Copper dissolves in dilute nitric acid but not in dilute `HCl`. Why?

Answer

To answer the question of why copper dissolves in dilute nitric acid (HNO3) but not in dilute hydrochloric acid (HCl), we can break down the explanation into several key points:

### Step-by-Step Solution:

1. **Understanding Oxidation Potential**:
- Copper (Cu) has a standard oxidation potential that is less than that of hydrogen (H). This means that copper cannot displace hydrogen from acids that release hydrogen ions (H+) in solution.

2. **Behavior in Dilute HCl**:
- When copper is placed in dilute hydrochloric acid, it does not react because it cannot displace hydrogen. The reaction would require copper to be oxidized to Cu²⁺, but since it has a lower oxidation potential than hydrogen, it remains unreacted.

3. **Behavior in Dilute HNO3**:
- In contrast, dilute nitric acid (HNO3) acts as a strong oxidizing agent. When copper is introduced to dilute HNO3, the nitric acid oxidizes copper to Cu²⁺ ions.
- The reaction can be represented as:
\[ 3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 4H_2O + 2NO_2 \]
- Here, copper is oxidized, and nitrogen dioxide (NO2) is produced as a byproduct along with copper(II) nitrate and water.

4. **Conclusion**:
- Therefore, the key difference lies in the nature of the acids: HCl does not oxidize copper due to its inability to displace hydrogen, while HNO3, being a strong oxidizing agent, successfully oxidizes copper, allowing it to dissolve.