Topic 1 of 6
Recognise oxidation and reduction
Use the model that makes the electron change easiest to see.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Recognise oxidation and reduction
Use the model that makes the electron change easiest to see.
| Model | Oxidation | Reduction |
|---|---|---|
| Oxygen transfer | Gain of oxygen | Loss of oxygen |
| Hydrogen transfer | Loss of hydrogen | Gain of hydrogen |
| Electron transfer | Loss of electrons | Gain of electrons |
| Oxidation state | Increase | Decrease |
Oxidation and reduction happen together: electrons lost by one species are gained by another. In CuO(s) + H2(g) -> Cu(s) + H2O(g), copper(II) oxide loses oxygen and is reduced; hydrogen gains oxygen and is oxidised. During addition of hydrogen to an alkene, the organic compound gains hydrogen and is reduced.
Oxidation state is an accounting number. An uncombined element has state 0. A monatomic ion has its ion charge. In the common compounds here, oxygen is usually -2 and hydrogen +1; exceptions must be supplied or recognised from context. The states sum to zero in a neutral compound or to the overall charge in an ion.
Worked example
Follow electrons and oxidation states
Explain Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s).
- The reacting ions give Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s).
- Zn -> Zn2+ + 2e-: oxidation state rises 0 to +2, so zinc is oxidised.
- Cu2+ + 2e- -> Cu: state falls +2 to 0, so copper ions are reduced.
Zinc transfers electrons to copper(II) ions. Sulfate is a spectator.