Which acid makes iron passive?

Options

A. Sulphuric acid
B. Fuming nitric acid
C. Hydrofluoric acid
D. Hydrochloric acid

Answer

To determine which acid makes iron passive, we can analyze the options provided:

1. **Identify the Acids**: We have four acids to consider:
- Sulfuric acid (H₂SO₄)
- Fuming nitric acid (HNO₃)
- Hydrofluoric acid (HF)
- Hydrochloric acid (HCl)

2. **Understand the Concept of Passivation**: Passivation refers to the 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 the environment.

3. **Analyze Each Acid**:
- **Sulfuric Acid (H₂SO₄)**: While it can react with iron, it does not form a protective oxide layer that passivates iron.
- **Fuming Nitric Acid (HNO₃)**: This acid is a strong oxidizing agent. When it reacts with iron, it oxidizes the iron and forms a layer of iron oxide (Fe₂O₃) on the surface. This oxide layer is non-reactive and prevents further reactions, effectively making the iron passive.
- **Hydrofluoric Acid (HF)**: HF is known for its ability to react with silicates and does not form a passivating layer on iron.
- **Hydrochloric Acid (HCl)**: HCl reacts with iron to produce iron chloride and hydrogen gas, but it does not create a protective oxide layer.

4. **Conclusion**: Based on the analysis, fuming nitric acid (HNO₃) is the acid that makes iron passive by forming a protective layer of iron oxide (Fe₂O₃) on its surface.

Therefore, the correct answer is **fuming nitric acid (HNO₃)**.