Metal oxides are basic in nature. But some metal
oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour.
Definition :
“Those metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides.“
e.g – Aluminium oxide reacts in the following manner with acids and bases –
Al2O3 + 6HCl → 2AlCl3 + 3H2O
Al2O3 + 2NaOH → 2NaAlO2 + H2O
(Sodium
aluminate)
Most metal oxides are insoluble in water but some of these dissolve n water to form alkalis. Sodium oxide and potassium oxide dissolve in water to produce alkalis as follows –
Na2O(s) + H2O (l) → 2 NaOH (aq)
K2O(s) + H2O (l) → 2 KOH (aq)