The element evolving two different gases on reaction with `conc. H_2 SO_4`.

Options

A. P
B. C
C. Hg
D. S

Explanation

To determine which element evolves two different gases when reacting with concentrated sulfuric acid (H₂SO₄), we can analyze the reactions of the given options: phosphorus, carbon, mercury, and sulfur.

### Step-by-Step Solution:

1. **Identify the Reactions**: We will examine the reactions of each element with concentrated H₂SO₄.

2. **Reaction with Phosphorus**:
- When phosphorus (P) reacts with concentrated H₂SO₄, it produces phosphoric acid (H₃PO₄) and sulfur dioxide (SO₂).
- The balanced equation is:
\[ 5 \text{H}_2\text{SO}_4 + 2 \text{P} \rightarrow 2 \text{H}_3\text{PO}_4 + 5 \text{SO}_2 + 2 \text{H}_2\text{O} \]
- **Gases Produced**: Only SO₂.

3. **Reaction with Carbon**:
- When carbon (C) reacts with concentrated H₂SO₄, it produces carbon dioxide (CO₂) and sulfur dioxide (SO₂).
- The balanced equation is:
\[ 2 \text{H}_2\text{SO}_4 + \text{C} \rightarrow \text{CO}_2 + 2 \text{SO}_2 + 2 \text{H}_2\text{O} \]
- **Gases Produced**: CO₂ and SO₂ (two different gases).

4. **Reaction with Mercury**:
- When mercury (Hg) reacts with concentrated H₂SO₄, it produces mercury(II) sulfate (HgSO₄) and sulfur dioxide (SO₂).
- The balanced equation is:
\[ 2 \text{H}_2\text{SO}_4 + \text{Hg} \rightarrow \text{HgSO}_4 + 2 \text{SO}_2 + 2 \text{H}_2\text{O} \]
- **Gases Produced**: Only SO₂.

5. **Reaction with Sulfur**:
- When sulfur (S) reacts with concentrated H₂SO₄, it produces sulfur dioxide (SO₂) and water.
- The balanced equation is:
\[ 2 \text{H}_2\text{SO}_4 + \text{S} \rightarrow 3 \text{SO}_2 + 2 \text{H}_2\text{O} \]
- **Gases Produced**: Only SO₂.

6. **Conclusion**:
- The only element that produces two different gases (CO₂ and SO₂) when reacting with concentrated H₂SO₄ is **Carbon** (option 2).

### Final Answer:
**Carbon (Option 2)** is the correct answer.