Types of Oxides

Classifying Oxides

Acid 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 nonmetal element
• The metallic character of the element influences the acidic or alkaline behaviour of the molecule

Metals form basic oxides and hydroxides while nonmetals form acidic oxides

Acidic oxides

• Acidic oxides are formed when a nonmetal 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 SO₂ 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 NaOH, KOH and Ca(OH)₂

Extended Only

Neutral & Amphoteric Oxides

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 a salt and water is formed
• Two most common amphoteric oxides are zinc oxide and aluminum oxide
• The hydroxides of both of these elements also behave amphoterically
• Example of aluminium oxide behaving as a base:

Al2O3 + 6HCl → 2AlCl3 + 3H2O

• Example for an aluminium oxide behaving as an acid:

Al2O3 + 2NaOH → 2NaAlO2 + H2O