Although aluminium is above hydrogen in the electrochemical series, it is stable in air and water. Why ?

Answer

### Step-by-Step Solution:

1. **Understanding the Electrochemical Series**:
- The electrochemical series ranks metals based on their tendency to lose electrons and form positive ions. Metals above hydrogen in this series are generally more reactive than hydrogen.

2. **Position of Aluminium**:
- Aluminium is positioned above hydrogen in the electrochemical series, which suggests that it should react with water and air, potentially leading to oxidation.

3. **Formation of Aluminium Oxide (Al2O3)**:
- When aluminium is exposed to air and moisture, it reacts to form a thin layer of aluminium oxide (Al2O3). This reaction occurs almost immediately upon exposure.

4. **Properties of Aluminium Oxide**:
- Aluminium oxide is a stable, protective layer that is both thin and continuous. This layer is resistant to further oxidation and prevents the underlying aluminium from reacting with air and water.

5. **Prevention of Further Oxidation**:
- The presence of this protective layer means that even though aluminium is more reactive than hydrogen, it does not continue to oxidize in air and water. The aluminium oxide layer acts as a barrier, preventing further chemical reactions.

6. **Conclusion**:
- Therefore, despite being above hydrogen in the electrochemical series, aluminium remains stable in air and water due to the formation of a protective aluminium oxide layer that inhibits further oxidation.