AQA · GCSE · Combined Science
Course version: 8464 · 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.
Small molecular substances such as H₂O, CO₂, I₂ and CH₄ contain strong covalent bonds holding atoms together inside each molecule, but the forces between separate molecules are weak.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Small molecular substances such as H₂O, CO₂, I₂ and CH₄ contain strong covalent bonds holding atoms together inside each molecule, but the forces between separate molecules are weak. When such a substance melts or boils, energy must overcome these weak intermolecular forces, not the covalent bonds. Consequently melting and boiling points are low, and molecular substances are often gases or liquids at room temperature. Molecular substances are also poor conductors of electricity, but this is because they have no overall electric charge and lack mobile charged particles, not due to weak forces.
For example: Iodine, I₂, is a solid at room temperature but sublimes readily on gentle heating because only weak forces between I₂ molecules must be overcome, while the I–I covalent bonds remain intact.
What earns the marks
- Identify that covalent bonds within a molecule are strong and hold atoms together.
- Identify that intermolecular forces act between separate molecules and are weak compared with covalent bonds.
- Explain low melting and boiling points by stating that little energy is needed to overcome the weak intermolecular forces.
- Explain that melting or boiling breaks intermolecular forces, not covalent bonds, so the molecules themselves remain unchanged.
- Use the weak intermolecular forces to explain why many molecular substances are gases or liquids at room temperature.
- Explain poor electrical conductivity by noting the absence of free ions or delocalised electrons in simple molecular substances, rather than weak intermolecular forces.
Common mistakes and how to put them right
Saying that boiling breaks covalent bonds.
Put it right by stating that boiling overcomes weak intermolecular forces while covalent bonds stay intact.
Confusing intermolecular forces with covalent bonds.
Put it right by describing covalent bonds as strong forces within a molecule and intermolecular forces as weak attractions between molecules.
Claiming that molecular substances conduct because they contain covalent bonds.
Put it right by explaining that no free charged particles are present, so they do not conduct.
Exam tips
- Link each bulk property directly to the weak intermolecular forces rather than describing the substance in general terms.
- Use a named example such as iodine or water and state clearly which forces are overcome on heating.
- When comparing molecular substances with giant covalent or ionic substances, refer explicitly to the strength of the forces that must be overcome.
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
Oxygen is a gas at room temperature and boils at -183 °C. Which statement explains why oxygen has such a low boiling point?
Listen
Small molecular substances such as H₂O, CO₂, I₂ and CH₄ contain strong covalent bonds holding atoms together inside each molecule, but the forces between separate molecules are weak. When such a substance melts or boils, energy must overcome these weak intermolecular forces, not the covalent bonds. Consequently melting and boiling points are low, and molecular substances are often gases or liquids at room temperature. Molecular substances are also poor conductors of electricity, but this is because they have no overall electric charge and lack mobile charged particles, not due to weak forces. For example, iodine, I₂, is a solid at room temperature but sublimes readily on gentle heating because only weak forces between I₂ molecules must be overcome, while the I–I covalent bonds remain intact. A strong answer earns marks for these points. Identify that covalent bonds within a molecule are strong and hold atoms together. Identify that intermolecular forces act between separate molecules and are weak compared with covalent bonds. Explain low melting and boiling points by stating that little energy is needed to overcome the weak intermolecular forces. Explain that melting or boiling breaks intermolecular forces, not covalent bonds, so the molecules themselves remain unchanged. A common slip: Saying that boiling breaks covalent bonds. Instead, stating that boiling overcomes weak intermolecular forces while covalent bonds stay intact. Exam tip: Link each bulk property directly to the weak intermolecular forces rather than describing the substance in general terms.
Key words
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Intermolecular forces
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Covalent bonds
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Melting and boiling points
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Electrical conductivity
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See every definition
- Intermolecular forces
- Weak forces acting between separate molecules, compared with the strong covalent bonds that hold atoms together inside each molecule.
- Covalent bonds
- Strong bonds within a molecule that hold atoms together and remain intact when a molecular substance melts or boils.
- Melting and boiling points
- Low for molecular substances because little energy is needed to overcome the weak intermolecular forces rather than the covalent bonds.
- Electrical conductivity
- Poor in simple molecular substances because they have no overall electric charge and lack mobile charged particles, not because of weak forces.
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