Oxidation and reduction Cambridge IGCSE Chemistry (9–1) revision
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In plain words
Oxidation first meant gaining oxygen, as when iron rusts, and reduction meant losing it. Chemists then spotted what is really going on underneath: electrons moving. A substance that loses electrons is oxidised; one that gains electrons is reduced. One can't happen without the other, so the pair is called redox.
Three things to know
- Oxidation is gain of oxygen, or loss of electrons. Reduction is loss of oxygen, or gain of electrons.
- An oxidising agent oxidises something else and is itself reduced. A reducing agent reduces something else and is itself oxidised.
- A Roman numeral in a name gives the oxidation number of the metal: iron(III) oxide contains Fe³⁺.
Worked example
Zn + Cu²⁺ → Zn²⁺ + Cu. Which substance is oxidised?
- Zinc starts as an atom, Zn, and ends as an ion, Zn²⁺.
- To become 2+ it must have lost two electrons.
- Losing electrons is oxidation, so zinc is oxidised (and the copper ions are reduced).
Tips and tricks
- OIL RIG: Oxidation Is Loss, Reduction Is Gain. It's about electrons.
- The agent is the opposite of what happens to it: the thing that gets oxidised is the reducing agent.
It lands in your notebook with its questions as flashcards.
Oxidation and reduction: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is oxidation, in terms of electrons?
OIL: Oxidation Is Loss.
2Mg + O₂ → 2MgO. What happens to the magnesium?
It gains oxygen (and loses electrons).
What happens to a reducing agent during a reaction?
It gives away electrons, so it is itself oxidised.
Fe₂O₃ + 3CO → 2Fe + 3CO₂. Which substance is reduced?
It loses its oxygen.
What does the (II) show in copper(II) oxide?
The copper is present as Cu²⁺.
2Na + Cl₂ → 2NaCl. What happens to the chlorine?
Each chlorine atom gains an electron to become Cl⁻.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Oxidation and reduction
Cambridge IGCSE Chemistry (9–1) 0971 · 8 marks · papermunch.org
Name ______________________________ Date ______________
CuO + H₂ → Cu + H₂O. State which substance is oxidised and which is reduced.[2]
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Hydrogen is oxidised (it gains oxygen). Copper(II) oxide is reduced (it loses oxygen).
Define oxidation and reduction in terms of electrons.[2]
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Oxidation is the loss of electrons. Reduction is the gain of electrons.
Mg + Cu²⁺ → Mg²⁺ + Cu. State which substance is oxidised, and explain why.[2]
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Magnesium: it loses two electrons to become Mg²⁺.
In the reaction above, identify the oxidising agent and explain your choice.[2]
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Cu²⁺ (copper ions): it takes electrons from the magnesium and is itself reduced.
Answers: Oxidation and reduction
- 1. Hydrogen is oxidised (it gains oxygen). Copper(II) oxide is reduced (it loses oxygen).
- 2. Oxidation is the loss of electrons. Reduction is the gain of electrons.
- 3. Magnesium: it loses two electrons to become Mg²⁺.
- 4. Cu²⁺ (copper ions): it takes electrons from the magnesium and is itself reduced.