What is an oxide?
An oxide is a compound formed when an element combines with oxygen. In Cambridge IGCSE Chemistry oxides are sorted into four classes by how they behave with acids and bases.
Key idea
Basic oxides are metal oxides that react with acids to give a salt and water. Acidic oxides are non-metal oxides that react with alkalis. Amphoteric oxides react with BOTH acids and alkalis. Neutral oxides react with neither.
The four classes
- Basic oxides – e.g. CuO, MgO, CaO. Soluble basic oxides (Na2O, K2O, CaO) dissolve in water to form alkalis.
- Acidic oxides – e.g. CO2, SO2, SO3. They dissolve in water to form acids, e.g. SO2 + H2O → H2SO3.
- Amphoteric oxides – e.g. Al2O3, ZnO, PbO. They react with acids and with alkalis.
- Neutral oxides – e.g. CO, NO, H2O. They show no acidic or basic behaviour.
Worked example
Copper(II) oxide is a basic oxide. Write the reaction with dilute sulfuric acid.
Metal oxide + acid → salt + water:
CuO + H2SO4 → CuSO4 + H2O
The black CuO dissolves to give a blue copper(II) sulfate solution.
Remember
- Metal oxide → usually basic; non-metal oxide → usually acidic.
- Amphoteric oxides (Al2O3, ZnO) sit at the metal/non-metal borderline.