Metal oxides are basic in nature. But some metal oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour. Such metal oxides which react with both acids as well as bases to produce salts and water, are known as amphoteric oxides.
Al2O3 + 6HCl ⇾ 2AlCl3 + 3H2O
Al2O3 + 2NaOH ⇾ 2NaAlO2(Sodium aluminate) + H2O
Notes: (i) When copper is heated in air, it combines with oxygen to form copper (II) oxide, a black oxide.
2Cu + O2⇾ 2CuO
Similarly, aluminium forms aluminium oxide 4Al + 3O2⇾ 2Al2O3
(ii) Most metal oxides are insoluble in water but some of these dissolves in water to form alkalis.
Sodium oxide and potassium oxide dissolve in water to produce alkalis as follows:
Na2O(s) + H2O(l) ⇾ 2NaOH (aq.) K2O(s) + H2O(l) ⇾ 2KOH(aq.)
(iii) Silver and gold do not react with oxygen even at high temperatures.