Classifying Oxides (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

What are oxides?

• Oxides are compounds made from one or more atoms of oxygen combined with one other element
• Examples of oxides include: MgO, ZnO, K₂O, CO₂, SO₂, H₂O
• Oxides can be classified based on their acid-base characteristics

Acidic oxides

• Acidic oxides are formed when a non-metal element combines with oxygen
• They react with bases to form a salt and water
• When dissolved in water they produce an acidic solution with a low pH
• Common examples include CO₂, SO_2, NO₂ and SiO₂

Basic oxides

• Basic oxides are formed when a metal element combines with oxygen
• They react with acids to form a salt and water
• When dissolved in water they produce a basic solution with a high pH
• Common examples include CuO and CaO

Basic and acidic oxides 

Metals form basic oxides while non-metals form acidic oxides

Extended tier only 

Neutral oxides

• Some oxides do not react with either acids or bases and thus are said to be neutral
• Examples include N₂O, NO and CO

Amphoteric oxides

• Amphoteric oxides are a curious group of oxides that can behave as both acidic and basic, depending on whether the other reactant is an acid or a base
• In both cases salt and water are formed
• Two of the most common amphoteric oxides are zinc oxide, ZnO and aluminium oxide, Al₂O₃
• The hydroxides of both of these elements also behave amphoterically
• Example of aluminium oxide behaving as a base:

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

• Example of aluminium oxide behaving as an acid:

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O