Which of the following can be oxidised by acidified `K_2Cr_2O_7` ?

Options

A. Nitrate ions
B. Sulphite ions
C. Ferric ions
D. Borate ions

Answer

To determine which of the given ions can be oxidized by acidified potassium dichromate (K₂Cr₂O₇), we need to analyze the oxidation states of the elements in each ion.

### Step-by-Step Solution:

1. **Identify the Ions:**
The ions given are:
- Nitrate ions (NO₃⁻)
- Sulphite ions (SO₃²⁻)
- Ferric ions (Fe³⁺)
- Borate ions (BO₃³⁻)

2. **Analyze Nitrate Ions (NO₃⁻):**
- The oxidation state of nitrogen in NO₃⁻ is +5.
- Since +5 is the maximum oxidation state for nitrogen, it cannot be oxidized further.
- **Conclusion:** Nitrate ions cannot be oxidized by K₂Cr₂O₇.

3. **Analyze Sulphite Ions (SO₃²⁻):**
- The oxidation state of sulfur in SO₃²⁻ is +4.
- The maximum oxidation state for sulfur is +6 (as in sulfate, SO₄²⁻).
- Therefore, sulphite ions can be oxidized to sulfate ions.
- **Conclusion:** Sulphite ions can be oxidized by K₂Cr₂O₇.

4. **Analyze Ferric Ions (Fe³⁺):**
- The oxidation state of iron in Fe³⁺ is +3.
- The maximum oxidation state for iron is also +3.
- Hence, ferric ions cannot be oxidized further.
- **Conclusion:** Ferric ions cannot be oxidized by K₂Cr₂O₇.

5. **Analyze Borate Ions (BO₃³⁻):**
- The oxidation state of boron in BO₃³⁻ is +3.
- The maximum oxidation state for boron is +3.
- Therefore, borate ions cannot be oxidized further.
- **Conclusion:** Borate ions cannot be oxidized by K₂Cr₂O₇.

### Final Conclusion:
Among the given options, only the sulphite ions (SO₃²⁻) can be oxidized by acidified potassium dichromate (K₂Cr₂O₇).

### Answer:
**The correct choice is the sulphite ions (SO₃²⁻).**

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