Lesson 2 of 2

Identify the agent from what it does

The oxidising agent is itself reduced.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

I can test redox agents.Syllabus
Syllabus K326 / K328, 7(c). Test redox agents

Identify the agent from what it does

The oxidising agent is itself reduced.

An oxidising agent accepts electrons and causes another species to be oxidised. It is reduced. A reducing agent supplies electrons and is oxidised. In the zinc/copper-ion reaction, Cu²⁺ is the oxidising agent and Zn is the reducing agent; copper metal is a product, not the agent added at the start.

Test an unknown redox agent
Reagent	Observation	Inference
Aqueous potassium iodide	Initially colourless solution becomes brown as iodine forms	Unknown oxidises I^- to I₂; it acts as an oxidising agent
Acidified potassium manganate(VII)	Purple solution becomes colourless	Unknown reduces manganate(VII); it acts as a reducing agent

For the iodide test, 2I^-(aq) -> I₂(aq) + 2e^- shows iodide losing electrons. For manganate(VII), use the acidified reagent specified: conditions affect its products and colours. A colour change is an observation; the assignment of oxidising or reducing behaviour is the inference.

Check your understanding

An unknown decolourises acidified potassium manganate(VII). Is the unknown oxidised or reduced?

Oxidised. It gives electrons to the manganate(VII), so it acts as a reducing agent.
Reduced. It removes the purple colour, so it acts as the oxidising agent here.
Neither. Losing the purple colour is a dilution effect, not an electron transfer.

Show the answer

Oxidised. It gives electrons to the manganate(VII), so it acts as a reducing agent.

Purple MnO₄^- is reduced to almost colourless Mn²⁺. The electrons for that reduction come from the unknown, so the unknown loses electrons: it is oxidised and is the reducing agent.