AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to use the idea that intermolecular forces are…
Full specification wording
Properties of small molecules: Students should be able to use the idea that intermolecular forces are weak compared with covalent bonds to explain the bulk properties of molecular substances.
Molecular substances have low melting and boiling points because the intermolecular forces between their molecules are weak compared with the covalent bonds inside each molecule.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Molecular substances have low melting and boiling points because the intermolecular forces between their molecules are weak compared with the covalent bonds inside each molecule. When a simple molecular substance melts or boils, energy is used to overcome the weak intermolecular forces, not to break the strong covalent bonds. This explains why substances such as iodine or carbon dioxide have low melting and boiling points despite containing strong covalent bonds. It also explains why molecular substances are usually gases or liquids at room temperature. Their inability to conduct electricity is due to the absence of mobile charge carriers, not weak forces. The key idea is the contrast: weak forces between molecules, strong bonds within molecules.
What earns the marks
- Intermolecular forces are weak compared with covalent bonds.
- Melting and boiling overcome intermolecular forces, not covalent bonds.
- Weak intermolecular forces explain low melting and boiling points.
- Strong covalent bonds remain intact when a molecular substance changes state.
- Bulk properties such as state at room temperature can be explained using this contrast.
Common mistakes and how to put them right
Stating that covalent bonds break when a molecular substance boils.
Put it right: Only intermolecular forces are overcome.
Saying intermolecular forces are strong because covalent bonds are strong.
Put it right: The two are different and intermolecular forces are much weaker.
Describing melting as breaking molecules apart.
Put it right: Molecules separate from each other but remain intact.
Exam tips
- Use the phrase weak intermolecular forces compared with strong covalent bonds to structure your answer.
- Explain bulk properties by referring to energy needed to overcome intermolecular forces.
- Check that your answer distinguishes between forces between molecules and bonds within molecules.
Try the lesson
The start of the Properties of small molecules: Students should be able to use the idea that intermolecular forces are weak compared with covalent bonds to explain the bulk properties of molecular substances. lesson, made from checked teaching notes for this specification point.
Have a go first
Simple molecular substances often have low melting points. Which statement explains this using the strength of the forces involved?
Listen
Molecular substances have low melting and boiling points because the intermolecular forces between their molecules are weak compared with the covalent bonds inside each molecule. When a simple molecular substance melts or boils, energy is used to overcome the weak intermolecular forces, not to break the strong covalent bonds. This explains why substances such as iodine or carbon dioxide have low melting and boiling points despite containing strong covalent bonds. It also explains why molecular substances are usually gases or liquids at room temperature. Their inability to conduct electricity is due to the absence of mobile charge carriers, not weak forces. The key idea is the contrast: weak forces between molecules, strong bonds within molecules. A strong answer earns marks for these points. Intermolecular forces are weak compared with covalent bonds. Melting and boiling overcome intermolecular forces, not covalent bonds. Weak intermolecular forces explain low melting and boiling points. Strong covalent bonds remain intact when a molecular substance changes state. A common slip: Stating that covalent bonds break when a molecular substance boils. Instead, only intermolecular forces are overcome. Exam tip: Use the phrase weak intermolecular forces compared with strong covalent bonds to structure your answer.
See it
Key words
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Intermolecular forces
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Covalent bonds
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Bulk properties
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See every definition
- Intermolecular forces
- Forces between molecules that are weak compared with covalent bonds, and are overcome when a molecular substance melts or boils.
- Covalent bonds
- Bonds inside each molecule that are strong compared with intermolecular forces and remain intact when a molecular substance changes state.
- Bulk properties
- Properties such as low melting and boiling points and state at room temperature, explained by weak intermolecular forces compared with covalent bonds.
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