Which of the following cations will turns a borax bead green in an oxidising flame?

Options

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

Answer

To determine which cation will turn a borax bead green in an oxidizing flame, we will follow these steps:

### Step 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 change of a borax bead when heated in a flame. Borax, chemically known as sodium borate (Na2B4O7·10H2O), is heated to form a glassy bead.

### Step 2: Formation of the Borax Bead
When borax is heated, it loses water and forms sodium metaborate (NaBO2) and boron trioxide (B2O3). The borax bead is formed when these compounds react with metal oxides.

### Step 3: Identify the Cations
The cations provided in the question are:
1. Fe²⁺ (Ferrous ion)
2. Mn²⁺ (Manganese ion)
3. Cr³⁺ (Chromium ion)
4. Cu²⁺ (Copper ion)

### Step 4: Determine the Color Change for Each Cation
In an oxidizing flame, different cations produce specific colors in the borax bead:
- **Fe²⁺**: Turns the bead yellow.
- **Mn²⁺**: Turns the bead violet.
- **Cr³⁺**: Turns the bead green.
- **Cu²⁺**: Turns the bead blue.

### Step 5: Conclusion
From the above observations, we can conclude that the cation that turns the borax bead green in an oxidizing flame is **Cr³⁺ (Chromium ion)**.

### Final Answer
The cation that will turn a borax bead green in an oxidizing flame is **Chromium (Cr³⁺)**.

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