How is the presence of `SO_2` detected?

Explanation

To detect the presence of sulfur dioxide (SO₂), we can use two common chemical tests involving potassium permanganate and potassium dichromate. Here’s a step-by-step solution:

### Step 1: Test with Potassium Permanganate (KMnO₄)

1. **Understanding the Reaction**: Sulfur dioxide acts as a reducing agent. When it reacts with potassium permanganate, it reduces the permanganate ion (MnO₄⁻) to manganese(II) ion (Mn²⁺), while itself being oxidized to sulfate ion (SO₄²⁻).

2. **Writing the Balanced Reaction**:
- The unbalanced reaction is:
\[ SO_2 + KMnO_4 \rightarrow Mn^{2+} + SO_4^{2-} \]
- Balancing the reaction gives:
\[ 5SO_2 + 2MnO_4^- + 4H^+ \rightarrow 2Mn^{2+} + 5SO_4^{2-} + 2H_2O \]
- This indicates that 5 moles of SO₂ react with 2 moles of KMnO₄ in an acidic medium.

3. **Observation**: Initially, the solution containing MnO₄⁻ is pink-violet due to the presence of permanganate ions. After the reaction, the solution becomes colorless due to the formation of Mn²⁺ ions, indicating the presence of sulfur dioxide.

### Step 2: Test with Potassium Dichromate (K₂Cr₂O₇)

1. **Understanding the Reaction**: Similar to the previous test, sulfur dioxide reduces the dichromate ion (Cr₂O₇²⁻) to chromium(III) ion (Cr³⁺) while being oxidized to sulfate ion (SO₄²⁻).

2. **Writing the Balanced Reaction**:
- The unbalanced reaction is:
\[ SO_2 + K_2Cr_2O_7 \rightarrow Cr^{3+} + SO_4^{2-} \]
- Balancing the reaction gives:
\[ 3SO_2 + K_2Cr_2O_7 + 4H^+ \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 2H_2O \]
- This indicates that 3 moles of SO₂ react with 1 mole of K₂Cr₂O₇ in an acidic medium.

3. **Observation**: Initially, the solution containing Cr₂O₇²⁻ is orange. After the reaction with sulfur dioxide, the solution changes to green due to the formation of Cr³⁺ ions, indicating the presence of sulfur dioxide.

### Conclusion:
The presence of sulfur dioxide can be detected using two tests:
1. **Potassium Permanganate Test**: Color change from pink-violet to colorless.
2. **Potassium Dichromate Test**: Color change from orange to green.