AIIMS 2003 Chemistry Solubility and Precipitation Assertion Reason Question

Let's break down the question regarding the solubility of barium carbonate (BaCO₃) in nitric acid (HNO₃) compared to plain water, along with the assertion and reason provided.

Assertion and Reason Analysis

Assertion: BaCO₃ is more soluble in HNO₃ than in plain water.
Reason: Carbonate is a weak base and reacts with the H⁺ from the strong acid, causing the barium salt to dissociate.

Explanation of the Assertion

1. Solubility in Water vs. Strong Acid:

   • BaCO₃ is a sparingly soluble salt in water. According to the solubility product constant (Ksp), the solubility of BaCO₃ in water can be described by the equilibrium: $\text{BaCO}_3 (s) \rightleftharpoons \text{Ba}^{2+} (aq) + \text{CO}_3^{2-} (aq)$
   • The value of Ksp for BaCO₃ is relatively low, indicating limited solubility in water.

2. Reaction with HNO₃:

   • When BaCO₃ is added to HNO₃, it reacts with the acid to form barium nitrate (Ba(NO₃)₂), carbon dioxide (CO₂), and water (H₂O): $\text{BaCO}_3 (s) + 2\text{HNO}_3 (aq) \rightarrow \text{Ba(NO}_3)_2 (aq) + \text{CO}_2 (g) + \text{H}_2\text{O} (l)$
   • The reaction produces carbon dioxide gas, which escapes from the solution. This removal of CO₂ shifts the equilibrium to the right, according to Le Chatelier's principle, thus driving the dissolution of more BaCO₃. Therefore, BaCO₃ is indeed more soluble in HNO₃ than in plain water.

Explanation of the Reason

• The reason states that carbonate ions (CO₃²⁻) act as a weak base and react with the H⁺ ions from HNO₃, a strong acid. This is correct because when CO₃²⁻ reacts with H⁺, it forms bicarbonate (HCO₃⁻) or CO₂: $\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{CO}_2 (g) + \text{H}_2\text{O}$
• The reaction with H⁺ ions decreases the concentration of CO₃²⁻ in solution, further driving the dissolution of solid BaCO₃.

Conclusion on the Correct Answer

Given that both the assertion and the reason are true, and the reason correctly explains the assertion, the correct answer is A.

Clarification of Other Options

• B) Assertion is true, Reason is false: This option is incorrect because the reason is accurate and supports the assertion.
• C) Assertion is false, Reason is true: This option is incorrect because the assertion is indeed true.
• D) Both Assertion and Reason are false: This option is incorrect as both the assertion and the reason are true.

Summary

In summary, BaCO₃ is more soluble in HNO₃ than in plain water due to its reaction with the strong acid, which shifts the equilibrium and allows more BaCO₃ to dissolve. The carbonate ion's behavior as a weak base and its reaction with H⁺ ions plays a crucial role in this increased solubility. Thus, the correct answer is A.