Which of the following cation (s) will turn blue in oxidising flame ?

Options

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

Answer

To determine which cations will turn blue in an oxidizing flame, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context**: The question pertains to the behavior of certain cations when subjected to an oxidizing flame, specifically in the context of the borax bead test.

2. **Identify the Cations**: The cations we need to analyze are:
- Cobalt (Co²⁺)
- Chromium (Cr³⁺)
- Nickel (Ni²⁺)
- Copper (Cu²⁺)

3. **Analyze Each Cation**:
- **Cobalt (Co²⁺)**: When cobalt is heated in an oxidizing flame, it imparts a blue color. This is due to the formation of cobalt borate (CoO·Na2B4O7), which appears blue.
- **Chromium (Cr³⁺)**: Chromium ions impart a yellow color to the flame when heated. Therefore, it does not turn blue.
- **Nickel (Ni²⁺)**: Nickel ions produce a yellow-brown color in the flame, indicating that it also does not turn blue.
- **Copper (Cu²⁺)**: Copper ions impart a blue color to the flame, similar to cobalt. The reaction forms copper borate (CuO·Na2B4O7), which appears blue.

4. **Conclusion**: Based on the analysis, the cations that turn blue in an oxidizing flame are:
- Cobalt (Co²⁺)
- Copper (Cu²⁺)

5. **Final Answer**: The correct cations that will turn blue in an oxidizing flame are cobalt (Co²⁺) and copper (Cu²⁺).