What does aluminium oxide form by reacting with sodium hydroxide?

Aluminium oxide has both an acidic and basic nature. It is also known as amphoteric oxide.

In reaction with the base sodium hydroxide, it acts as an acid and forms sodium aluminate. In this reaction, as Al is less reactive than Na, so it cannot displace Na from the base. Hence, it combines with NaOH to form a complex salt known as sodium aluminate.

The chemical reaction is represented as:

${\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{NaOH}+3{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{Sodium} \mathrm{Aluminate}}{2\mathrm{NaAl}{\left(\mathrm{OH}\right)}_4}$