Calcium and its compounds

Calcium and its compounds

Calcium is a typical alkali earth metal. It occurs as calcium trioxocarbonate (IV) in different forms: chalk, limestone, marble, calcite, Iceland spar and aragonite. It also occurs as calcium tetraoxosulphate (VI) e.g. gypsum, anhydrite. However, it is extracted industrially by the electrolysis of calcium chloride (a by-product from Solvay process), using graphite anode (also the container) and iron cathode. It is a silvery white metal. The density is 1.55g per cm3 and the melting point is about 850°C. It is not as reactive as sodium and so it is not necessary to keep it below the surface of petroleum oil (in contrast to sodium). However, a white film of oxide is formed on the surface on exposure to air. Calcium compounds generally burn with s brick-red flame.

Some of the important reaction of calcium are:

i. In air: it forms calcium oxide (quicklime).

ii. With water: calcium metal is used in the extraction of thorium and in steel casting (as deoxidizer). The important compounds of calcium are calcium oxide (CaO), calcium hydroxide (Ca(OH)2), calcium trioxocarbonate (IV) (CaCO3), calcium tetraoxosulphate (VI) (CaSO4) and calcium chloride (CaCl2).

1. Calcium oxide (lime, quicklime) CaO, is a white solid, made industrially by the action of strong heat on limestone (CaCO3).

It is a refractory substance (it will not melt even when heated to a very high temperature) hence its use in producing light (limelight). Its important reactions are:

i. With water, slaked lime Ca(OH)2 is formed.

This reaction is called slaking. The slaked lime is changed to lime-water if water is added and filtered..

Because quicklime is basic and hygroscopic, it is used as the drying agent for ammonia gas.

ii. With silica: CaO + SiO2 —> CaSiO3

2. Calcium hydroxide, Ca(OH)2 is a white powder, prepared by addition of water to calcium oxide. Its dissolution in water is exothermic. It is sparingly soluble in water but the solubility decreases with increasing temperature. It is basic hence gives reactions expected of bases (e.g. neutralization with aqueous mineral acids). The compound is useful in many ways:

i. In agriculture, to reduce acidity in soils (liming).

ii. In preparation of calcium trioxocarbonate (IV) e.g. chalk.

iii. In the making of mortar by builders.

3. Calcium trioxocarbonate (IV), CaCO3, occurs naturally as marble, chalk, limestone and in many other forms such as stalagmites and stalactites from the roof and floor of carvers. It is a white solid which is practically insoluble in water. Some of its reactions are:

i. With dilute acids (HCl and HNO3); it evolves carbon (IV) oxide (CO2).

ii. With water containing dissolved CO2, it dissolves to give soluble calcium hydrogentrioxocarbonate(IV):

Calcium trioxocarbonate (IV) is used:

i. In the extraction of iron using the blast furnace.

ii. In the manufacture of cement.

iii. In the Solvay process of manufacturing sodium trioxocarbonate (IV).

4. Calcium tetraoxosulphate (VI), CaSO4, is a white solid. It occurs naturally as anhydrite (CaSO4) and gypsum (CaSO4.2H2O), known as plaster of Paris (POP). This involves heating gypsum to a temperature between 100°C and 200°C.

Plaster of Paris is used medically in making casts to maintain joints in a fixed position. It is used industrially for cement and wall-plasters.

5. Calcium chloride, CaCl2, is a white solid. The solution is prepared by adding marble to dilute HCl until a little of marble remains.

The mixture is filtered. The evaporation of the filtrate to dryness gives the fused solid (anhydrous CaCl2). This is very deliquescent hence its use as a drying agent for most gases (except ammonia with which it reacts).