CO2 and NO2 are acidic oxides as non-metal form acidic oxides. Al2O3 is an amphoteric oxide as it is a lewis acid but forms Al(OH)3 (which is basic) in water. CaO is basic oxide as metals form basic oxides Solution CO2 and NO2 are acidic oxides as non-metal form acidic oxides. Al2O3 is an amphoteric oxide as it is a lewis acid but forms Al(OH)3 (which is basic) in water. CaO is basic oxide as metals form basic oxides.