Conc. `HNO_(3)` is treated with iron. The metal is passive because

Options

A. It is a transition metal
B. It forms protective oxide film
C. It is reduced
D. It liberates laughing gas

Answer

To solve the question "Conc. HNO3 is treated with iron. The metal is passive because," we need to analyze the behavior of iron when it comes into contact with concentrated nitric acid (HNO3).

### Step-by-Step Solution:

1. **Understanding the Reaction**: When concentrated HNO3 is added to iron, a chemical reaction occurs. Iron, being a reactive metal, would typically react with acids. However, in this case, the reaction leads to the formation of a protective layer.

2. **Formation of Protective Oxide Layer**: As iron reacts with concentrated nitric acid, it forms iron oxides (such as Fe2O3 or Fe3O4) on its surface. This layer acts as a barrier that prevents further reaction between the iron and the nitric acid.

3. **Passive State of Iron**: Once this protective oxide layer is formed, the underlying iron is no longer exposed to the nitric acid. This means that the metal becomes passive, as it does not react further with the acid.

4. **Conclusion**: The reason why iron becomes passive when treated with concentrated HNO3 is that it forms a protective oxide filling over its surface, which inhibits any further reaction.

### Final Answer:
The metal is passive because it forms a protective oxide filling.