Question

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.

Answer 1

Calcium hydroxide

• The chemical compound which is used for white washing is Calcium hydroxide.
• This substance is obtained when the element of calcium is made to react with water.
• The equation involved in the reaction is as follows:

$\mathrm{Ca}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

So, element A is calcium, and compound B is Calcium hydroxide$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$.

Compound C

• When compound B is heated, it leads to the formation of two products namely Calcium oxide and water.
• The equation involved in the reaction is as follows:

$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

• From the above equation, oxide C is identified as Calcium oxide. When it is treated with water, the formation of Calcium hydroxide takes place.

$\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)$

• So, element A is calcium, compound B is Calcium hydroxide, and Compound C is Calcium oxide.