These properties influence how hydrocarbons are used as fuels.

The idea

Hydrocarbons are used as fuels because burning them transfers energy usefully, but their physical properties decide which fuel suits which job. Short-chain hydrocarbons such as methane and propane have low boiling points, so they are gases that vaporise readily and burn with a clean flame; they suit domestic heating and cooking. Longer-chain hydrocarbons have higher boiling points and are more viscous, so they are liquids that are easier to store and transport but harder to ignite; they suit petrol, diesel and heavier fuel oils. Flammability also falls as molecular size rises, so a fuel must be matched to its use: a gas for a quick, controllable flame, a liquid for a slower, sustained release of energy.

What earns the marks

• Burning hydrocarbon fuels transfers energy to the surroundings, which is why they are useful as fuels.
• Boiling point increases with molecular size, so small hydrocarbons are gases and large ones are liquids or solids at room temperature.
• Viscosity increases with molecular size, so larger hydrocarbons flow less easily and are more difficult to pump or vaporise.
• Flammability decreases with molecular size, so smaller hydrocarbons ignite more readily than larger ones.
• The choice of fuel depends on matching these properties to the intended use, for example a gas for a quick hot flame and a liquid for storage and transport.

Common mistakes and how to put them right

• Thinking that larger hydrocarbons burn more easily.

  Put it right: In fact flammability decreases as molecular size increases, so smaller molecules ignite more readily.

• Confusing viscosity with flammability.

  Put it right: Viscosity describes resistance to flow, while flammability describes how easily a fuel ignites.

• Assuming all hydrocarbons are gases.

  Put it right: Only the smallest are gases at room temperature, while larger ones are liquids or solids.

Exam tips

• Link each property to a named use, such as methane for cooking or diesel for road transport, to show understanding rather than listing properties.
• Use the phrase 'as molecular size increases' when describing trends so the direction of change is unambiguous.
• Check whether the question asks about a trend or about a use, and answer the one asked rather than describing both.