Explain why alkenes are classified as unsaturated hydrocarbons.

Alkenes are classified as 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 double bond allows alkenes to undergo addition reactions, distinguishing them from saturated hydrocarbons like alkanes.

This question tests the student's understanding of the definition of unsaturation in hydrocarbons and the structural differences between alkenes and alkanes. A strong answer clearly connects the presence of the double bond to the reduction in hydrogen atoms. This response is aligned to Structure and formulae of alkenes because it explains explain that alkenes are unsaturated because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms 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.