Despite the fact that aluminium is a reactive metal, it is a stable in air as well as in water. This is due to the formation of thin layer of oxide on the surface of aluminium metal which makes it passive for further attack. The layer is so useful, that in industry, it is purposely deposited by an electrolytic process called anodising. Reaction of aluminium with oxygen is highly exothermic and is called thermite reaction.
`2Al_((s)) +(3)/(2) O_(2(g)) rarr Al_(2)O_(3(s)) Delta_(f) H^(Ө) = -1670 kJmol^-1`
This reaction finds application for welding of metal and in the metallurgical extraction of many metals from their oxides.
The limitation is that to start the reaction, high temperature is required for which an ignition mixture is required.
Aluminium becomes passive in :

Options

A. `H_(2)CrO_(4)`
B. `conc HNO_(3)`
C. `HClO_(4)`
D. `NaOH`

Explanation

To determine in which substance aluminium becomes passive, we need to analyze the options provided and understand the behavior of aluminium in the presence of each substance.

### Step-by-Step Solution:

1. **Understanding Aluminium's Reactivity**:
Aluminium is a reactive metal that typically forms an oxide layer (Al₂O₃) on its surface when exposed to air or water. This oxide layer protects the underlying metal from further reactions, making aluminium passive.

2. **Analyzing the Options**:
- **Option 1: H₂CrO₄ (Chromic Acid)**:
When aluminium reacts with chromic acid, it does not form an oxide layer. Instead, it forms aluminium chromate (AlCrO₄) and liberates hydrogen gas. Therefore, aluminium does not become passive in this case.

- **Option 2: Concentrated HNO₃ (Nitric Acid)**:
Concentrated nitric acid reacts with aluminium to form a protective oxide layer (Al₂O₃) on the surface. This layer prevents further reaction with the acid, thus making aluminium passive in the presence of concentrated HNO₃.

- **Option 3: HClO₄ (Perchloric Acid)**:
Similar to chromic acid, when aluminium reacts with perchloric acid, it does not form an oxide layer. Instead, it forms aluminium perchlorate (AlClO₄) and liberates hydrogen gas. Therefore, aluminium does not become passive in this case either.

- **Option 4: NaOH (Sodium Hydroxide)**:
Aluminium reacts with sodium hydroxide in the presence of water to form sodium aluminate. This reaction does not lead to the formation of an oxide layer, so aluminium does not become passive in this case.

3. **Conclusion**:
Based on the analysis, aluminium becomes passive only in the presence of concentrated HNO₃ due to the formation of a protective oxide layer.

### Final Answer:
Aluminium becomes passive in concentrated HNO₃.

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