Redox through oxygen transfer and Roman numerals
Identify oxidation and reduction together, and distinguish an oxidation number from a formula subscript.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Electron transfer and oxidation-number calculations follow in Extended lessons. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read formulas and balanced equations without changing element identities.
- Distinguish an element from a compound and recognise oxygen in formulas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the subscript 2 in FeCl₂ count?
What you will learn
- Identify oxidation as oxygen gain and reduction as oxygen loss in a suitable reaction.
- Explain why both changes occur together in redox and interpret Roman numerals in compound names.
Follow oxygen from one substance to another
In the oxygen-transfer description, oxidation is gain of oxygen and reduction is loss of oxygen. In CuO+H₂→Cu+H₂O, copper(II) oxide loses oxygen and is reduced to copper; hydrogen gains oxygen and is oxidised to water. Identify the changing substance on each side rather than calling every reactant oxidised just because the reaction contains oxygen.
Oxidation and reduction occur simultaneously in a redox reaction: one substance's oxygen gain accompanies another's oxygen loss in these examples. Some redox reactions do not involve oxygen, so failure to find oxygen transfer does not prove a reaction is non-redox. Extended electron and oxidation-number definitions provide the wider test. Melting a metal oxide changes its state without transferring oxygen between substances.
Use the name as information, not an atom count
A Roman numeral in a name states the oxidation number of the named element in that compound. Iron(II) chloride is FeCl₂ and iron(III) chloride is FeCl₃; II and III distinguish iron's oxidation numbers, not the number of iron atoms in the formula. Copper(II) oxide is CuO, which has one copper and one oxygen atom per formula ratio, even though its name contains II.
For an oxygen-transfer example such as 2Mg+CO₂→2MgO+C, magnesium gains oxygen and is oxidised, while carbon dioxide loses oxygen and is reduced to carbon. The balanced coefficients conserve atoms; they are separate from Roman numerals and formula subscripts. State both changes and the substances involved to demonstrate that the reaction is redox.
Pause and explain
Which statement correctly describes CuO+H₂→Cu+H₂O?
Worked example
Use CuO+H₂→Cu+H₂O to identify the oxidised and reduced substances, then explain II in copper(II) oxide.
Show the worked solution
- CuO loses its oxygen to form Cu, so copper(II) oxide is reduced.
- H₂ gains oxygen to form H₂O, so hydrogen is oxidised; the two changes occur together.
- II states copper's oxidation number +2 in CuO; it does not mean CuO contains two copper atoms.
Answer Hydrogen is oxidised; copper(II) oxide is reduced. II denotes copper's +2 oxidation number.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For CuO+CO→Cu+CO₂, identify both oxygen-transfer changes and explain why the equation is redox.
Give me a hint
Compare the oxygen associated with each carbon-containing and copper-containing substance before and after.
Compare my reasoning
- CuO loses oxygen when it forms Cu, so copper(II) oxide is reduced.
- CO gains oxygen when it forms CO₂, so carbon monoxide is oxidised.
- Oxidation and reduction occur simultaneously, making the reaction redox.
CuO is reduced; CO is oxidised; the simultaneous changes make it redox.
Look for these in your work
- Both changing substances are named.
- Each label is justified by oxygen gain or loss.
For Fe₂O₃+3CO→2Fe+3CO₂, identify the reduced reactant and oxidised reactant. Explain III in iron(III) oxide.
Give me a hint
Keep oxidation numbers, atom subscripts and balanced coefficients separate.
Compare my reasoning
- Fe₂O₃ loses oxygen to become Fe and is reduced.
- CO gains oxygen to become CO₂ and is oxidised.
- III means iron has oxidation number +3 in the oxide; the subscript 2 separately specifies two iron atoms per three oxygen atoms.
Fe₂O₃ is reduced; CO is oxidised; III denotes iron's +3 oxidation number.
Look for these in your work
- Oxygen transfer supports both labels.
- The Roman numeral is distinguished from the subscript.
A student says melting CuO is reduction because the solid disappears, and says CuO+H₂→Cu+H₂O cannot be redox because no O₂ gas is used. Correct both claims.
Give me a hint
A state change and oxygen transfer ask different questions; oxygen need not enter as O₂.
Compare my reasoning
- Melting retains the CuO composition, so the solid-to-liquid change alone does not demonstrate oxygen loss or reduction.
- In the reaction, oxygen already in CuO transfers to hydrogen, reducing CuO and oxidising H₂.
- Redox does not require O₂ gas as a reactant; suitable oxygen-transfer evidence establishes both changes here.
Melting alone is not reduction; the equation is redox through oxygen transfer from CuO to H₂.
Look for these in your work
- Composition is separated from physical state.
- The source and recipient of oxygen are identified.
Common mistakes
- Calling reduction oxygen gain rather than loss.
- Naming only one change when asked to explain redox.
- Reading a Roman numeral as the number of atoms of that element.
- Assuming every redox reaction must contain oxygen.
What can you explain now?
In 2Mg+CO₂→2MgO+C, which reactant gains oxygen and which loses it?
Compare with the explanation
Magnesium gains oxygen and is oxidised; carbon dioxide loses oxygen and is reduced.
Oxygen transfers from carbon dioxide to magnesium while atom identities are conserved, so both changes occur together.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Choose an oxygen-transfer equation and label both changes with a reason. Explain what a Roman numeral, a subscript and a coefficient each communicate.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.