AQA · GCSE · Combined Science
Course version: 8464 · 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). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
In small-molecule substances, each molecule is held together by strong covalent bonds, but the forces between separate molecules are weak.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
In small-molecule substances, each molecule is held together by strong covalent bonds, but the forces between separate molecules are weak. These weak intermolecular forces must be overcome when the substance melts or boils. The covalent bonds within the molecules are not broken during these physical changes. For example, when ice melts, the hydrogen bonds between water molecules are overcome, but the O–H covalent bonds remain intact. Similarly, when iodine sublimes, the weak forces between I₂ molecules are overcome, not the covalent bonds within each I₂ molecule. This explains why small-molecule substances have relatively low melting and boiling points: little energy is required to separate the molecules. Understanding this distinction is essential for explaining physical properties and changes of state.
What earns the marks
- State that small-molecule substances have weak intermolecular forces between molecules.
- Explain that melting and boiling involve overcoming these weak intermolecular forces.
- Emphasise that covalent bonds within molecules are not broken during melting or boiling.
- Use examples such as water or iodine to illustrate that intermolecular forces are overcome, not covalent bonds.
- Relate the weak intermolecular forces to the low energy required for melting and boiling.
- Distinguish clearly between intermolecular forces (between molecules) and covalent bonds (within molecules).
Common mistakes and how to put them right
Saying that covalent bonds break when a small-molecule substance boils.
Put it right: Only intermolecular forces are overcome; covalent bonds remain intact.
Using 'intermolecular forces' and 'covalent bonds' interchangeably.
Put it right: Intermolecular forces act between molecules; covalent bonds act within molecules.
Thinking that melting involves breaking bonds within molecules.
Put it right: Melting overcomes forces between molecules, not bonds within them.
Exam tips
- Always specify that weak intermolecular forces are overcome, not covalent bonds, when explaining melting or boiling.
- Use the correct terminology: 'intermolecular forces' for between molecules and 'covalent bonds' for within molecules.
- Link the energy required for melting or boiling to the strength of intermolecular forces.
Try the lesson
The start of the Properties of small molecules: These substances have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils. lesson, made from checked teaching notes for this specification point.
Have a go first
A simple molecular substance boils. Which forces are overcome when it boils?
Listen
In small-molecule substances, each molecule is held together by strong covalent bonds, but the forces between separate molecules are weak. These weak intermolecular forces must be overcome when the substance melts or boils. The covalent bonds within the molecules are not broken during these physical changes. For example, when ice melts, the hydrogen bonds between water molecules are overcome, but the O–H covalent bonds remain intact. Similarly, when iodine sublimes, the weak forces between I₂ molecules are overcome, not the covalent bonds within each I₂ molecule. This explains why small-molecule substances have relatively low melting and boiling points: little energy is required to separate the molecules. Understanding this distinction is essential for explaining physical properties and changes of state. A strong answer earns marks for these points. State that small-molecule substances have weak intermolecular forces between molecules. Explain that melting and boiling involve overcoming these weak intermolecular forces. Emphasise that covalent bonds within molecules are not broken during melting or boiling. Use examples such as water or iodine to illustrate that intermolecular forces are overcome, not covalent bonds. A common slip: saying that covalent bonds break when a small-molecule substance boils. Instead, only intermolecular forces are overcome; covalent bonds remain intact. Exam tip: Always specify that weak intermolecular forces are overcome, not covalent bonds, when explaining melting or boiling.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Properties of small molecules: These substances have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge