Explain why alkenes are considered unsaturated hydrocarbons.

Alkenes are considered unsaturated hydrocarbons because they contain a carbon-carbon double bond, which means they have two fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms.

This question assesses understanding of the structural characteristics of alkenes and their classification as unsaturated, which is crucial for understanding their reactivity and properties. This response is aligned to Structure and formulae of alkenes because it explains recall the first four members of the homologous series of alkenes as ethene, propene, butene and pentene using the correct AQA GCSE Chemistry organic context. Keep molecular formula, structural formula, displayed formula, and general formula distinct. Do not confuse alkanes with alkenes, saturated with unsaturated, cracking with combustion, polymers with monomers, or hydrocarbons with oxygen-containing alcohols and carboxylic acids. When formulae are used, preserve the stored notation exactly and explain the GCSE chemistry idea in words rather than using unsupported displayed-formula diagrams.