OXIDES

(i)  Acidic oxides :

The covalent oxides of non-metal are usually acidic; dissolves in water to produce solutions of acids e.g., CO2, SO2 , SO3, N2O5 , N2O3 , P4O6 , P4O10, Cl2O7, CrO3 ,  Mn2O7 etc. They are termed as acid anhydride.

Cl2O7 + H2O  ®  2 HClO4

Mn2O7 + H2O  ® 2 HMnO4

(ii) Basic oxides :

Metallic oxides are generally basic oxides. They either dissolve in water to form alkalies or combine with acids to form salts and water or combine with acidic oxides to form salts; e.g., Na2O, CaO. CuO, FeO, BaO etc.

Na2O + H2O  ® 2 NaOH

CaO + H2 ® Ca(OH)2

CuO + H2SO4   ® CuSO4 + H2O

The metallic oxides with lowest oxidation sate is the most ionic and the most basic but with increasing oxidation sate the acidic character increases e.g., CrO is basic, Cr2O3 amphoteric and CrO3 acidic.

(iii) Amphoteric Oxides :

 Many metals yield oxides which combine with both strong acid as well as strong bases e.g., ZnO, Al2O3, BeO, Sb2O3, Cr2O3, PbO, SnO, SnO2, Ga2O3 etc.

Cr2O3 + 2 NaOH ® Na2Cr2O4 + H2O

Cr2O3 + 3 H2SO4 ® Cr2(SO4)3 + 3 H2O