When conc. `H_2 SO_4` is added to dry `KNO_3` brown fumes evolve. These fumes are of

Options

A. `SO_(2)`
B. `SO_(3)`
C. `NO`
D. `NO_(2)`

Answer

To solve the question, we need to analyze the reaction that occurs when concentrated sulfuric acid (H₂SO₄) is added to dry potassium nitrate (KNO₃) and identify the gas that produces brown fumes.

### Step-by-Step Solution:

1. **Identify the Reactants**:
- The reactants in this scenario are concentrated sulfuric acid (H₂SO₄) and dry potassium nitrate (KNO₃).

2. **Write the Initial Reaction**:
- When concentrated H₂SO₄ is added to KNO₃, a reaction occurs that produces potassium hydrogen sulfate (KHSO₄) and nitric acid (HNO₃):
\[ \text{KNO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{KHSO}_4 + \text{HNO}_3 \]

3. **Decomposition of Nitric Acid**:
- The nitric acid (HNO₃) produced can further decompose. The decomposition of nitric acid can be represented as follows:
\[ 4 \text{HNO}_3 \rightarrow 2 \text{H}_2\text{O} + 4 \text{NO}_2 + \text{O}_2 \]
- This reaction indicates that nitric acid decomposes to produce nitrogen dioxide (NO₂), water (H₂O), and oxygen (O₂).

4. **Identify the Gas**:
- Among the products of the decomposition, nitrogen dioxide (NO₂) is the gas that is known to produce brown fumes.

5. **Conclusion**:
- Therefore, when concentrated H₂SO₄ is added to dry KNO₃, the brown fumes that evolve are due to nitrogen dioxide (NO₂). The correct answer is option D: NO₂.

### Final Answer:
The brown fumes evolved are of nitrogen dioxide (NO₂).