AQA · GCSE · Chemistry
Course version: 8462 · 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 in a liquid or solid by intermolecular forces acting between whole molecules.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 6
Study notes
From the checked teaching notes for this specification point.
The idea
Small covalent molecules are held together in a liquid or solid by intermolecular forces acting between whole molecules. These forces are much weaker than the covalent bonds inside each molecule. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome them when a substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, whereas larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds within each molecule are not broken during melting or boiling; only the forces between molecules are overcome.
What earns the marks
- Intermolecular forces act between separate molecules, not within them.
- The strength of intermolecular forces increases as molecular size increases.
- Larger molecules therefore need more energy to overcome these forces.
- Melting and boiling points increase with molecular size.
- Covalent bonds within molecules remain intact during melting and boiling.
- A suitable comparison, such as methane and octane, can illustrate the trend.
Common mistakes and how to put them right
Saying that covalent bonds break when a molecular substance boils.
Put it right: Intermolecular forces are overcome, while covalent bonds stay intact.
Confusing intermolecular forces with covalent bonds.
Put it right: Intermolecular forces are between molecules and are much weaker.
Assuming all small molecules have low boiling points regardless of size.
Put it right: Larger molecules have stronger intermolecular forces and higher boiling points.
Exam tips
- State clearly that it is the intermolecular forces that are overcome, not the covalent bonds.
- Link molecular size to the strength of intermolecular forces before explaining the effect on boiling point.
- Use a named example, such as methane versus octane, to support the trend.
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
Molecule X has a smaller molecular size than molecule Y. Both are simple molecular substances. Which statement about their boiling points is correct?
Listen
Small covalent molecules are held together in a liquid or solid by intermolecular forces acting between whole molecules. These forces are much weaker than the covalent bonds inside each molecule. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome them when a substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, whereas larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds within each molecule 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 molecules, not within them. The strength of intermolecular forces increases as molecular size increases. Larger molecules therefore need more energy to overcome these forces. Melting and boiling points increase with molecular size. A common slip: Saying that covalent bonds break when a molecular substance boils. Instead, intermolecular forces are overcome, while covalent bonds stay intact. Exam tip: State clearly that it is the intermolecular forces that are overcome, not the covalent bonds.
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