Crude oil is separated into fractions by fractional distillation. Describe how the properties of hydrocarbon molecules change as the chain length increases, and explain how these properties affect the suitability of different fractions as fuels.

As the carbon chain length increases, the molecules become larger and heavier. 1. Boiling point rises – longer chains have more London dispersion forces, so they require more heat to vapourise. 2. Viscosity rises – the longer chains are more entangled and flow more slowly, giving a thicker liquid. 3. Flammability falls – the larger molecules contain more carbon per hydrogen, so they release less energy per unit mass and ignite less readily. 4. Short‑chain fractions (e.g. gasoline, kerosene) are therefore better fuels because they are highly flammable and have low boiling points, allowing them to vapourise and ignite easily. 5. Long‑chain fractions (e.g. lubricating oils) are less suitable as fuels because their high viscosity makes them difficult to pump and their lower flammability reduces combustion efficiency.