AIIMS 2009 Chemistry Solubility Assertion Reason Question

To solve this question, we need to analyze both the assertion and the reason provided and determine their truthfulness and relationship.

Assertion:

Cu(OH)₂ is soluble in NH₄OH but not in NaOH.

Explanation: Copper(II) hydroxide, Cu(OH)₂, is a sparingly soluble compound. In the presence of ammonium hydroxide (NH₄OH), Cu(OH)₂ can dissolve due to the formation of a soluble complex. The relevant reaction can be illustrated as follows:

$$\text{Cu(OH)}_2 + 4 \text{NH}_3 \rightleftharpoons \text{[Cu(NH}_3\text{)}_4]^{2+} + 2 \text{OH}^-$$

This reaction shows that Cu(OH)₂ interacts with ammonia (NH₃) to form a complex ion, tetraamminecopper(II), which is soluble in solution. The presence of NH₄OH facilitates this reaction since NH₄OH dissociates to provide NH₃ in solution.

Conversely, when Cu(OH)₂ is added to sodium hydroxide (NaOH), it does not dissolve because NaOH does not form a similar complex. Instead, Cu(OH)₂ remains in a solid state due to a lack of a complexing agent that can solubilize it effectively.

Reason:

Cu(OH)₂ forms a soluble complex with NH₃.

Explanation: This reason provides a proper explanation for the assertion. The formation of the soluble complex [Cu(NH₃)₄]²⁺ from Cu(OH)₂ and NH₃ is indeed the key factor that accounts for the solubility of Cu(OH)₂ in NH₄OH. Therefore, the reason supports the assertion.

Conclusion:

Since both the assertion and the reason are true and the reason correctly explains the assertion, the correct answer is:

A) If both assertion and reason are true and reason is the correct explanation of assertion.

Why Other Options Are Incorrect:

• B) If both assertion and reason are true but reason is not the correct explanation of assertion: This option cannot be correct because the reason does indeed explain why Cu(OH)₂ is soluble in NH₄OH.
• C) If assertion is true but reason is false: This option is incorrect as both the assertion and the reason are true.
• D) If both assertion and reason are false: This is not correct since both statements are verified to be true.

Thus, the detailed analysis confirms that the correct choice is indeed A, as both statements are true and the reason explains the assertion effectively.