A compound 'X' on heating gives a colourless gas. The residue is dissolved in water to obtain 'Y'. Excess CO2 is passed through aqueous solution of 'Y' when 'Z' is formed. 'Z' on gentle heating gives

Question

A compound 'X' on heating gives a colourless gas. The residue is dissolved in water to obtain 'Y'. Excess CO2 is passed through aqueous solution of 'Y' when 'Z' is formed. 'Z' on gentle heating gives back 'X'. The compound 'X' is:

Prashant Kotian · Accepted Answer

Correct answer: D. Step 1: Calcium carbonate (CaCO3, 'X') decomposes upon heating to form calcium oxide residue ('Y') and carbon dioxide gas: CaCO3 --Δ--> CaO + CO2. Step 2: Dissolving CaO ('Y') in water yields limewater, Ca(OH)2. Passing excess CO2 through Ca(OH)2 forms soluble calcium bicarbonate, Ca(HCO3)2 ('Z'): Ca(OH)2 + 2CO2 -> Ca(HCO3)2. Step 3: Heating Ca(HCO3)2 ('Z') decomposes it back into calcium carbonate (CaCO3, 'X'), water, and CO2. Therefore, compound 'X' is CaCO3, matching option (d).

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