When nitric acid reacts with metals, nitrogen dioxide is usually evolved if the acid is

Options

A. dilute
B. very dilute
C. moderately strong
D. concentrated

Answer

### Step-by-Step Solution:

1. **Understanding Nitric Acid (HNO3)**:
- Nitric acid is a strong acid and can exist in various concentrations: dilute, moderately strong, and concentrated.

2. **Reactivity of Nitric Acid with Metals**:
- When nitric acid reacts with metals, it typically leads to the evolution of nitrogen dioxide (NO2) gas. This is a characteristic reaction of nitric acid with metals.

3. **Conditions for NO2 Evolution**:
- The evolution of nitrogen dioxide occurs when nitric acid is either dilute, moderately strong, or concentrated.
- The concentration of the acid influences the products formed during the reaction.

4. **Oxidation of Hydrogen**:
- In the presence of nitric acid, hydrogen gas (H2) can be oxidized. This means that when metals react with nitric acid, they can release hydrogen gas, which is then oxidized by the nitric acid.

5. **Conclusion**:
- Therefore, when nitric acid reacts with metals, nitrogen dioxide is usually evolved if the acid is dilute, moderately strong, or concentrated.