Question

In acidic medium, which of the following does not change its colour?

• A. \( \text{MnO}_4^- \)
• B. \( \text{MnO}_4^{2-} \)
• C. \( \text{CrO}_4^{2-} \)
• D. \( \text{FeO}_4^{2-} \)

Hint:

In acidic conditions, \( \text{MnO}_4^- \) is stable and retains its purple colour, making it a reliable indicator in redox reactions.

Answer 1

The Correct Option is A

To analyze the behavior of different ions in an acidic medium, consider the following: 1. Stability of \( \text{MnO}_4^- \): - \( \text{MnO}_4^- \) is stable in acidic conditions and retains its characteristic purple color because it does not undergo any further reactions under these conditions. 2. Behavior of \( \text{MnO}_2^{2-} \): - \( \text{MnO}_2^{2-} \) undergoes disproportionation in an acidic medium to form \( \text{MnO}_4^- \) (purple) and \( \text{Mn}^{2+} \) (colorless). 3. Conversion of \( \text{CrO}_4^{2-} \): - \( \text{CrO}_4^{2-} \) converts to \( \text{Cr}_2\text{O}_7^{2-} \), resulting in a color change from yellow to orange due to the equilibrium shift in acidic conditions. 4. Decomposition of \( \text{FeO}_4^{2-} \): - \( \text{FeO}_4^{2-} \) decomposes into \( \text{Fe}^{3+} \), which is colorless or pale yellow, depending on the concentration. Therefore, among the given species, \( \text{MnO}_4^- \) retains its purple color in an acidic medium.

 Final Answer: \[ \boxed{\text{MnO}_4^-} \]