Question

Base found in lime water is ___________.

Answer 1

Hint: Base is a substance that, upon dissolution in water, gives hydroxide ion. Usually oxides and hydroxides of alkali metal or alkaline earth metals are bases. The base present in lime water is hydroxide of alkaline earth metal.

Complete answer:
Lime water contains dilute aqueous calcium hydroxide solution. The chemical formula of base 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 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, calcium oxide is treated with water.
${CaO\; + \;{H_2}O\; \to\; Ca(OH)_2}$
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, it is called base. Calcium hydroxide dissociates in aqueous solution to produce hydroxide ions. Hence, calcium hydroxide is a base.

Hence, the base found in lime water is calcium hydroxide.

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.