Why does aluminium not react with water under ordinary conditions?

Aluminium does not readily react with water under ordinary conditions due to the presence of a protective oxide layer that forms on its surface. This oxide layer, primarily composed of aluminum oxide (Al2O3), is very stable and acts as a barrier, preventing further reaction between the metal and water.

When exposed to air, aluminum reacts with oxygen to form this oxide layer. This layer is very thin, transparent, and adheres tightly to the surface of the aluminum. It effectively prevents water molecules from directly contacting the underlying aluminum metal, thereby inhibiting the typical metal-water reaction.

The oxide layer on aluminum is also known to be chemically inert and doesn't dissolve in water. This further contributes to the metal's resistance to reacting with water. If this protective oxide layer were to be disrupted or removed, then aluminum might react with water, generating aluminum hydroxide and hydrogen gas:

2 Al + 6 H2O → 2 Al(OH)3 + 3 H2

However, this reaction requires a significant amount of energy to overcome the barrier of the oxide layer, and in everyday conditions, this energy is not readily available, making the reaction negligible.

In summary, the presence of a stable and protective oxide layer on the surface of aluminum prevents it from reacting with water under ordinary conditions.