Oxidation

Summary: Loss of electrons. Also: gain of oxygen, increase in oxidation number. OIL RIG: Oxidation Is Loss. Tags: igcse chemistry definition reactions Created: 2026-07-14 Last Updated: 2026-07-14

Oxidation is defined at IGCSE level in three complementary ways: as the loss of electrons (recalled by the mnemonic OIL RIG — Oxidation Is Loss), as the gain of oxygen, and as an increase in oxidation number. Oxidation never occurs in isolation; it is always coupled with reduction in a simultaneous process called a redox reaction, where one species loses electrons (is oxidised) while another gains them (is reduced). The substance that is oxidised is termed the reducing agent because it donates electrons and brings about reduction in the other species — a conceptual distinction that is frequently tested in exams. Common IGCSE examples include the rusting of iron (Fe → Fe²⁺ + 2e⁻, with iron being oxidised), the displacement of copper from copper(II) sulfate by zinc (Zn + Cu²⁺ → Zn²⁺ + Cu), and the combustion of fuels where carbon and hydrogen are oxidised by atmospheric oxygen. When writing half-equations in electrochemistry, identifying which electrode is the anode is straightforward: oxidation always takes place at the anode, whether in an electrolytic cell or a simple displacement reaction.

Key term — IGCSE Chemistry (0620/0971) glossary.