Metal oxides — AQA GCSE Combined Science
Test yourself on Metal oxides with AQA GCSE practice questions.
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Metal oxides explained
When a metal reacts with oxygen, the product is a metal oxide.
Read the full explanation
For example, magnesium burns in oxygen to form magnesium oxide, and iron reacts slowly with oxygen and water to form iron oxide in rusting. These are oxidation reactions because the metal gains oxygen. In terms of particles, the metal atoms lose electrons to oxygen atoms, forming metal ions and oxide ions in an ionic compound; the metal is oxidised. The reaction can be represented by a word equation such as magnesium + oxygen → magnesium oxide, or by a balanced symbol equation such as 2Mg + O₂ → 2MgO. Oxidation is not limited to oxygen gain, but at this level the definition is the gain of oxygen by a substance. Metal oxides are often basic, and some react with acids to form salts and water.
Students should be able to explain reduction and oxidation in terms of loss or gain of oxygen.
Oxidation is the gain of oxygen and reduction is the loss of oxygen. In metal oxide reactions, these processes occur simultaneously. For example, in the reaction 2CuO + C → 2Cu + CO₂, copper(II) oxide loses oxygen and is reduced to copper, while carbon gains oxygen and is oxidised to carbon dioxide. A substance removing oxygen is a reducing agent; one supplying it is an oxidising agent. Explaining these reactions requires naming the specific reactant gaining or losing oxygen and stating the consequence. Always refer to the full name of the reactant, such as copper(II) oxide, rather than just the metal, to ensure accurate identification of the species involved.
Your focus
- Describe the reaction of a metal with oxygen to produce a metal oxide.
- Explain why the reaction of a metal with oxygen is an oxidation reaction in terms of the metal gaining oxygen.
- Write word equations and balanced symbol equations for the formation of metal oxides.
Show all 6 objectives
- Define oxidation as gain of oxygen and reduction as loss of oxygen.
- Identify which reactant is oxidised and which is reduced in a given metal–oxygen reaction.
- Use a balanced equation to justify an explanation of oxidation and reduction in terms of oxygen.
Metal oxides exam tips
Marking Points
- Metals react with oxygen to produce metal oxides, for example magnesium + oxygen → magnesium oxide.
- The reaction is an oxidation reaction because the metal gains oxygen.
- Oxidation can be described as the gain of oxygen by a substance, and the metal is the substance being oxidised.
- Word equations and balanced symbol equations can represent the reaction, such as 2Mg + O₂ → 2MgO.
- Metal oxides are compounds of a metal and oxygen, and many are basic and can neutralise acids to form a salt and water.
- Oxidation is defined as the gain of oxygen by a substance during a chemical reaction.
- Reduction is defined as the loss of oxygen from a substance during a chemical reaction.
- In the reaction 2CuO + C → 2Cu + CO₂, copper(II) oxide is reduced because it loses oxygen.
- In the same reaction, carbon is oxidised because it gains oxygen to form carbon dioxide.
- A reducing agent removes oxygen from another substance, while an oxidising agent gives oxygen to another substance.
Examiner Tips
- 💡State the definition of oxidation as the gain of oxygen, then apply it directly to the metal in the reaction you are describing.
- 💡When writing a symbol equation for a metal oxide formation, check that the formula of the oxide has the correct ratio of metal ions to oxide ions before balancing.
- 💡If asked to explain why a reaction is oxidation, refer to the metal gaining oxygen rather than giving a general statement about oxygen being present.
- 💡Underline 'gain' and 'loss' in the question to ensure you correctly identify both the substance gaining and the substance losing oxygen.
- 💡When asked to explain which species is reduced, explicitly state 'it loses oxygen' rather than just identifying the substance.
Common Mistakes
- Writing the product as ‘magnesium oxygen’ instead of magnesium oxide; correction: use the naming rule that a metal plus oxygen gives the metal oxide, and write the correct formula, MgO.
- Saying that oxygen is oxidised because it gains electrons; correction: at this level, identify the metal as the substance that gains oxygen and is therefore oxidised.
- Balancing equations by changing subscripts, such as writing MgO₂ for magnesium oxide; correction: balance by placing whole-number coefficients in front of the correct formulae, as in 2Mg + O₂ → 2MgO.
- Writing 'copper is reduced' instead of 'copper(II) oxide is reduced'; always name the full reactant losing oxygen.
- Confusing the products with the reactants when identifying what has been oxidised or reduced; correct this by always selecting the substance from the left-hand side of the equation.
- Stating that oxygen itself is oxidised or reduced; correct this by identifying the element or compound that is gaining or losing the oxygen atoms.