Why Aluminium Oxide and Zinc Oxide is called Amphoteric Oxides?
An amphoteric oxide is an oxide which reacts which both acids and bases. Generally metal form basic oxides while non-metals form acidic oxides. However this trend is not always followed. Many elements form amphoteric oxides. Thus the nature of oxide formed by an element is a characteristic property of the element, which does not depend upon any other factor.
For example both zinc and aluminium are metals but their oxides, ZnO and Al2O3 are amphoteric oxides as they react with both acids and bases.
Al2O3 + 6HCl ---------- 2AlCl3 + 3H2
Al2O3 + 6 NaOH + 3H2O ------- 2Na3Al (OH)6
ZnO + 2HCl ---- ZnCl2 + H2O
ZnO + 2NaOH + H2O ----- Na2 [Zn (OH)4]
Generally metallic oxides are basic in nature butAn amphoteric oxide is an oxide that can act both as an acid and as a base. An example is aluminium oxide, Al2O3. It reacts with acids to form the corresponding aluminium salt, and with bases to form salts called aluminates.
Example reactionsAl2O3 + 6HCl → 2AlCl3 + 3H2OAl2O3 + 2NaOH → Na2Al2O4 + H2O