A dark green bead in the borax bead test (in oxidising flame) indicates the presence of:

Options

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

Answer

To determine the presence of ions based on the color of the bead formed in the borax bead test when subjected to an oxidizing flame, we can analyze the different metal ions mentioned in the question. Here's a step-by-step solution:

### Step-by-Step Solution:

1. **Understanding the Borax Bead Test**:
- The borax bead test involves heating borax (sodium borate) and then dipping it into a metal salt. When heated again, the metal ions react with borax to form a colored bead.

2. **Identifying the Metal Ions**:
- The question provides four options: Chromium (Cr), Manganese (Mn), Cobalt (Co), and Nickel (Ni). We need to determine which of these ions produces a dark green bead in the borax bead test.

3. **Analyzing Each Metal Ion**:
- **Chromium (Cr)**: When chromium (specifically Cr³⁺) is heated with borax, it forms chromium metaborate (Cr₂B₂O₇), which gives a dark green color in the oxidizing flame.
- **Manganese (Mn)**: Manganese (Mn²⁺) when heated with borax forms manganese metaborate (MnB₂O₄), which produces a violet coloration.
- **Cobalt (Co)**: Cobalt (Co²⁺) reacts with borax to form cobalt metaborate (CoB₂O₄), resulting in a blue coloration.
- **Nickel (Ni)**: Nickel (Ni²⁺) reacts with borax to form nickel metaborate (NiB₂O₄), which gives a grey coloration.

4. **Conclusion**:
- Based on the analysis, the only metal ion that produces a dark green bead in the borax bead test in an oxidizing flame is Chromium (Cr³⁺).

### Final Answer:
The dark green bead in the borax bead test (in oxidizing flame) indicates the presence of **Chromium (Cr³⁺)**.

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