Metal which become passive with conc. `HNO_(3)`

Options

A. `Cr`
B. Zn
C. Al
D. Both (1) & (3)

Answer

To determine which metal becomes passive with concentrated HNO₃ (nitric acid), we can analyze the properties of the given metals: chromium, zinc, and aluminum.

### Step-by-Step Solution:

1. **Understanding Passivation**:
- Passivation is a process where a metal becomes less reactive due to the formation of a protective oxide layer on its surface. This layer prevents further corrosion or reaction with acids.

2. **Analyzing the Metals**:
- **Chromium (Cr)**: When chromium is treated with concentrated HNO₃, it reacts to form chromium nitrate (Cr(NO₃)₃) and nitrogen oxides (NO). The reaction also leads to the formation of a protective oxide layer, which makes chromium passive.
- **Zinc (Zn)**: Zinc does not become passive with concentrated HNO₃. Instead, it reacts vigorously and does not form a protective layer.
- **Aluminum (Al)**: Aluminum also reacts with concentrated HNO₃ to form aluminum nitrate (Al(NO₃)₃) and nitrogen oxides (NO). Similar to chromium, aluminum can also become passive due to the formation of a protective oxide layer.

3. **Conclusion**:
- Both chromium and aluminum can become passive when treated with concentrated HNO₃. Therefore, the correct answer to the question is that both chromium and aluminum are metals that become passive with concentrated HNO₃.

### Final Answer:
The metals which become passive with concentrated HNO₃ are **Chromium and Aluminum**.