Which gives blue bead in borax bead test ?

Options

A. `Cr^(3+)`
B. `Co^(2+)`
C. `Ni^(2+)`
D. `Cd^(2+)`

Explanation

To solve the question "Which gives blue bead in borax bead test?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Borax Bead Test**: The borax bead test is a qualitative analysis method used to identify certain metal ions based on the color of the bead formed when a metal salt is heated with borax (sodium borate).

2. **Identify the Metal Ions**: The metal ions that are commonly tested in the borax bead test include cobalt (Co), nickel (Ni), and others. Each metal ion produces a characteristic color when heated.

3. **Focus on Cobalt**: Among the common metal ions, cobalt is known to produce a blue bead when subjected to the borax bead test. This is due to the formation of cobalt borate, which has a blue color.

4. **Chemical Composition**: The chemical formula for the compound formed in the borax bead test with cobalt is \( \text{Co}_2\text{B}_2\text{O}_7 \) or similar forms, but the important point is that it produces a blue color.

5. **Conclusion**: Therefore, the metal ion that gives a blue bead in the borax bead test is \( \text{Co}^{2+} \) (cobalt ion).

### Final Answer:
The ion that gives a blue bead in the borax bead test is \( \text{Co}^{2+} \).