Pure conc. `HNO_(3)` makes iron passive as the surface is covered with protective layer of

Options

A. `Fe_(2)O_(3)`
B. `FeO`
C. `Fe_(3)O_(4)`
D. `Fe(NO_(3))_(3)`

Explanation

To solve the question regarding the protective layer formed on iron when treated with pure concentrated HNO₃, we can follow these steps:

### Step 1: Understanding the Reaction
Pure concentrated nitric acid (HNO₃) is a strong oxidizing agent. When it comes into contact with iron (Fe), it reacts with the metal.

**Hint:** Recall that strong oxidizing agents can cause metals to oxidize, leading to the formation of metal oxides.

### Step 2: Formation of Iron Oxide
During the reaction, iron is oxidized to form iron oxide. The specific oxide formed in this case is ferric oxide (Fe₂O₃). This occurs because HNO₃ oxidizes iron, and the reaction can be represented as:

\[ 4Fe + 6HNO₃ \rightarrow 4Fe(NO₃)₃ + 3H₂O + 2NO_2 \]

**Hint:** Remember that the oxidation state of iron increases when it reacts with an oxidizing agent.

### Step 3: Protective Layer
The ferric oxide (Fe₂O₃) that forms creates a protective layer on the surface of the iron. This layer is non-reactive and prevents further oxidation of the underlying iron.

**Hint:** Consider how protective coatings can prevent further reactions in metals.

### Step 4: Conclusion
Thus, the protective layer that forms on the surface of iron when treated with concentrated HNO₃ is ferric oxide (Fe₂O₃).

**Final Answer:** The protective layer is Fe₂O₃.

### Summary of Steps:
1. Recognize that HNO₃ is a strong oxidizing agent.
2. Identify the reaction of iron with HNO₃ leading to the formation of iron oxide.
3. Understand that Fe₂O₃ acts as a protective layer.
4. Conclude that the protective layer is Fe₂O₃.