Concentrated `HNO_3`, upon long standing, turns yellow-brown due to the formation of

Options

A. `NO`
B. `NO_2`
C. `N_2 O`
D. `N_2 O_4`

Explanation

To solve the question about why concentrated nitric acid (HNO3) turns yellow-brown upon long standing, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Composition of Concentrated Nitric Acid**:
- Concentrated nitric acid is a strong oxidizing agent and is typically colorless when fresh.

2. **Identifying the Change Over Time**:
- When left standing for a long time, concentrated nitric acid undergoes a change in color, turning yellow-brown.

3. **Recognizing the Reaction**:
- The color change is due to a decomposition reaction. In this reaction, one reactant (HNO3) breaks down into multiple products.

4. **Writing the Decomposition Reaction**:
- The decomposition of concentrated nitric acid can be represented by the following reaction:
\[ 4 \text{HNO}_3 \rightarrow 4 \text{NO}_2 + 2 \text{H}_2\text{O} + \text{O}_2 \]
- In this reaction, nitric acid decomposes to form nitrogen dioxide (NO2), water (H2O), and oxygen (O2).

5. **Identifying the Product Responsible for Color Change**:
- The nitrogen dioxide (NO2) produced in this reaction is a brown gas. Its presence in the solution is what causes the yellow-brown color of the concentrated nitric acid.

6. **Conclusion**:
- Therefore, the yellow-brown color observed in concentrated nitric acid upon long standing is due to the formation of nitrogen dioxide (NO2) as a result of its decomposition.

### Final Answer:
Concentrated HNO3, upon long standing, turns yellow-brown due to the formation of nitrogen dioxide (NO2).

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