IGCSE Organic Chemistry Practice Test 2026 – Comprehensive Exam Prep

The key idea is saturation: carbon wants four bonds. In saturated hydrocarbons, every carbon–carbon bond is a single bond, so each carbon can form enough single bonds to attach as many hydrogens as needed to reach four total bonds. This is why alkanes like methane and ethane are saturated. If there were any carbon–carbon double or triple bonds, some of carbon’s valence would be used in those bonds, leaving fewer sites for hydrogen, making the molecule unsaturated (alkenes or alkynes). Also, hydrocarbons contain only carbon and hydrogen, so a molecule with heteroatoms would not be a hydrocarbon.