AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Substances that consist of small molecules are usually gases or liquids…
Full specification wording
Properties of small molecules: Substances that consist of small molecules are usually gases or liquids that have relatively low melting points and boiling points.
Small-molecule substances contain discrete molecules held together by covalent bonds, but the forces between separate molecules are weak.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Small-molecule substances contain discrete molecules held together by covalent bonds, but the forces between separate molecules are weak. Because melting and boiling require overcoming these weak intermolecular forces rather than breaking strong covalent bonds, little energy is needed. Consequently, such substances are usually gases or liquids at room temperature and have relatively low melting and boiling points. For example, oxygen (O₂) is a gas at room temperature, bromine (Br₂) is a liquid, and iodine (I₂) is a solid with a low melting point. The small size of the molecules means the intermolecular forces are weak, so the particles can separate easily when heated. They do not conduct electricity because the molecules have no overall electric charge and there are no mobile charged particles.
What earns the marks
- Identify that small-molecule substances consist of discrete molecules with strong covalent bonds within each molecule.
- Explain that the forces between separate molecules (intermolecular forces) are weak.
- State that melting and boiling involve overcoming these weak intermolecular forces, not breaking covalent bonds.
- Relate weak intermolecular forces to relatively low melting points and boiling points.
- Give examples such as oxygen (gas), bromine (liquid) or iodine (solid) to illustrate typical states at room temperature.
- Explain that because little energy is needed to separate molecules, these substances are usually gases or liquids at room temperature.
- State that small-molecule substances do not conduct electricity because they lack mobile charged particles.
Common mistakes and how to put them right
Thinking that covalent bonds break when a small-molecule substance boils.
Put it right: Only weak intermolecular forces are overcome; covalent bonds remain intact.
Assuming all small-molecule substances are gases.
Put it right: Some are liquids or low-melting solids, such as bromine and iodine.
Confusing intermolecular forces with covalent bonds.
Put it right: Intermolecular forces act between molecules; covalent bonds act within molecules.
Exam tips
- When explaining low boiling points, always state that weak intermolecular forces are overcome, not covalent bonds.
- Use the correct terms: 'intermolecular forces' for between molecules and 'covalent bonds' for within molecules.
- Link the state at room temperature to the strength of intermolecular forces and the energy needed to separate molecules.
Try the lesson
The start of the Properties of small molecules: Substances that consist of small molecules are usually gases or liquids that have relatively low melting points and boiling points. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following best describes the usual state of matter of a substance made of small molecules at room temperature?
Listen
Small-molecule substances contain discrete molecules held together by covalent bonds, but the forces between separate molecules are weak. Because melting and boiling require overcoming these weak intermolecular forces rather than breaking strong covalent bonds, little energy is needed. Consequently, such substances are usually gases or liquids at room temperature and have relatively low melting and boiling points. For example, oxygen (O₂) is a gas at room temperature, bromine (Br₂) is a liquid, and iodine (I₂) is a solid with a low melting point. The small size of the molecules means the intermolecular forces are weak, so the particles can separate easily when heated. They do not conduct electricity because the molecules have no overall electric charge and there are no mobile charged particles. A strong answer earns marks for these points. Identify that small-molecule substances consist of discrete molecules with strong covalent bonds within each molecule. Explain that the forces between separate molecules (intermolecular forces) are weak. State that melting and boiling involve overcoming these weak intermolecular forces, not breaking covalent bonds. Relate weak intermolecular forces to relatively low melting points and boiling points. A common slip: thinking that covalent bonds break when a small-molecule substance boils. Instead, only weak intermolecular forces are overcome; covalent bonds remain intact. Exam tip: When explaining low boiling points, always state that weak intermolecular forces are overcome, not covalent bonds.
Key words
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Intermolecular forces
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Melting and boiling points
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States of matter
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Structure and bonding
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See every definition
- Intermolecular forces
- The forces between separate molecules, which are weak, so the particles can separate easily when heated.
- Melting and boiling points
- Relatively low for small-molecule substances because melting and boiling involve overcoming weak intermolecular forces rather than breaking strong covalent bonds.
- States of matter
- Small-molecule substances are usually gases or liquids at room temperature, and some are solids with a low melting point.
- Structure and bonding
- Small-molecule substances consist of discrete molecules with strong covalent bonds within each molecule.
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