3.12 - Acidic & Basic Oxides

Acidic and basic oxides

• An oxide is a compound of an element with oxygen.
• Metal oxides are basic - They neutralise acids to form a salt and water (e.g. CuO, CaO, MgO).
• Most non-metal oxides are acidic - They neutralise alkalis to form a salt and water (e.g. CO₂, SO₂).
• An element's position in the periodic table predicts the type of oxide it forms: metals on the left form basic oxides, while non-metals on the right form acidic oxides.

Solubility and pH

• An oxide only shows an acidic or alkaline pH if it dissolves in water.
• Most basic oxides are insoluble in water (e.g. CuO, MgO), so they don't form alkaline solutions, but they still neutralise acids.
• Soluble metal oxides react with water to form alkaline solutions:
  • metal oxide + water → metal hydroxide
  • Na₂O(s) + H₂O(l) → 2NaOH(aq)

Soluble non-metal oxides react with water to form acidic solutions:

CO₂(g) + H₂O(l) → H₂CO₃(aq)