Unsaturated

Summary: Containing at least one carbon-carbon double bond (C=C). Alkenes are unsaturated hydrocarbons. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

In organic chemistry, an unsaturated compound is one that contains at least one carbon-carbon double bond (C=C) or triple bond, meaning that not all carbon atoms are bonded to the maximum possible number of hydrogen atoms and the compound has the capacity to take on additional atoms. Unsaturated hydrocarbons known as alkenes follow the general formula CₙH₂ₙ and include ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈), each containing one C=C double bond that serves as the functional group and site of chemical reactivity. Because of the presence of this electron-rich double bond, unsaturated compounds readily undergo addition reactions, in which small molecules such as hydrogen (hydrogenation), bromine (bromination), or water (hydration) add across the double bond to form a single saturated product — these are the reactions that make alkenes such versatile industrial feedstocks. The standard chemical test for unsaturation at IGCSE is the bromine water test: when a few drops of orange-brown bromine water are shaken with an unsaturated compound, the bromine adds across the C=C double bond and the solution turns colourless, whereas saturated compounds leave the bromine water unchanged. Unsaturated compounds are also the essential starting materials for addition polymerisation, where the C=C bonds open and monomers link together to form polymers such as poly(ethene) and poly(propene), making alkenes among the most economically important chemicals produced by the petrochemical industry.

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