Reduction

Summary: Gain of electrons. Also: loss of oxygen, decrease in oxidation number. OIL RIG: Reduction Is Gain. Tags: igcse chemistry definition reactions Created: 2026-07-14 Last Updated: 2026-07-14

Reduction is defined in chemistry as the gain of electrons by an atom, ion, or molecule, and is memorised alongside oxidation by the mnemonic OIL RIG — Oxidation Is Loss, Reduction Is Gain of electrons. In terms of oxygen transfer, which is the definition typically introduced first at IGCSE, reduction is the removal of oxygen from a substance, such as when copper(II) oxide is reduced to copper metal by hydrogen gas or when iron(III) oxide is reduced to iron in the blast furnace. Reduction can also be defined as a decrease in oxidation number, which provides a consistent framework covering reactions where oxygen is not directly involved. Reduction never occurs in isolation; it is always coupled with oxidation in what are called redox reactions, and the substance that is reduced (gains electrons) is itself acting as the oxidising agent because it causes the other species to be oxidised. Common IGCSE examples include the extraction of iron from haematite, the Thermit reaction between aluminium and iron(III) oxide, and electrolysis where reduction always occurs at the cathode.

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