Preparation of Calcium Oxide: Calcium oxide, CaO, is obtained by the thermal decomposition of calcium trioxocarbonate(IV). Industrially, lumps of limestone are heated strongly in a furnace called a lime kiln leading to the decomposition of the limestone at around a temperature of 900C to yield calcium oxide and carbon(IV) oxide. This reaction is a reversible reaction and the forward reaction is favored by the continuous removal of the products and addition of reactants. To achieve this, the kiln is opened at the top and air is blown in from the bottom. Calcium oxide is removed from the bottom of the kiln as it is formed, while limestone is added from the top.
CaCO3(s) CaO(s) + CO2(g)
Properties of Calcium Oxide: Calcium oxide is also known as quicklime. It is a white solid that can easily be ground into powder. It is a very refractory solid which can be attributed to its ability to withstand very high temperatures. It just merely becomes incandescent when heated strongly, emitting an intensely bright light, which is known as limelight. Calcium oxide melts at 2600C.
Calcium oxide is very hygroscopic and is used to dry ammonia gas. When water is added in drops to the oxide, it cracks with a hissing noise and breaks up into a powdery form accompanied by the liberation of heat. This process is known as slaking, and the end-point is calcium hydroxide, Ca(OH)2, which is more commonly known as slaked lime.
CaO(s) + H2O(l) → Ca(OH)2(s)
If more water is added to the slaked lime, a suspension known as milk of lime is obtained. On standing, a supernatant known as lime water is obtained. Calcium oxide is a strong base which reacts vigorously with acids to form calcium salts. It also reacts with all ammonium salts to liberate ammonia gas.
Uses of Calcium Oxide
Below are some of the very important uses of Calcium oxide
- Calcium oxide is used in the manufacture of slaked lime, mortar, cement and calcium carbide.
- Calcium oxide is used to make refractory furnace linings, in melting processes and in glass-making.
- In Industry, lime is used to neutralize acidic waste substances. The treated waste can then be emptied into natural waterways without harming aquatic life.
- In agriculture, lime is used to treat acidic soils. If the pH of the soil is too low, most soil bacteria and other soil microflora would not survive or function properly in enhancing soil fertility. Lime which is a cheap base, is used to reduce soil acidity.
- In the laboratory, calcium oxide is used for drying ammonia gas and for making soda-lime, which is made by slaking calcium oxide with concentrated sodium hydroxide.