Question 53
An element A reacts with water to form a compound B which is used in whitewashing. The compound B on heating forms an oxide C which on treatment with water gives back B. Identify A, B and C and give the reactions involved.

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Solution

The compound that is used in whitewashing is slaked lime with chemical formula ${Ca(OH)}_{2}$ . This, on heating, gives the calcium oxide which again gives calcium hydroxide on treatment with water.

Therefore, $A - Ca; B - {Ca(OH)}_{2}; C - CaO$

The reactions involved are:
$Ca(s) + 2 H_{2} O \to {Ca(OH)}_{2} (aq) + H_{2} (g)$
${Ca(OH)}_{2} (aq) Heat - - \to CaO (aq) + H_{2} O (l)$

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