AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Some properties of hydrocarbons depend on the size of their molecules,…
Full specification wording
Properties of hydrocarbons: Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability.
The physical properties of hydrocarbons change predictably with molecular size.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The physical properties of hydrocarbons change predictably with molecular size. As the carbon chain gets longer, the molecules become larger and have stronger intermolecular forces. This raises the boiling point, making larger hydrocarbons less volatile. Larger molecules also flow less easily, so viscosity increases, and they become less flammable. For example, methane (CH₄) is a small gas molecule that vaporises and ignites easily, while a long-chain hydrocarbon such as fuel oil is a viscous liquid that is harder to vaporise and burn. Fractions collected lower down in a fractionating column consist of larger molecules, meaning they are more viscous, have higher boiling points, and are less flammable.
What earns the marks
- State that boiling point increases as molecular size increases because stronger intermolecular forces require more energy to overcome.
- Explain that flammability decreases as molecular size increases because larger molecules are less volatile and harder to vaporise.
- Describe how viscosity increases with molecular size, meaning longer chain hydrocarbons flow less easily.
- Link the position of a fraction in a fractionating column to the size and properties of its hydrocarbon molecules.
Common mistakes and how to put them right
Stating larger hydrocarbons are less flammable because they need more oxygen.
Put it right: They are less volatile and harder to vaporise.
Saying boiling point decreases as molecules get larger.
Put it right: Boiling point increases with molecular size due to stronger intermolecular forces.
Confusing intermolecular forces with covalent bonds.
Put it right: State that intermolecular forces are overcome during boiling, not the covalent bonds within the molecule.
Exam tips
- Link decreased flammability explicitly to higher boiling points and lower volatility.
- State the direction of each trend clearly, for example 'boiling point increases as molecular size increases'.
Try the lesson
The start of the Properties of hydrocarbons: Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability. lesson, made from checked teaching notes for this specification point.
Have a go first
As the molecules of a hydrocarbon become larger, what happens to its boiling point?
Listen
The physical properties of hydrocarbons change predictably with molecular size. As the carbon chain gets longer, the molecules become larger and have stronger intermolecular forces. This raises the boiling point, making larger hydrocarbons less volatile. Larger molecules also flow less easily, so viscosity increases, and they become less flammable. For example, methane (CH₄) is a small gas molecule that vaporises and ignites easily, while a long-chain hydrocarbon such as fuel oil is a viscous liquid that is harder to vaporise and burn. Fractions collected lower down in a fractionating column consist of larger molecules, meaning they are more viscous, have higher boiling points, and are less flammable. A strong answer earns marks for these points. State that boiling point increases as molecular size increases because stronger intermolecular forces require more energy to overcome. Explain that flammability decreases as molecular size increases because larger molecules are less volatile and harder to vaporise. Describe how viscosity increases with molecular size, meaning longer chain hydrocarbons flow less easily. Link the position of a fraction in a fractionating column to the size and properties of its hydrocarbon molecules. A common slip: Stating larger hydrocarbons are less flammable because they need more oxygen. Instead, they are less volatile and harder to vaporise. Exam tip: Link decreased flammability explicitly to higher boiling points and lower volatility.
See it
Key words
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Boiling point
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Viscosity
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Flammability
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Intermolecular forces
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See every definition
- Boiling point
- Increases as molecular size increases because stronger intermolecular forces require more energy to overcome.
- Viscosity
- Increases with molecular size, meaning longer chain hydrocarbons flow less easily.
- Flammability
- Decreases as molecular size increases because larger molecules are less volatile and harder to vaporise.
- Intermolecular forces
- Become stronger as molecules become larger, and are overcome during boiling rather than the covalent bonds within the molecule.
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