Question

Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed?

(i) It is an endothermic reaction

(ii) It is an exothermic reaction

(iii) The pH of the resulting solution will be more than seven

(iv) The pH of the resulting solution will be less than seven

• A. (i) and (ii)
• B. (ii) and (iii)
• C. (i) and (iv)
• D. (iii) and (iv)

Answer 1

The correct option is B

(ii) and (iii)

Explanation:

Reaction of calcium oxide with water:

• When solid calcium oxide is reacted with the water, it will lead to the formation of the Calcium hydroxide, and the process is known as slaking of lime.
• Calcium hydroxide dissolves in water to form its solution called lime water.

$\underset{\mathrm{Calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\underset{(\mathrm{Slaked}\mathrm{lime})}{\mathrm{Calcium}\mathrm{hydroxide}}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}+\mathrm{Heat}$

• This slaking process is exothermic in nature because heat is evolved during reaction.
• Since Lime water is a metallic hydroxide, which is basic in nature.
• The dissociation of Lime water can be represented as:

$\underset{\mathrm{Calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}\to \underset{\mathrm{Calcium}\mathrm{ion}}{{\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydroxide}\mathrm{ion}}{2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)}$

• When phenolphthalein is added to a basic solution it turns pink and the pH of the solution would be greater than that of 7.

Therefore, the correct answer is (B).