Topic 2 of 6
Identify the agent from what it does
The oxidising agent is itself reduced.
O-Level 6092 (2026) / SEC G3 K324 (2027)
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.
| 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 understandingAn unknown decolourises acidified potassium manganate(VII). Is the unknown oxidised or reduced?Think it through, then reveal the answer
It is oxidised: it supplies electrons and acts as a reducing agent. The manganate(VII) reagent is reduced.