Calcium

SOME IMPORTANT COMPOUNDS OF CALCIUM

Quick Lime (solid Calcium Oxide or CaO)

It is prepared on a commercial scale by heating limestone (CaCO₃) in a rotary kiln at    1070-1270K.

                                  CaCO₃ ⇋ CaO + CO₂:

Slaked lime (solid Calcium Hydroxide Ca(OH)₂) Calcium hydroxide is prepared by adding water to quick lime, CaO. It is a white amorphous powder. It is sparingly soluble in water.

Lime water is the aqueous solution of Ca(OH)₂

Milk of lime is the suspension of slaked lime in water.  When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.

                   Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

On passing excess of carbon dioxide, the precipitate dissolves to form calcium bicarbonate.

                    CaCO₃ + CO₂ +H₂O→Ca(HCO₃)₂

CALCIUM CARBONATE CaCO₃

Calcium carbonate occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride.

Ca(OH)₂  + CO₂  → CaCO₃ + H₂O               and    CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl

Calcium carbonate is a white fluffy powder. It is almost insoluble in water. It reacts with dilute acid to liberate carbon dioxide.

Calcium Sulphate (Plaster of Paris), CaSO₄·½H₂O

It is a hemihydrate of calcium sulphate. It is obtained when gypsum, CaSO₄·2H₂O, is heated to 393K.

          2(CaSO₄.2H₂O)  → 2(CaSO₄).H₂O + 3H₂O

Cement: is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO2 along with the oxides of aluminium, iron and magnesium. The average composition of Portland cement is: CaO, 50-60%; SiO₂, 20-25%; Al₂O₃, 5-10%; MgO, 2-3%; Fe₂O₃, 1-2% and SO₃, 1-2%. For good quality cement, the ratio of silica (SiO₂) to alumina (Al₂O₃) should be between 2.5 and 4 and the ratio of lime (CaO) to the total of the oxides of silicon (SiO₂) aluminium (Al₂O₃) and iron (Fe₂O₃) should be as close as possible to 2