Question

The reaction of water with slaked lime is studied by______
(A) putting slaked lime into water taken in a test tube.
(B) Putting a lot of water into slaked lime.
(C) Sprinkling a little water on slaked lime.
(D) None of the above methods.

Answer 1

Hint: Slaked lime is hydroxide of calcium (an alkaline earth metal). Thus, slaked lime is calcium hydroxide. The reaction between water and calcium hydroxide is an exothermic reaction.

Complete answer:
Lime water contains dilute aqueous calcium hydroxide solution. The chemical formula of calcium hydroxide is \[{\text{Ca}}{\left( {{\text{OH}}} \right)_2}\]. At room temperature, calcium hydroxide is sparingly soluble in water. At 298 K, the solubility of calcium hydroxide in water is \[ \text {1.5 g/L } \]. 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.
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 base is dissolved in water and produces hydroxide ions. Calcium hydroxide is a base.
Slaked lime does not form aqueous solution. It is present as suspension in water.
To prepare lime water, excess of water is added to solid calcium hydroxide. The solution is filtered to remove the excess of undissolved calcium hydroxide.
The reaction between water and calcium hydroxide to form slaked lime is an exothermic reaction. Lot of heat energy is evolved during reaction. Hence, it is not advised that slaked lime should be added to water. Instead, water should be added to slaked lime.
This is similar to the procedure followed for the dilution of concentric sulphuric acid with water.

Hence, the correct option is the option B ).

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.