AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
These substances have only weak forces between the molecules…
Full specification wording
Properties of small molecules: These substances have only weak forces between the molecules (intermolecular forces).
In a simple molecular substance, each molecule is held together internally by strong covalent bonds, but separate molecules attract one another only by weak intermolecular forces.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a simple molecular substance, each molecule is held together internally by strong covalent bonds, but separate molecules attract one another only by weak intermolecular forces. These forces act between molecules, not within them. For example, in liquid oxygen, O₂ molecules are held near each other by weak intermolecular forces, while the O=O covalent bond within each molecule remains strong. The weakness of these intermolecular forces explains why simple molecular substances melt and boil at relatively low temperatures: only a small amount of energy is needed to separate the molecules from one another. When answering, always specify that the forces are between molecules and describe them as weak, rather than saying the covalent bonds are weak.
What earns the marks
- State that intermolecular forces act between separate molecules, not within a molecule.
- Describe intermolecular forces in simple molecular substances as weak.
- Contrast weak intermolecular forces with the strong covalent bonds inside each molecule.
- Use a named example, such as O₂ or CH₄, to show that molecules remain intact when the substance melts or boils.
- Link the weakness of intermolecular forces to the low energy needed for melting or boiling.
Common mistakes and how to put them right
Saying that covalent bonds between molecules are weak.
Put it right by stating that intermolecular forces between molecules are weak while covalent bonds within molecules are strong.
Describing intermolecular forces as bonds.
Put it right by calling them forces of attraction between molecules.
Stating that molecules break apart when a simple molecular substance boils.
Put it right by explaining that molecules separate from one another but covalent bonds remain intact.
Exam tips
- Use the term intermolecular forces explicitly and state that they are weak.
- Contrast the weak forces between molecules with the strong covalent bonds within molecules in the same answer.
- When explaining boiling, say that energy is used to overcome intermolecular forces, not to break covalent bonds.
Try the lesson
The start of the Properties of small molecules: These substances have only weak forces between the molecules (intermolecular forces). lesson, made from checked teaching notes for this specification point.
Have a go first
What type of forces exist between the molecules in a substance made of small molecules?
Listen
In a simple molecular substance, each molecule is held together internally by strong covalent bonds, but separate molecules attract one another only by weak intermolecular forces. These forces act between molecules, not within them. For example, in liquid oxygen, O₂ molecules are held near each other by weak intermolecular forces, while the O=O covalent bond within each molecule remains strong. The weakness of these intermolecular forces explains why simple molecular substances melt and boil at relatively low temperatures: only a small amount of energy is needed to separate the molecules from one another. When answering, always specify that the forces are between molecules and describe them as weak, rather than saying the covalent bonds are weak. A strong answer earns marks for these points. State that intermolecular forces act between separate molecules, not within a molecule. Describe intermolecular forces in simple molecular substances as weak. Contrast weak intermolecular forces with the strong covalent bonds inside each molecule. Use a named example, such as O₂ or CH₄, to show that molecules remain intact when the substance melts or boils. A common slip: Saying that covalent bonds between molecules are weak. Instead, stating that intermolecular forces between molecules are weak while covalent bonds within molecules are strong. Exam tip: Use the term intermolecular forces explicitly and state that they are weak.
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Key words
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Intermolecular forces
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Weak forces between molecules
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Strong covalent bonds within molecules
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Energy changes during state changes
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See every definition
- Intermolecular forces
- Forces of attraction that act between separate molecules, not within a molecule.
- Weak forces between molecules
- The attraction between separate molecules in a simple molecular substance, which is weak.
- Strong covalent bonds within molecules
- The bonds that hold each molecule together internally, which remain strong.
- Energy changes during state changes
- Only a small amount of energy is needed to separate the molecules from one another when a substance melts or boils.
Label the diagram
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