Which of the following statements correctly defines oxidation in terms of electrons?
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Practise IGCSE 0620 questions in the style of recent Extended past papers, organised by syllabus subtopic. Each set comes with an examiner-style mark scheme and a downloadable worksheet.
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These Redox Topical Past Paper Questions are written in the style of recent Cambridge IGCSE Chemistry 0620 Extended papers and grouped by the Chemical reactions (C6.4) section of the 2025–2027 syllabus. Use them to revise the exact skills examiners test in this part of the course.
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Define oxidation and reduction in terms of oxygen and electrons. Identify oxidising and reducing agents. Extended candidates use oxidation numbers, write the oxidation number in Roman numerals after the element name (e.g. iron(III)) and explain redox in terms of electron transfer.
Which of the following statements correctly defines oxidation in terms of electrons?
Iron(III) oxide is reduced in a blast furnace by carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
State which substance has been REDUCED and explain in terms of oxygen.
[1 mark]State which substance has been OXIDISED and explain in terms of oxygen.
[1 mark]In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, which species is oxidised and which is the oxidising agent?
Magnesium burns in oxygen to form magnesium oxide: 2Mg + O₂ → 2MgO.
State which substance has been oxidised in terms of BOTH oxygen and electrons.
[2 marks]Identify the reducing agent in this reaction.
[1 mark]What is the oxidation number of nitrogen in HNO₃?
In the reaction Cl₂ + 2KBr → 2KCl + Br₂, chlorine displaces bromine from potassium bromide.
Write the half-equation showing what happens to chlorine.
[1 mark]Write the half-equation showing what happens to bromide.
[1 mark]Identify the oxidising agent and the reducing agent.
[1 mark]Explain why chlorine is able to displace bromine, but bromine cannot displace chlorine.
[1 mark]Acidified potassium manganate(VII) is a strong oxidising agent. The half-equation is: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
Use oxidation numbers to show that manganese is reduced in this half-equation.
[2 marks]State a visible colour change you would expect when potassium manganate(VII) is reduced.
[1 mark]What is the oxidation number of sulfur in the thiosulfate ion S₂O₃²⁻?