On strongly heating, a blue salt leaves a black residue. Which of the following cations can be present in the salt?

Options

A. `Fe^(2+)`
B. `Fe^(3+)`
C. `Cu^(2+)`
D. `Zn^(2+)`

Answer

To solve the question regarding which cation can be present in a blue salt that leaves a black residue upon strong heating, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Color of the Salt**:
- The problem states that the salt is blue. We need to identify which cations typically form blue salts.

2. **Analyze the Given Cations**:
- The cations provided are:
- Fe²⁺ (ferrous)
- Fe³⁺ (ferric)
- Cu²⁺ (cupric)
- Zn²⁺ (zinc)
- We know that:
- Fe²⁺ salts are usually green.
- Fe³⁺ salts are typically yellow or brown.
- Cu²⁺ salts are blue (e.g., copper(II) sulfate).
- Zn²⁺ salts are white (e.g., zinc sulfide).

3. **Determine the Likely Cation**:
- From the analysis, only Cu²⁺ (cupric) forms blue salts, such as copper(II) sulfate (CuSO₄·5H₂O).

4. **Heating the Blue Salt**:
- When copper(II) sulfate is strongly heated, it undergoes decomposition:
- The hydrated form (blue) loses water and becomes anhydrous (white).
- Further heating leads to the formation of copper(II) oxide (CuO), which is black, and sulfur trioxide (SO₃) gas is released.

5. **Identify the Black Residue**:
- The black residue formed after heating is copper(II) oxide (CuO), confirming that the cation present in the original blue salt is Cu²⁺.

6. **Conclusion**:
- Therefore, the cation that can be present in the blue salt is Cu²⁺.

### Final Answer:
The cation that can be present in the blue salt is **Cu²⁺ (cupric cation)**.

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