SEC G3 Combined Science Chemistry component K326/K328
C7: Redox Chemistry
Recognise oxidation and reduction through oxygen, hydrogen, electrons, oxidation states, and the named redox tests.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
A redox explanation must show that oxidation and reduction occur together and identify the transferred substance, electrons, or oxidation-state change.
Oxygen and hydrogen definitions
Oxidation can be described as gain of oxygen or loss of hydrogen. Reduction can be described as loss of oxygen or gain of hydrogen.
Apply the definition to the named substance rather than to the equation as an unnamed whole.
Electrons and oxidation states
Oxidation is electron loss and an increase in oxidation state. Reduction is electron gain and a decrease in oxidation state.
An oxidising agent accepts electrons and is reduced; a reducing agent supplies electrons and is oxidised.
Tests for redox agents
Aqueous potassium iodide is colourless before the test. An oxidising agent converts iodide ions to iodine, changing the mixture from colourless to brown. Acidified potassium manganate(VII) is purple before the test; a reducing agent changes it from purple to colourless.
State the initial colour, final colour, and whether the tested substance acts as an oxidising or reducing agent.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: A colourless sample turns aqueous potassium iodide brown. What does this show?
- Brown colour indicates iodine formation.
- Iodide was oxidised to iodine.
- The sample therefore acted as an oxidising agent.
Answer: The sample is an oxidising agent.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Oxygen and hydrogen definitions with the named terms, evidence, and causal links kept distinct.
- Use Electrons and oxidation states to interpret the evidence given and justify each conclusion.
- Apply Tests for redox agents to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C7: 3 mapped outcomes, references C7(a), C7(b), C7(c). Check the official K326 syllabus.
K328 C7: 3 mapped outcomes, references C7(a), C7(b), C7(c). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret oxidising and reducing-agent tests using acidified potassium manganate(VII) or aqueous potassium iodide where specified. State the reagent, initial and final observation, and electron-transfer inference.
Exam traps and retrieval check
Avoid these traps
- Describing only oxidation and omitting the paired reduction.
- Reversing electron gain and loss.
- Naming a reagent without its colour evidence.
Check from memory
What electron change is oxidation?
Loss of electrons.
What happens to an oxidising agent?
It is reduced.
What colour change indicates a reducing agent with acidified manganate(VII)?
Purple to colourless.
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. Oxidation and reduction by oxygen or hydrogen transfer, electron transfer, oxidation state, and the two prescribed redox tests.