AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The intermolecular forces increase with the size of the molecules, so…
Full specification wording
Properties of small molecules: The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
Small covalent molecules are held together by weak intermolecular forces, while the atoms inside each molecule are joined by strong covalent bonds.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Small covalent molecules are held together by weak intermolecular forces, while the atoms inside each molecule are joined by strong covalent bonds. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome these forces when the substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, but larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds themselves are not broken during melting or boiling; only the forces between molecules are overcome.
What earns the marks
- Intermolecular forces act between separate small molecules, not between the atoms within a molecule.
- The strength of intermolecular forces increases as the size of the molecules increases.
- Larger molecules need more energy to overcome their intermolecular forces during melting or boiling.
- Melting and boiling points therefore increase as molecular size increases.
- Covalent bonds within each molecule remain intact when a simple molecular substance melts or boils.
- A suitable comparison, such as methane being a gas and octane being a liquid at room temperature, supports the trend.
Common mistakes and how to put them right
Saying that covalent bonds break when a simple molecular substance melts or boils.
Put it right by stating that only the weak intermolecular forces are overcome.
Confusing intermolecular forces with covalent bonds.
Put it right by describing covalent bonds as strong forces within molecules and intermolecular forces as weaker attractions between molecules.
Claiming that all small molecules have low melting points regardless of size.
Put it right by explaining that the trend depends on molecular size, so larger molecules have higher melting and boiling points.
Exam tips
- Use the phrase intermolecular forces when explaining melting and boiling, and keep it distinct from covalent bonds.
- Link molecular size to the energy needed to overcome the forces, then state the effect on melting and boiling points.
- Give a named example, such as methane compared with a larger alkane, to make the trend concrete.
Try the lesson
The start of the Properties of small molecules: The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points. lesson, made from checked teaching notes for this specification point.
Have a go first
Small molecules are held together in a liquid by intermolecular forces. What happens to the strength of these intermolecular forces as the molecules get larger?
Listen
Small covalent molecules are held together by weak intermolecular forces, while the atoms inside each molecule are joined by strong covalent bonds. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome these forces when the substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, but larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds themselves are not broken during melting or boiling; only the forces between molecules are overcome. A strong answer earns marks for these points. Intermolecular forces act between separate small molecules, not between the atoms within a molecule. The strength of intermolecular forces increases as the size of the molecules increases. Larger molecules need more energy to overcome their intermolecular forces during melting or boiling. Melting and boiling points therefore increase as molecular size increases. A common slip: Saying that covalent bonds break when a simple molecular substance melts or boils. Instead, stating that only the weak intermolecular forces are overcome. Exam tip: Use the phrase intermolecular forces when explaining melting and boiling, and keep it distinct from covalent bonds.
See it
Key words
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Intermolecular forces
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Trends in physical properties
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Energy and changes of state
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See every definition
- Intermolecular forces
- Forces that act between separate small molecules, not between the atoms within a molecule, and become stronger as the molecules get larger.
- Trends in physical properties
- Melting and boiling points rise as molecular size increases, because larger molecules need more energy to overcome their intermolecular forces.
- Energy and changes of state
- More energy is needed to overcome intermolecular forces when the substance melts or boils, while the covalent bonds within each molecule remain intact.
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