Oxides that act as acids as well as bases in a chemical reaction are called amphoteric oxides. Many metallic oxides exhibit properties of both acids and bases. Oxides of metals are able to receive or donate a proton or H+ ion, and this gives such oxides their amphoteric character.
Aluminum, copper, zinc and tin form amphoteric oxides when they react with oxygen. For instance, the oxide of aluminum is Al2O3. This oxide is amphoteric and acts as an acid when it reacts with a base like Sodium hydroxide or NaOH. This reaction produces the salt NaAlO2. When the same oxide Al2O3 reacts with an acid such as H2SO4 it acts as a base and produces a salt Al2(SO4)3.