Question

The compound $A$ on heating gives a colourless gas and a residue that is dissolved in water to obtain $B$. Excess of $C{O}_2$ is bubbled through an aqueous solution of $B,C$ is formed which is recovered in the solid form. Solid $C$ on gentle heating gives back $A$. The compound $A$ is:

• A. $CaC{O}_3$
• B. $N{a}_{2}C{O}_3$
• C. $K_{2}C{O}_3$
• D. $CaS{O}_4.2H_2$O

Answer 1

The correct option is A $CaC{O}_3$

The reactions can be summarized as follows :

$A$ $\stackrel{\Delta }{\to }$ colorless gas + residue

residue + $H_{2}O$ $\to$ $B$ $\stackrel{ExcessC{O}_2}{\to }$ $C$ $\stackrel{\Delta }{\to }$ $A$

This is possible only when A is CaCO${}_3$. The reactions are as follows

$CaCO$${}_3$ $\stackrel{\Delta }{\to }$ $\underset{\left(colourlessgas\right)}{C{O}_2}+\underset{\left(residue\right)}{CaO}$

$CaO+$ $H_{2}O$ $\to$ $\underset{\left(B\right)}{Ca(OH{)}_2}$ $\underset{\left(excess\right)}{\overset{C{O}_2}{\to }}$ $\underset{\left(C\right)}{Ca(HC{O}_3{)}_2}$ $\stackrel{\Delta }{\to }CaC{O}_3$

Thus, calcium carbonate on heating gives colourless gas carbon dioxide and residue calcium oxide. Calcium oxide reacts with water to form calcium hydroxide. Calcium hydroxide reacts with excess carbon dioxide to form calcium hydrogen carbonate. This on heating gives calcium carbonate.