Aluminium becomes passive in :

Options

A. `Cr_(2)O_(3)`
B. `conc HNO_(3)`
C. `HClO_(4)`
D. All of the above

Explanation

To determine when aluminum becomes passive, we need to analyze the options provided and understand the concept of passivity in aluminum.

### Step-by-Step Solution:

1. **Understanding Passivity**:
- Aluminum can become passive when it forms a thin layer of aluminum oxide (Al2O3) on its surface. This oxide layer protects the aluminum from further reactions, making it resistant to corrosion.

2. **Analyzing the Options**:
- **Option 1: Cr2O3**:
- When aluminum reacts with chromium(III) oxide (Cr2O3), it can produce chromium metal and aluminum oxide. The formation of aluminum oxide indicates that aluminum becomes passive in this reaction.

- **Option 2: Concentrated HNO3**:
- Concentrated nitric acid (HNO3) can also react with aluminum. In this case, the reaction leads to the formation of a protective oxide layer (Al2O3) on the aluminum surface, which makes it passive. This is why aluminum can be safely stored in containers made of aluminum when using concentrated HNO3.

- **Option 3: HClO4**:
- Perchloric acid (HClO4) can similarly react with aluminum, leading to the formation of aluminum oxide on its surface. This also results in passivity, protecting the aluminum from further corrosion.

3. **Conclusion**:
- Since aluminum becomes passive in the presence of Cr2O3, concentrated HNO3, and HClO4, the correct answer is that aluminum becomes passive in **all of the above**.

### Final Answer:
Aluminum becomes passive in **all of the above** (Cr2O3, concentrated HNO3, and HClO4).

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