When aqueous NaOH is added to an aqueous solution of chromium (III ) ions, a green blue precipitate is first formed which re - dissolves to give a green solution. The green colour is due to

Options

A. `[Cr(H_(2)O)_(6)]^(3+)`
B. `CrO_(4)^(2-)`
C. `[Cr(OH)_(4)]^(-)`
D. `[Cr(OH)_(3)(H_(2)O)_(3)]`

Answer

To solve the question regarding the color change when aqueous NaOH is added to an aqueous solution of chromium (III) ions, we can break down the process step by step.

### Step-by-Step Solution:

1. **Identify the Initial Species**:
- We start with chromium (III) ions in an aqueous solution, represented as \( \text{Cr}^{3+} \) in water, which can be written as \( \text{Cr(H}_2\text{O)}_6^{3+} \).

2. **Reaction with Sodium Hydroxide (NaOH)**:
- When aqueous NaOH is added to the solution, hydroxide ions (\( \text{OH}^- \)) react with the chromium (III) ions. The reaction can be represented as:
\[ \text{Cr(H}_2\text{O)}_6^{3+} + 3 \text{OH}^- \rightarrow \text{Cr(OH)}_3 \cdot \text{H}_2\text{O} + 3 \text{H}_2\text{O} \]
- This results in the formation of a green-blue precipitate of chromium (III) hydroxide, \( \text{Cr(OH)}_3 \).

3. **Observation of Precipitate**:
- The green-blue precipitate is observed initially when the hydroxide ions are added.

4. **Dissolution of Precipitate**:
- Upon further addition of NaOH, the precipitate can dissolve, leading to the formation of a soluble complex ion. The reaction can be represented as:
\[ \text{Cr(OH)}_3 + \text{OH}^- \rightarrow \text{Cr(OH)}_4^{-} \]
- This results in a green solution due to the formation of the complex ion \( \text{Cr(OH)}_4^{-} \).

5. **Conclusion**:
- The green color of the solution is attributed to the presence of the complex ion \( \text{Cr(OH)}_4^{-} \).

### Final Answer:
The green color is due to the formation of the complex ion \( \text{Cr(OH)}_4^{-} \).

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