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 the student's understanding of the concept of saturation in hydrocarbons. It requires them to connect the structure of alkenes to their chemical properties, demonstrating comprehension of molecular composition. This response is aligned to Structure and formulae of alkenes because it explains state the general formula for the homologous series of alkenes as CnH2n 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.