Properties of hydrocarbons: Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability. These properties influence how hydrocarbons are used as fuels.

The idea

This statement explains that the physical and chemical behaviour of hydrocarbons is linked to molecule size. As the number of carbon atoms in a hydrocarbon chain increases, the molecules become larger and heavier. This changes three key properties: boiling point increases, viscosity increases and flammability decreases. These trends arise because larger molecules have stronger intermolecular forces between them, so more energy is needed to separate them and they flow less easily. The properties then determine how a hydrocarbon is used. For example, short-chain hydrocarbons such as methane and propane have low boiling points and are highly flammable, making them useful as gases for heating and cooking. Longer-chain hydrocarbons are more viscous and less flammable, so they are used as lubricants or as fuels where a slower, steadier burn is needed.

What earns the marks

• State that as hydrocarbon molecules get larger, boiling point increases.
• Explain that larger molecules have stronger intermolecular forces, so more energy is needed to overcome them.
• Describe that viscosity increases as molecule size increases, meaning larger hydrocarbons flow less easily.
• Explain that flammability decreases as molecule size increases, so larger hydrocarbons are harder to ignite.
• Link these properties to uses: small molecules as fuels for burning, larger molecules as lubricants or for slower-burning fuels.
• Use the trends to predict the behaviour of an unfamiliar hydrocarbon when its size is given.

Common mistakes and how to put them right

• Saying that boiling point decreases as molecule size increases.

  Put it right: Boiling point increases because larger molecules have stronger intermolecular forces.

• Confusing viscosity with density.

  Put it right: Viscosity is resistance to flow, not mass per unit volume.

• Thinking all hydrocarbons are equally flammable.

  Put it right: Flammability decreases as molecule size increases.

• Assuming larger hydrocarbons are always better fuels.

  Put it right: Larger hydrocarbons are less flammable and may be used as lubricants instead.

Exam tips

• When explaining a trend, always refer to the size of the molecule and the strength of intermolecular forces.
• Use comparative language such as 'higher', 'lower', 'more viscous' and 'less flammable' to make your answers precise.
• If asked to suggest a use, match the property to the job: highly flammable for easy burning, viscous for lubrication.
• Practise drawing or interpreting a graph of boiling point against number of carbon atoms to reinforce the trend.