A salt gives white residue in charcoal cavity test but in cobalt nitrate test it gives pink mass. It represents :

Options

A. `Zn^(+2)`
B. `Al^(+3)`
C. `Mg^(+2)`
D. `PO_(4)^(-3)`

Answer

To solve the problem, we need to analyze the given information about the salt and the tests it undergoes.

### Step-by-Step Solution:

1. **Understanding the Tests**:
- The **charcoal cavity test** is used to determine the presence of certain metal ions. A white residue indicates the presence of specific metal ions, such as magnesium.
- The **cobalt nitrate test** involves heating the salt with cobalt nitrate, which can produce a pink mass if certain ions are present, particularly magnesium ions.

2. **Analyzing the Given Information**:
- The salt gives a **white residue** in the charcoal cavity test. This suggests the presence of magnesium ions (Mg²⁺).
- The salt also gives a **pink mass** in the cobalt nitrate test, which further supports the presence of magnesium ions.

3. **Evaluating the Options**:
- **Zinc (Zn²⁺)**: Does not produce a white residue in the charcoal cavity test and does not give a pink mass with cobalt nitrate. So, it can be eliminated.
- **Aluminum (Al³⁺)**: Does not produce a white residue in the charcoal cavity test and also does not give a pink mass. So, it can also be eliminated.
- **Magnesium (Mg²⁺)**: Produces a white residue in the charcoal cavity test and gives a pink mass with cobalt nitrate. This matches our observations.
- **Phosphate ion (PO₄³⁻)**: Does not produce a white residue in the charcoal cavity test and does not give a pink mass. So, it can be eliminated.

4. **Conclusion**:
- The salt that gives a white residue in the charcoal cavity test and a pink mass in the cobalt nitrate test is **Magnesium (Mg²⁺)**.

### Final Answer:
The salt represents **Magnesium ion (Mg²⁺)**.