Preparation of Calcium Hydroxide
From quick lime
Calcium hydroxide is prepared on a commercial scale by adding water to quicklime. This process is called slaking of lime.
CaO + H2O —-> Ca(OH)2
From calcium chloride
It is also obtained by treating calcium chloride with caustic soda.
Properties of Calcium Hydroxide
1) It is a white amorphous powder sparingly soluble in water, the solubility decreases with rise in temperature. The suspension of slaked lime in water is called milk of lime while the filtered and clear solution is known as lime water.
2) Action of heat: On heating, slaked lime loses water only at temperature greater than 700 K.
Ca(OH)2 ———> CaO + H2O
3) Reaction with chlorine : Slaked lime reacts with chlorine to form calcium hypochlorite, a constituent of bleaching powder.
Ca(OH)2 + 2 Cl2 ——> CaCl2 + Ca(OCl)2 + 2 H2O
4) Reaction with carbon dioxide
When carbon dioxide is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)2 + CO2 ——-> CaCO3 ↓ + H2O
On passing excess of carbon dioxide, the precipitate of calcium carbonate dissolve to form soluble calcium bicarbonate and hence the milkiness disappears.
CaCO3 + CO2 + H2O——–> Ca(HCO3)2
If this clear solution of calcium bicarbonate is heated, the solution again turns milky due to the decomposition of calcium bicarbonate back to calcium carbonate.
Ca(HCO3)2 (aq)——-> CaCO3 ↓ + H2O + CO2
5) Reaction with acids: Slaked lime being a strong base reacts with acids and acidic gases forming salts.
Ca(OH)2 + 2 HCl —-> CaCl2 + H2O
Uses of Calcium Hydroxide
1) It is used as a building material in form of mortar.
2) It is used in white washing because of its disinfectant properties.
3) It is used in the preparation of ammonia from ammonium chloride and in the softening of hard water.
4) It is used in the manufacture of bleaching powder.
5) It is used in glass making, tanning industry and purification of sugar.
6) It is used for absorbing acidic gases.
7) It form lime of water, it is used for detection of carbon dioxide.