AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Properties of hydrocarbons: Students should be able to recall how boiling point, viscosity and flammability change with increasing molecular size.
As hydrocarbon molecules get larger, the chains become longer and the molecules have a greater surface area of contact.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
As hydrocarbon molecules get larger, the chains become longer and the molecules have a greater surface area of contact. This strengthens the intermolecular forces between molecules, so more energy is needed to separate them. Therefore boiling point increases with increasing molecular size. Larger molecules also flow less easily because the stronger intermolecular forces resist sliding, so viscosity increases. Flammability decreases because larger molecules need more energy and oxygen to vaporise and burn, so they ignite less readily.
For example: Methane (CH₄) is a gas that ignites easily, whereas larger alkanes such as decane (C₁₀H₂₂) are viscous liquids that are harder to ignite.
What earns the marks
- Boiling point increases as molecular size increases because intermolecular forces become stronger with longer chains.
- Viscosity increases as molecular size increases because larger molecules flow less easily.
- Flammability decreases as molecular size increases because larger molecules are harder to ignite and burn.
- The trend applies across a homologous series such as the alkanes, where each member differs by a CH₂ unit.
- Intermolecular forces, not covalent bonds, are overcome during boiling; covalent bonds within molecules remain intact.
- A named example can support the trend, such as methane being a flammable gas and decane being a viscous, less flammable liquid.
Common mistakes and how to put them right
Saying boiling point decreases with molecular size.
Put it right by linking longer chains to stronger intermolecular forces and more energy needed to boil.
Confusing viscosity with flammability.
Put it right by stating that viscosity is resistance to flow, while flammability is how easily a substance catches fire.
Claiming covalent bonds break during boiling.
Put it right by explaining that only intermolecular forces are overcome when a simple molecular substance boils.
Exam tips
- Use the phrase 'increasing molecular size' consistently and state the direction of each trend clearly.
- Link each property to intermolecular forces rather than to covalent bond strength.
- Give a specific example from the alkanes to show the trend, such as methane compared with a larger alkane.
Try the lesson
The start of the Properties of hydrocarbons: Students should be able to recall how boiling point, viscosity and flammability change with increasing molecular size. lesson, made from checked teaching notes for this specification point.
Have a go first
As the molecules of a hydrocarbon get larger, what happens to its boiling point?
Listen
As hydrocarbon molecules get larger, the chains become longer and the molecules have a greater surface area of contact. This strengthens the intermolecular forces between molecules, so more energy is needed to separate them. Therefore boiling point increases with increasing molecular size. Larger molecules also flow less easily because the stronger intermolecular forces resist sliding, so viscosity increases. Flammability decreases because larger molecules need more energy and oxygen to vaporise and burn, so they ignite less readily. For example, methane (CH₄) is a gas that ignites easily, whereas larger alkanes such as decane (C₁₀H₂₂) are viscous liquids that are harder to ignite. A strong answer earns marks for these points. Boiling point increases as molecular size increases because intermolecular forces become stronger with longer chains. Viscosity increases as molecular size increases because larger molecules flow less easily. Flammability decreases as molecular size increases because larger molecules are harder to ignite and burn. The trend applies across a homologous series such as the alkanes, where each member differs by a CH₂ unit. A common slip: Saying boiling point decreases with molecular size. Instead, linking longer chains to stronger intermolecular forces and more energy needed to boil. Exam tip: Use the phrase 'increasing molecular size' consistently and state the direction of each trend clearly.
See it
Key words
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Intermolecular forces
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Viscosity
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Flammability
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Homologous series
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See every definition
- Intermolecular forces
- Forces between molecules that become stronger as hydrocarbon chains get longer and have a greater surface area of contact.
- Viscosity
- Resistance to flow that increases with increasing molecular size because stronger intermolecular forces resist sliding.
- Flammability
- How easily a substance catches fire, which decreases with increasing molecular size because larger molecules need more energy and oxygen to vaporise and burn.
- Homologous series
- A series such as the alkanes, where each member differs by a CH₂ unit and the trend applies across it.
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