Iron is rendered passive by treatment with:

Options

A. `H_(2)SO_(4)`(dil)
B. `H_(2)PO_(4)`
C. conc.`HNO_(3)`
D. HCl

Answer

To solve the question "Iron is rendered passive by treatment with:", we will analyze each of the given options and determine which one is correct based on the properties of iron and its reactions with different substances.

### Step-by-Step Solution:

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

2. **Analyzing the Options**:
- **Option 1: Dilute Sulfuric Acid (H2SO4)**:
- Dilute sulfuric acid reacts with iron, leading to the formation of iron sulfate and hydrogen gas. This does not lead to passivation.
- **Option 2: Phosphoric Acid (H3PO4)**:
- Phosphoric acid can react with iron but does not effectively create a protective oxide layer that would render iron passive.
- **Option 3: Concentrated Nitric Acid (HNO3)**:
- Concentrated nitric acid oxidizes iron and forms a protective layer of iron oxide (Fe2O3). This oxide layer prevents further reaction of iron with the environment, thus rendering it passive.
- **Option 4: Hydrochloric Acid (HCl)**:
- Hydrochloric acid reacts with iron to produce iron chloride and hydrogen gas, which does not lead to passivation.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Concentrated Nitric Acid (HNO3)**, as it effectively renders iron passive by forming a protective oxide layer.

### Final Answer:
Iron is rendered passive by treatment with **Concentrated Nitric Acid (HNO3)**.

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