Chemistry
Redox reactions & oxidation states — quick study summary
Redox = REDuction + OXidation. Oxidation is losing electrons; reduction is gaining them — and they always happen together. The oxidising agent is what gets reduced (it took the electrons); the reducing agent is what gets oxidised. Use oxidation states (a bookkeeping tool) to spot which atoms gained or lost electrons. Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons).
Key points
- OIL RIG: Oxidation = Loss of electrons; Reduction = Gain of electrons
- Oxidising agent is reduced; reducing agent is oxidised
- Pure elements have oxidation state 0; group 1 metals +1; group 2 metals +2; O usually −2; H usually +1
- Half-equations show electron transfer separately for each side
- Common redox: combustion, corrosion (rust), respiration, photosynthesis, batteries
Practice quiz
Click each question to reveal the answer.
1. In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, which species is oxidised?
- Zn
- Cu²⁺
- Zn²⁺
- Cu
Answer: Zn
Zn loses 2 electrons (0 → +2). Cu²⁺ gains 2 electrons (+2 → 0). Zn is oxidised; Cu²⁺ is reduced.
2. What is the oxidation state of sulfur in SO₄²⁻?
Answer: +6
O is −2 ×4 = −8. Total charge of ion is −2, so S = −2 − (−8) = +6.
3. Why are oxidation and reduction always coupled?
Answer: Electrons lost by one substance must be gained by another — they can't just disappear
Conservation of charge means every electron leaving one atom has to land on another. Hence 'redox'.
Last reviewed: May 2026