Which of the following pairs of cations will turn borax beads blue in an oxidising flame?

Options

A. `Fe^(2+) and Co^(2+)`
B. `Co^(2+) and Cu^(2+)`
C. `Cu^(2+) and Mn^(2+)`
D. `Cu^(2+) and Cr^(3+)`

Answer

To determine which pair of cations will turn borax beads blue in an oxidizing flame, we will analyze the behavior of each cation when heated in the presence of borax.

### Step-by-Step Solution:

1. **Understanding Borax Bead Test**:
- The borax bead test is used to identify metal ions based on the color change of the borax bead when heated in a flame. The color of the bead can change depending on the cation present and the temperature of the bead (hot or cold).

2. **Analyzing Each Cation**:
- **Cobalt (Co²⁺)**:
- Hot: Blue
- Cold: Blue
- **Copper (Cu²⁺)**:
- Hot: Green
- Cold: Blue
- **Iron (Fe²⁺)**:
- Hot: Yellowish-brown
- Cold: Yellow
- **Manganese (Mn²⁺)**:
- Hot: Violet
- Cold: Amethyst
- **Chromium (Cr³⁺)**:
- Hot: Yellow
- Cold: Green

3. **Identifying Pairs**:
- We need to find pairs of cations that will result in a blue color in the borax bead when heated.
- From the analysis:
- **Cobalt (Co²⁺)** consistently gives a blue color both hot and cold.
- **Copper (Cu²⁺)** gives a blue color when cold, but green when hot.

4. **Evaluating the Options**:
- **Option 1**: Iron (Fe²⁺) and Cobalt (Co²⁺) - Cobalt is blue, Iron does not contribute to blue.
- **Option 2**: Cobalt (Co²⁺) and Copper (Cu²⁺) - Cobalt is blue, Copper turns blue when cold.
- **Option 3**: Copper (Cu²⁺) and Manganese (Mn²⁺) - Copper turns blue when cold, Manganese does not contribute to blue.
- **Option 4**: Copper (Cu²⁺) and Chromium (Cr³⁺) - Copper turns blue when cold, Chromium does not contribute to blue.

5. **Conclusion**:
- The only pair that consistently leads to a blue borax bead in an oxidizing flame is **Cobalt (Co²⁺) and Copper (Cu²⁺)**. Therefore, the correct answer is **Option 2**.