Classifying Oxides (Cambridge O Level Chemistry)

• 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 and basic oxides

• Acidic and basic oxides have different properties and values of pH
• The difference in their pH stems from whether they are bonded to a metal or a non-metal element
• The metallic character of the element influences the acidic or basic behaviour of the molecule

Metals form basic oxides while non-metals form acidic oxides

 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

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, a salt and water are formed
• Two of the most common amphoteric oxides are zinc oxide, ZnO and aluminum 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

• This acidic and basic behaviour is not easily explained by donating or accepting protons. A separate theory called the Lewis acid-base theory can identify acids or bases in these situations, but is not required for this course