Chemical Properties of Calcium

Calcium is a highly electropositive element, and combines directly with non-metals, although Goodwin considers that it is more inert than is generally supposed. In many cases reaction takes place with considerable vigour. For example, if calcium and fluorine are brought together at ordinary temperatures, the rate of reaction is sufficiently great to raise the mass to incandescence, and the resulting fluoride is molten.

The readiness with which calcium combines with the nitrogen of the air, forming a yellow coating over the surface, resulted in an error in the description of the colour of the metal by the earlier investigators, the pure untarnished metal being silvery white in appearance.

At 300° C. it burns in oxygen with a brilliant light and an intense heat, which volatilises the lime. Combination with the other halogens, with hydrogen, and with sulphur, selenium, tellurium, phosphorus, antimony, boron, and silicon, can also be effected with varying facility.

Dilute acids are decomposed by calcium with vigorous evolution of hydrogen. Water is attacked more slowly owing to the formation of an insoluble coating of hydroxide on the surface. Carbon dioxide, silica, fuming sulphuric acid, sulphuretted hydrogen, borates, oxides, sulphides, many fused salts, and various organic compounds may also be decomposed. Calcium will even decompose some alkali salts at bright red heat.

Moisture appears to have the customary influence, since calcium is unaffected by dry oxygen.

The reactivity of calcium depends largely on its physical state. The fused mass of the metal is less readily attacked than the well- defined crystals. Sieverts showed that some samples of commercial calcium combine with nitrogen only above 800° C., but the majority have a second temperature range of activity between 300° and 660° C., the optimum temperature being 440° C. The inactive calcium may be changed to the active form by melting, and then cooling slowly to produce a relatively coarse-grained crystalline structure. It will then absorb nitrogen even between 150° and 300° C. The reverse change from the active to the inactive modification may be brought about by chilling quickly from 840° C. or by mechanical powdering. The absorption of hydrogen also depends on crystalline structure, but no samples are quite indifferent to hydrogen below 800° C.

Calcium reacts with gaseous ammonia, or dissolves in liquid ammonia to form an addition compound which gradually gives off ammonia and hydrogen at room temperature and forms an amide.

With alcoholic ammonia it forms calcium ethylate, the amide probably being an intermediate product.

General Properties of Compounds of Calcium

With certain minor exceptions, calcium behaves as a divalent element. The calcium ion is colourless, so that a coloured salt is, in general, only obtained with a coloured anion; for example, calcium chromate is yellow. There is a decided tendency to form double salts, especially with the salts of the alkali metals.