which process is aided by mechanical friction. Hence, as is the case with magnesium sulphate (Note [27]), one and the same crystallo-hydrate appears in two forms—the β, which is easily produced but is unstable, and the α, which is stable. The solubility of the above-mentioned hydrates of chloride of calcium, or amount of calcium chloride per 100 parts of water, is as follows:—
| 0° | 20° | 30° | 40° | 60° | ||
| CaCl2,6H2O | 60 | 75 | 100 | (102·8) | ||
| CaCl2,4H2Oα | — | 90 | 101 | 117 | (154·2) | |
| CaCl2,4H2Oβ | — | 104 | 114 | — | ||
| CaCl2,2H2O | — | — | (308·3) | 128 | 137 | |
The amount of calcium chloride to 100 parts of water in the crystallo-hydrate is given in brackets. The point of intersection of the curves of solubility lies at about 30° for the first two salts and about 45° for the salts with 4H2O and 2H2O. The crystals CaCl2,2H2O may, however, be obtained (Ditte) at the ordinary temperature from solutions containing hydrochloric acid. The vapour tension of this crystallo-hydrate equals the atmospheric at 165°, and therefore the crystals may be dried in an atmosphere of steam and obtained without a mother liquor, whose vapour tension is greater. This crystallo-hydrate decomposes at about 175° into CaCl2,H2O and a solution; this is easily brought about in a closed vessel when the pressure is greater than the atmosphere. This crystallo-hydrate is destroyed at temperatures above 260°, anhydrous calcium chloride being formed.
Neglecting the unstable modification CaCl2,4H2Oβ, we will give the temperatures t at which the passage of one hydrate into another takes place and at which the solution CaCl2 + nH2O, the two solids A and B and aqueous vapour, whose tension is given as p in millimetres, are able to exist together in stable equilibrium, according to Roozeboom's determinations:
| t | n | A | B | p |
| -55° | 14·5 | ice | CaCl2,6H2O | 0 |
| +29·8° | 6·1 | CaCl2,6H2O | CaCl2,4H2O | 6·8 |
| 45·3° | 4·7 | CaCl2,4H2O | CaCl2,2H2O | 11·8 |
| 175·5° | 2·1 | CaCl2,2H2O | CaCl2,H2O | 842 |
| 200° | 1·8 | CaCl2,H2O | CaCl2 | Several atmospheres |
Solutions of calcium chloride may serve as a convenient example for the study of the supersaturated state, which in this case easily occurs, because different hydrates are formed. Thus at 25° solutions containing more than 83 parts of anhydrous calcium chloride per 100 of water will be supersaturated for the hydrate CaCl2,6H2O.
On the other hand, Hammerl showed that solutions of calcium chloride, when frozen, deposit ice if they contain less than 43 parts of salt per 100 of water, and if more the crystallo-hydrate CaCl2,6H2O separates, and that a solution of the above composition (CaCl2,14H2O requires 44·0 parts calcium chloride per 100 of water) solidifies as a cryohydrate at about -55°.
[51] The action of barium sulphate on sodium and potassium carbonates is given on p. 437.
[52] Barium sulphide is decomposed by water, BaS + 2H2O = H2S + Ba(OH)2 (the reaction is reversible), but both substances are soluble in water, and their separation is complicated by the fact that barium sulphide absorbs oxygen and gives insoluble barium sulphate. The hydrogen sulphide is sometimes removed from the solution by boiling with the oxides of copper or zinc. If sugar be added to a solution of barium sulphide, barium saccharate is precipitated on heating; it is decomposed by carbonic anhydride, so that barium carbonate is formed. An equivalent mixture of sodium sulphate with barium or strontium sulphates when ignited with charcoal gives a mixture of sodium sulphide and barium or strontium sulphide, and if this mixture be dissolved in water and the solution evaporated, barium or strontium hydroxide crystallises out on cooling, and sodium hydrosulphide, NaHS, is obtained in solution. The hydroxides BaH2O2 and SrH2O2 are prepared on a large scale, being applied to many reactions; for example, strontium hydroxide is prepared for sugar works for extracting crystallisable sugar from molasses.