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. $CaS{O_4}.2{H_2}O$

Answer 1

The carbonates of the alkaline earth metals are thermally unstable. They readily decompose on heating to form gaseous products.

Complete step by step answer:
The given substances are $CaC{O_3}$, $N{a_2}C{O_3}$, ${K_2}C{O_3}$ and $CaS{O_4}.2{H_2}O$. The $N{a_2}C{O_3}$ and ${K_2}C{O_3}$ are carbonates of alkali earth metals of sodium and potassium. $CaC{O_3}$ is the carbonate of calcium which is an alkaline earth metal. $CaS{O_4}.2{H_2}O$ is known as gypsum which is the hydrated salt of calcium sulfate.
The thermal stability of these substances varies differently. The sodium and potassium carbonate are stable and do not decompose on heating. The calcium carbonate decomposes on heating to generate calcium oxide and carbon dioxide. The $CaS{O_4}.2{H_2}O$ undergoes decomposition on heating to generate calcium sulfate hemihydrate ($CaS{O_4}.\frac{1}{2}{H_2}O$) and water molecule.
The sequence of the reactions provided as follows:
$A\xrightarrow{\Delta }B\xrightarrow[{water}]{{C{O_2}}}C\xrightarrow{\Delta }A$
Of the given substances only $CaC{O_3}$ decomposes on heating to produce a colourless gas and a residue. Thus the given transformation is explained as below:
$CaC{O_3}\xrightarrow{\Delta }CaO(residue) + C{O_2}(colourless{\text{ }}gas)$
When $CaO$ is treated with water $Ca{\left( {OH} \right)_2}$ is formed. Thus B is calcium hydroxide ($Ca{\left( {OH} \right)_2}$).
$CaO + {H_2}O \to Ca{(OH)_2}$
When $C{O_2}$ is bubbled in the aqueous solution of $CaO$, the product obtained is calcium hydrogen carbonate, i.e. C.
$Ca{(OH)_2}\xrightarrow{{2C{O_2}}}Ca{(HC{O_3})_2}$
The calcium hydrogen carbonates on heating produces calcium carbonate i.e. A.
$Ca{(HC{O_3})_2}\xrightarrow{\Delta }CaC{O_3}$
Hence option (A) is the correct answer, i.e. the compound A is $CaC{O_3}$.

Note:
The thermal decomposition of calcium carbonate is also known as calcination. The calcium carbonate also reacts with acids to produce calcium ions. Hence it is used as calcium supplements.