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Acids, Bases and Salts

Cambridge O Level Chemistry 5070 — 7.2 Oxides — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

5 min read 5 parts Lesson 2 of 3

📚 Cambridge O Level Chemistry 5070 Hub / Topical Revision
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Lesson summary

7.2 Oxides — Cambridge O Level Chemistry: classify oxides as acidic, basic or amphoteric using their reactions with acids and bases. An oxide is a compound of oxygen with one other element.

  • Example reaction
  • Aluminium oxide examples

Part 1 of 5What is an oxide?

What is an oxide?
An oxide is a compound of oxygen with one other element. Examples include magnesium oxide (MgO), calcium oxide (CaO), carbon dioxide (CO2) and sulfur dioxide (SO2). Oxides can be classified by how they react with acids and bases.
A useful first prediction is that many metal oxides are basic, while many non-metal oxides are acidic. This is a pattern, not an absolute rule: zinc oxide and aluminium oxide are important amphoteric exceptions.

Part 2 of 5Acidic oxides

Acidic oxides are commonly oxides of non-metals. They react with bases to form salts and water. Some dissolve in water and make acidic solutions. Examples include CO2 and SO2.
Example reaction
Carbon dioxide reacts with calcium hydroxide
  • CO2 + Ca(OH)2 → CaCO3 + H2O.
  • The calcium carbonate is a white precipitate.
  • This reaction shows CO2 behaving as an acidic oxide toward a base.

Part 3 of 5Basic oxides

Basic oxides are commonly oxides of metals. They react with acids to form a salt and water. Copper(II) oxide, CuO, and calcium oxide, CaO, are examples. Some metal oxides dissolve or react with water to give alkaline solutions, but not every basic oxide is soluble.
Example reaction
CuO + 2HCl → CuCl2 + H2O. Copper(II) oxide reacts with hydrochloric acid to make copper(II) chloride and water, demonstrating its basic behaviour.

Part 4 of 5Amphoteric oxides

An amphoteric oxide reacts with both acids and bases, forming a salt and water in each type of reaction. Zinc oxide (ZnO) and aluminium oxide (Al2O3) are common examples. Their behaviour cannot be described simply as always acidic or always basic.
Aluminium oxide examples
With an acid, Al2O3 + 6HCl → 2AlCl3 + 3H2O. With sodium hydroxide, aluminium oxide also reacts to form an aluminate salt and water. Recognising that it reacts with both reagents is the key classification point.

Part 5 of 5Check your understanding

    • 1.
    • Define an oxide.
    • 2.
    • Classify CO2, CuO and ZnO.
    • 3.
    • Complete: MgO + 2HCl → ____ + ____.
    • 4.
    • Explain how one could demonstrate that an oxide is amphoteric.
    • 5.
    • Why is metal versus non-metal only a first prediction?

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