Question

The suspension of slaked lime in water is known as:
A ) milk of lime
B ) aqueous solution of slaked lime
C ) lime water
D ) quick lime

Answer 1

Lime water is hydroxide of calcium (an alkaline earth metal). Thus, lime water is calcium hydroxide. The chemical formula of calcium hydroxide is ${\text{Ca}}{\left( {{\text{OH}}} \right)_2}$.

Complete answer:

Lime water contains dilute aqueous calcium hydroxide solution. At room temperature, calcium hydroxide is sparingly soluble in water. At 298 K, the solubility of calcium hydroxide in water is 1.5 g/L. Lime water is colorless and clear. It has a slight earthy smell. It also has an astringent/bitter test. Lime water is basic in nature and has a pH of 12.4.
To prepare lime water, excess of solid calcium hydroxide is added to water and the solution is filtered to remove the excess of undissolved calcium hydroxide.
In aqueous solution, calcium hydroxide dissociates to provide calcium cations and hydroxide anions.
${\text{Ca}}{\left( {{\text{OH}}} \right)_2}{\text{ }} \to {\text{ C}}{{\text{a}}^{2 + }}{\text{ + 2O}}{{\text{H}}^ - }{\text{ }}$
When a substance dissolves in water and produces hydroxide ions then is known as a base.
Slaked lime does not form aqueous solution. It is present as suspension in water.
Quicklime is calcium oxide.
The suspension of slaked lime in water is known as milk of lime.

Hence, the correct option is the option A ).

Additional Information: When carbon dioxide gas is passed through lime water, it turns milky due to formation of precipitate of calcium carbonate. This precipitate is present in the form of insoluble suspension. But if excess carbon dioxide gas is passed through milky lime water, the milkiness disappears as water insoluble calcium carbonate is converted to water soluble calcium bicarbonate.

Note: Lime water is used to detect the presence of carbon dioxide gas. This is because, when carbon dioxide gas is passed through lime water, a milky suspension is obtained. Sulphur dioxide gas present in the industrial exhaust can be trapped as a precipitate of calcium sulphite by passing through lime water.