Question

A metal carbonate X on treatment with a mineral acid liberates a gas which when passed through aqueous solution of a substance Y gives back X. The substance Y on reaction with the gas obtained at anode during electrolysis of brine gives a compound Z which can decolourise fabrics. The compounds X, Y and Z respectively are:
(a) ${ CaCO }_{ 3 },{ Ca(OH) }_{ 2 }, Ca(ClO)_{ 2 }$
(b) ${ Ca(OH) }_{ 2 },CaO, Ca(ClO)_{ 2 }$
(c) ${ CaCO }_{ 3 }, Ca(ClO)_{ 2 },{ Ca(OH) }_{ 2 }$
(d) ${ { Ca(OH) }_{ 2 },CaCO }_{ 3 }, Ca(ClO)_{ 2 }$

Answer 1

When metal carbonates are treated with mineral acids, a brisk effervescence is seen which is due to the evolution of carbon dioxide gas. This carbon dioxide gas can turn lime water milky. Also brine solution is aqueous solution of sodium chloride.

Complete step by step answer:
Let us consider the statement: “A metal carbonate X on treatment with a mineral acid liberates a gas which when passed through aqueous solution of a substance Y gives back X”.
When metal carbonates are treated with mineral acids, they produce carbon dioxide which if passes through the lime water turns the lime water milky due to the formation of calcium carbonate. Therefore the compound X should be calcium carbonate. The reactions are given below:
$\begin{matrix} CaC{ O }_{ 3 }(s) \\\ Calcium\quad carbonate \\\ (X) \end{matrix}+\begin{matrix} 2HCl(aq) \\\ Hydrochloric \\\ acid \end{matrix}\rightarrow \begin{matrix} { CaCl }_{ 2 }(aq) \\\ Calcium \\\ Chloride \end{matrix}+\begin{matrix} { CO }_{ 2 }(g) \\\ Carbon \\\ dioxide \end{matrix}+\begin{matrix} { H }_{ 2 }O(l) \\\ Water \end{matrix}$
$\begin{matrix} Ca{ (OH{ ) }_{ 2 }(aq) } \\\ lime\quad water \\\ Y \end{matrix}+\begin{matrix} { CO }_{ 2 }(g) \\\ carbon \\\ dioxide \end{matrix}\xrightarrow { the\quad solution\quad turns\quad milky } \begin{matrix} Ca{ CO }_{ 3 }(s) \\\ calcium\quad carbonate \\\ X \end{matrix}$
Brine is an aqueous solution of sodium chloride. In electrolysis a non-spontaneous reaction is carried out by supplying current. The electrolysis of brine is used for the production of chlorine gas. The chloride ions present in the solution ate oxidised to chlorine gas while the water molecules are reduced to hydrogen gas. The reactions are given below:
$\begin{matrix} at\quad the \\\ anode \end{matrix}:\begin{matrix} { 2Cl }^{ - }(aq) \\\ Chloride\quad ions \end{matrix}\rightarrow \begin{matrix} { Cl }_{ 2 }(g) \\\ Chlorine\quad gas \end{matrix}+\begin{matrix} { 2e }^{ - } \\\ electrons \end{matrix}$
$\begin{matrix} at\quad the \\\ cathode \end{matrix}:\begin{matrix} { 2H }_{ 2 }O(l) \\\ water \end{matrix}+\begin{matrix} 2{ e }^{ - } \\\ electrons \end{matrix}\rightarrow \begin{matrix} { H }_{ 2 }(g) \\\ Chlorine\quad gas \end{matrix}+\begin{matrix} { 2OH }^{ - }(aq) \\\ Hydroxide\quad ions \end{matrix}$
When calcium hydroxide is reacted with chlorine gas, it produces Calcium hypochlorite which is the main active ingredient of the bleaching powder. It is a yellow powder and has a distinct smell of chlorine. If applied to fabrics, it will decolourise them. The reaction is given below:
$\begin{matrix} { 2Ca(OH) }_{ 2 } \\\ Calcium\quad Hydroxide \\\ Y \end{matrix}+\begin{matrix} { 2Cl }_{ 2 }(g) \\\ Chlorine \\\ gas \end{matrix}\rightarrow \begin{matrix} Ca({ ClO) }_{ 2 } \\\ Calcium\quad Hypochlorite \\\ Z \end{matrix}+\begin{matrix} { CaCl }_{ 2 } \\\ Calcium \\\ Chloride \end{matrix}+\begin{matrix} { 2H }_{ 2 }O(l) \\\ water \end{matrix}$

Hence the correct answer is (a) ${ CaCO }_{ 3 },{ Ca(OH) }_{ 2 }, Ca(ClO)_{ 2 }$.

Note: Do not get confused between slaked lime and lime water. Calcium hydroxide is called slaked lime and the aqueous solution of calcium hydroxide is called lime water.