Homologous Series

Summary: A family of organic compounds with the same functional group and general formula, where each member differs by —CH₂—. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

A homologous series is a family of organic compounds that share the same functional group and general formula, with each successive member differing by a —CH₂— unit. Because they possess the same functional group, all members of a given series undergo the same characteristic chemical reactions, such as combustion and substitution for alkanes or addition for alkenes. Physical properties such as boiling point, viscosity, and melting point show a gradual trend along the series, increasing with chain length due to growing intermolecular forces. IGCSE candidates are expected to recognise the alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH), and carboxylic acids (CₙH₂ₙ₊₁COOH) as the four key homologous series. In exam questions, identifying the functional group is the most reliable way to assign an organic molecule to its correct homologous series.

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