Note 50 — Chapter XIV · THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM. CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM

Calcium chloride has a specific gravity 2·20, or, when fused, 2·12, and the sp. gr. of the crystallised salt CaCl2,6H2O is 1·69. If the volume of the crystals at 0° = 1, then at 29° it is 1·020, and the volume of the fused mass at the same temperature is 1·118 (Kopp) (specific gravity of solutions, see Note 27). The solution containing 50 p.c. CaCl2 boils at 130°, 70 p.c. at 158°. Superheated steam decomposes calcium chloride with more difficulty than magnesium chloride and with greater ease than barium chloride (Kuhnheim). Sodium does not decompose fused calcium chloride even on prolonged heating (Liés-Bodart), but an alloy of sodium with zinc, lead, and bismuth decomposes it, forming an alloy of calcium with one of the above-named metals (Caron). The zinc alloy may be obtained with as much as 15 p.c. of calcium. Calcium chloride is soluble in alcohol and absorbs ammonia.

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°.