Identify oxidising and reducing agents
Name the agents in a redox reaction by tracking which species accepts or donates electrons and which is changed.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Check atom balance, total charge and oxidation-state changes separately. Written practice is self-checked
Before you begin
- Recognise common ions and write formulas for simple compounds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to an oxidising agent during a redox reaction?
What you will learn
- Define oxidising agent and reducing agent
- Identify each agent from oxidation-number changes
- Link agent role to whether the agent is reduced or oxidised
The oxidising agent accepts electrons
An oxidising agent causes another species to be oxidised by accepting electrons itself. It is therefore reduced and its oxidation number decreases.
An oxidising agent can be an element or an ion. Identify it from the species whose oxidation number falls, rather than relying on the word 'oxygen' in the reaction name.
The reducing agent donates electrons
A reducing agent causes another species to be reduced by donating electrons itself. It is therefore oxidised and its oxidation number increases.
A useful paired check: oxidising agent is reduced; reducing agent is oxidised. The agent name describes its effect on the other reactant, so it is easy to reverse the labels if you follow only the agent's own change.
Pause and explain
A reducing agent donates electrons. What happens to its oxidation number?
Worked example
For Zn + Cu²⁺ → Zn²⁺ + Cu, identify both agents.
Show the worked solution
- Zn changes 0→+2 and loses electrons, so it is oxidised.
- Cu²⁺ changes +2→0 and gains electrons, so it is reduced.
- The donor Zn is the reducing agent; the acceptor Cu²⁺ is the oxidising agent.
Answer Zn is the reducing agent; Cu²⁺ is the oxidising agent.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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 Mg + 2Ag⁺ → Mg²⁺ + 2Ag, identify the oxidising and reducing agents.
Give me a hint
Track which reactant gains electrons and which loses them.
Compare my reasoning
- Mg goes 0→+2 and loses electrons.
- Ag⁺ goes +1→0 and gains electrons.
- The electron acceptor is the oxidising agent; the donor is the reducing agent.
Ag⁺ is the oxidising agent and Mg is the reducing agent.
Look for these in your work
- Mg oxidation identified
- Ag⁺ reduction identified
- Agent roles assigned correctly
In the reaction Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O, explain why chlorine acts as both agents.
Give me a hint
One part of the chlorine is reduced while another part is oxidised.
Compare my reasoning
- Some chlorine goes 0→−1 and accepts electrons.
- Other chlorine goes 0→+1 and loses electrons.
- The same reactant is reduced and oxidised, so it has both agent roles in the disproportionation.
Cl₂ is both: the portion reduced to Cl⁻ acts as oxidising agent, while the portion oxidised to ClO⁻ acts as reducing agent.
Look for these in your work
- Both chlorine changes tracked
- Electron acceptor role identified
- Electron donor role identified
A learner labels the species that is oxidised as the oxidising agent. Correct the reasoning for Fe²⁺ + Ce⁴⁺ → Fe³⁺ + Ce³⁺.
Give me a hint
An oxidising agent is reduced while it accepts electrons.
Compare my reasoning
- Fe²⁺ becomes Fe³⁺, so it loses an electron and is oxidised.
- Ce⁴⁺ becomes Ce³⁺, so it gains an electron and is reduced.
- Fe²⁺ is the reducing agent; Ce⁴⁺ is the oxidising agent.
The labels are reversed in the learner's claim. Fe²⁺ is the reducing agent because it donates electrons and is oxidised; Ce⁴⁺ is the oxidising agent because it accepts electrons and is reduced.
Look for these in your work
- Electron directions correct
- Oxidation/reduction correct
- Both agent labels corrected
Common mistakes
- Calling the species reduced the reducing agent
- Calling the species oxidised the oxidising agent
- Confusing the agent's effect on another species with its own change
- Identifying agents without checking electron direction
What can you explain now?
What happens to an oxidising agent during a redox reaction?
Compare with the explanation
It accepts electrons and is reduced
The oxidising agent causes another species to lose electrons and gains them itself.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Why is the oxidising agent reduced while the reducing agent is oxidised?
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.