Classifying Oxides (Cambridge International IGCSE Chemistry)

Spec point — 7.1.4 · Classify oxides as acidic, including SO2 and CO2, or basic, including CuO and CaO, related to metallic and non-metallic character

Classifying oxides

What are oxides?

• Oxides are compounds made from one or more atoms of oxygen combined with one other element

• Examples of oxides include:

  • Magnesium oxide - MgO
  • Zinc oxide - ZnO
  • Potassium oxide - K₂O
  • Carbon dioxide - CO₂
  • Sulfur dioxide - SO₂
  • Water - 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

Amphoteric oxides

Spec point — 7.1.4 · Describe amphoteric oxides as oxides that react with acids and with bases to produce a salt and water Classify Al2O3 and ZnO as amphoteric oxides

Amphoteric 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