Calcium Oxide (Quick Lime) and Calcium Hydroxide (Slaked lime) - Practice Questions & MCQ

Calcium Oxide(CaO)
It is called burnt lime or lime also.

Preparation
It is prepared by heating limestone in a rotary kiln at 800-1000^oC. Since it is a reversible reaction the evolved CO₂ must be removed so that the equilibrium may shift in the forward direction.

slaked lime

Physical Properties

• It is a white amorphous solid with a melting point of 287K.
• On exposure to the atmosphere, it absorbs moisture and CO₂.
• On hearing in an oxyhydrogen flame, it becomes incandescent and releases a bright white light called limelight.

Chemical Properties

• Being a basic oxide it reacts with acidic oxides to form salts as follows:
• On reaction with water, it forms calcium hydroxide with a hissing sound. II is an exothermic reaction and known as slaking of lime.
• When it is heated with carbon at 3000^oC, calcium carbide is formed. Calcium carbide on reaction with water gives acetylene and with nitrogen gives CaCN₂.

Uses

• It is used in the manufacture of glass, cement, bleaching powder, soda-lime etc.
• It is used as a basic flux.
• It is used as a drying agent for gases and alcohols.
• It is used in the refining of sugar.
• It is also used as a disinfectant and germicide.
• It is used for whitewashing.

Calcium Hydroxide(CaOH)₂
It is also known as slaked lime

Preparation
It is prepared by the reaction of lime with water as follows:

Physical Properties

• It is a white amorphous powder and sparingly soluble in water. Its suspension in water is called milk of lime while the clear solution is called lime water but chemically both are same.

Chemical Properties

• Its slowly absorbs CO₂ from air forming insoluble milky solution of calcium carbonate. If more CO₂ is passed in this milky solution, soluble calcium bicarbonate is formed and milkiness gets disappeared.
• When it is reacted with chlorine in cold milk of lime form, calcium hypochlorite is formed bur when it is hot, calcium chlorate is formed.

Uses

• It is used in softening of water, purification of coal gas, sugar etc.
• It is used in the manufacturing of bleaching powder, sodium carbonate etc.
• It is also used in the making of mortar and plaster used as building materials.