AIPMT PRELIMS 2004 Chemistry Carbonates and Bicarbonates MCQ Question

To solve the question, we need to analyze the behavior of the solid compound 'X' upon heating and its interactions with water and carbon dioxide (CO₂) as described in the problem.

Step 1: Heating of Compound 'X'

The first step mentions that solid compound 'X' produces CO₂ gas and a residue upon heating. Among the options, we need to identify compounds that decompose to produce CO₂ when heated.

• Calcium Carbonate (CaCO₃): When heated, it decomposes as follows:

  $\text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g)$

  This reaction produces carbon dioxide gas and calcium oxide as a residue.

• Sodium Carbonate (Na₂CO₃), Potassium Carbonate (K₂CO₃), and Calcium Bicarbonate (Ca(HCO₃)₂) do not produce CO₂ upon heating in the same manner.

  • Na₂CO₃ and K₂CO₃ are stable and do not decompose to release CO₂ upon heating.

  • Ca(HCO₃)₂, on the other hand, will decompose to produce CO₂ but is not a solid compound that would leave a solid residue like CaO.

Step 2: Formation of 'Y'

The residue formed in the case of compound 'X' is calcium oxide (CaO) from the decomposition of CaCO₃. When CaO is mixed with water, it forms calcium hydroxide:

$\text{CaO} (s) + \text{H}_2O (l) \rightarrow \text{Ca(OH)}_2 (aq)$

Thus, 'Y' is calcium hydroxide, which is soluble in water.

Step 3: Passing CO₂ through 'Y' to form 'Z'

When an excess of CO₂ is passed through the solution of calcium hydroxide (Y), it reacts to form calcium carbonate:

$\text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2O (l)$

In this reaction, calcium carbonate precipitates out of the solution, but if we consider the presence of excess CO₂, it can also form calcium bicarbonate, which remains in solution:

$\text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) + \text{H}_2O (l) \rightarrow \text{Ca(HCO}_3)_2 (aq)$

Thus, 'Z' is a clear solution of calcium bicarbonate.

Step 4: Boiling 'Z' to reform 'X'

When we boil the solution 'Z' (calcium bicarbonate), it decomposes to form calcium carbonate and releases CO₂:

$\text{Ca(HCO}_3)_2 (aq) \rightarrow \text{CaCO}_3 (s) + \text{CO}_2 (g) + \text{H}_2O (l)$

This process allows us to regenerate compound 'X' (CaCO₃).

Conclusion

Thus, the correct answer is A) CaCO₃ as it fits all the described transformations.

Explanation of Incorrect Options

• B) Na₂CO₃: Does not decompose to release CO₂ upon heating.
• C) K₂CO₃: Similar to Na₂CO₃, it is stable and does not yield CO₂ upon heating.
• D) Ca(HCO₃)₂: While it can produce CO₂ upon heating, it does not leave a solid residue like CaO and does not match the sequence of reactions leading to the formation of 'Y' and 'Z' as described.

In summary, the behavior of calcium carbonate upon heating, its reaction with water to form calcium hydroxide, and subsequent reactions with CO₂ lead us to conclude that the solid compound 'X' is indeed calcium carbonate (CaCO₃).