What is passivity of Aluminium in presence of `HNO_3` ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Passivity:**
- Passivity refers to the phenomenon where a metal becomes resistant to further corrosion or reaction due to the formation of a protective layer on its surface.

2. **Reaction of Aluminium with Concentrated HNO3:**
- When aluminium (Al) comes into contact with concentrated nitric acid (HNO3), a chemical reaction occurs. The balanced chemical equation for this reaction is:
\[ 2Al + 6HNO_3 \rightarrow 2Al_2O_3 + 6NO_2 + 3H_2O \]
- In this reaction, aluminium oxide (Al2O3), nitrogen dioxide (NO2), and water (H2O) are produced.

3. **Formation of Aluminium Oxide:**
- The aluminium oxide (Al2O3) that is formed during the reaction creates a thin, protective coating over the surface of the aluminium metal.

4. **Effect of the Protective Coating:**
- This coating of aluminium oxide acts as a barrier, preventing further contact between the aluminium and the nitric acid. As a result, the aluminium does not continue to react with the acid.

5. **Conclusion on Passivity:**
- The process where the formation of aluminium oxide inhibits further reaction with HNO3 is known as passivity. This means that once the coating is formed, the aluminium metal becomes passive and resistant to further corrosion by the acid.