Which one of the following metal salts produces a blue coloured bead in cobalt nitrate charcoal cavity test ?

Options

A. `Zn^(2+)`
B. `Mg^(2+)`
C. `Sn^(2+)`
D. `Al^(3+)`

Answer

To determine which metal salt produces a blue-colored bead in the cobalt nitrate charcoal cavity test, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Cobalt Nitrate Charcoal Cavity Test**:
- This test is used to identify certain metal ions based on the color of the bead formed when the metal salt is heated with cobalt nitrate and charcoal.

2. **Identify the Metal Salts**:
- The question provides several options for metal salts. We need to analyze which of these salts is known to produce a blue-colored bead.

3. **Know the Color Indicators**:
- In the cobalt nitrate charcoal cavity test, certain metal ions produce characteristic colors. For example, aluminum ions (Al³⁺) are known to produce a blue bead when heated with cobalt nitrate.

4. **Determine the Relevant Metal Salt**:
- Among the options given, we need to find the one that contains aluminum ions. The metal salt that produces a blue-colored bead is Al₂O₃·Coo (aluminum oxide combined with cobalt).

5. **Conclusion**:
- The metal salt that produces a blue-colored bead in the cobalt nitrate charcoal cavity test is Al₂O₃·Coo. Therefore, the correct answer is option D.

### Final Answer:
- The metal salt that produces a blue-colored bead in the cobalt nitrate charcoal cavity test is **Al₂O₃·Coo** (option D).