Question

The compound A on heating gives a colourless gas and a residue that is dissolved in water to obtain B. Excess of ${CO_2}$ is bubbled through 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. ${Na_2 CO_3}$
• B. ${K_2 CO_3}$
• C. ${CaSO4.2H2O}$
• D. ${CaCO_3}$

Answer 1

The Correct Option is D

1. The problem involves identifying a compound A that decomposes upon heating to give a colorless gas and a solid residue.
2. Let's analyze the options:
   • Na_2CO_3 decomposes with difficulty above 850°C, yielding a color gas CO_2.
   • K_2CO_3 does not decompose readily on heating.
   • CaSO_4 \cdot 2H_2O is gypsum, which gives off water upon heating, not a gas.
   • CaCO_3 on heating decomposes to give a colorless gas CO_2 and CaO as residue.
3. The equation for this decomposition reaction is:
   CaCO_3(s) \rightarrow CaO(s) + CO_2(g)
4. The residue, CaO (quicklime), dissolves in water to form Ca(OH)_2 (slaked lime):
   CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)
5. When excess CO_2 is bubbled through Ca(OH)_2, the precipitate CaCO_3 (solid) is formed:
   Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)
6. On gentle heating, this solid CaCO_3 can revert back to CaO and CO_2:
   CaCO_3(s) \rightarrow CaO(s) + CO_2(g)
7. Thus, the compound A is indeed CaCO_3 as it satisfies all the given conditions.

Therefore, the correct answer is: CaCO_3.