AQA · GCSE · Chemistry
Course version: 8462 · 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.
Substances made of small molecules, such as hydrogen, chlorine, oxygen, nitrogen, carbon dioxide and methane, contain a limited number of atoms joined by covalent bonds.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Substances made of small molecules, such as hydrogen, chlorine, oxygen, nitrogen, carbon dioxide and methane, contain a limited number of atoms joined by covalent bonds. Because the forces between separate molecules are weak, these substances usually exist as gases or liquids at room temperature and have relatively low melting and boiling points. For example, oxygen is a gas at room temperature and boils at about −183 °C, while water is a liquid with a boiling point of 100 °C. The small size of the molecules is not the reason for the low melting and boiling points; the key factor is the weak intermolecular forces that must be overcome when the substance changes state. This idea lets you predict and explain physical properties of simple molecular substances.
What earns the marks
- Identify small-molecule substances as those containing a limited number of atoms joined by covalent bonds, for example H₂, Cl₂, O₂, N₂, CO₂ and CH₄.
- State that small-molecule substances are usually gases or liquids at room temperature.
- Relate relatively low melting points and boiling points to the weak forces between molecules.
- Use a named example, such as oxygen or methane, to illustrate the link between small molecules and low boiling point.
- Distinguish between the strong covalent bonds within molecules and the weak intermolecular forces between molecules.
Common mistakes and how to put them right
Claiming that small molecules have low boiling points because the covalent bonds are weak.
Put it right by stating that the weak intermolecular forces are overcome, not the covalent bonds.
Confusing the size of a molecule with the strength of the forces between molecules.
Put it right by focusing on intermolecular forces rather than molecular size.
Assuming all small-molecule substances are gases.
Put it right by noting that some, such as water and ethanol, are liquids at room temperature.
Exam tips
- Name a specific small-molecule substance and quote its state or approximate boiling point to support your answer.
- Use the phrase weak intermolecular forces when explaining low melting and boiling points.
- Avoid saying bonds are broken during melting or boiling of small-molecule substances; state that forces between molecules are overcome.
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 statement describes the usual physical state of substances made of small molecules at room temperature?
Listen
Substances made of small molecules, such as hydrogen, chlorine, oxygen, nitrogen, carbon dioxide and methane, contain a limited number of atoms joined by covalent bonds. Because the forces between separate molecules are weak, these substances usually exist as gases or liquids at room temperature and have relatively low melting and boiling points. For example, oxygen is a gas at room temperature and boils at about −183 °C, while water is a liquid with a boiling point of 100 °C. The small size of the molecules is not the reason for the low melting and boiling points; the key factor is the weak intermolecular forces that must be overcome when the substance changes state. This idea lets you predict and explain physical properties of simple molecular substances. A strong answer earns marks for these points. Identify small-molecule substances as those containing a limited number of atoms joined by covalent bonds, for example H₂, Cl₂, O₂, N₂, CO₂ and CH₄. State that small-molecule substances are usually gases or liquids at room temperature. Relate relatively low melting points and boiling points to the weak forces between molecules. Use a named example, such as oxygen or methane, to illustrate the link between small molecules and low boiling point. A common slip: Claiming that small molecules have low boiling points because the covalent bonds are weak. Instead, stating that the weak intermolecular forces are overcome, not the covalent bonds. Exam tip: Name a specific small-molecule substance and quote its state or approximate boiling point to support your answer.
See it
Key words
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Simple molecular substances
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Weak intermolecular forces
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States of matter at room temperature
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Melting and boiling points
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See every definition
- Simple molecular substances
- Substances that contain a limited number of atoms joined by covalent bonds, such as hydrogen, chlorine, oxygen, nitrogen, carbon dioxide and methane.
- Weak intermolecular forces
- The weak forces between separate molecules that must be overcome when the substance changes state, and which are the key factor in low melting and boiling points.
- States of matter at room temperature
- Small-molecule substances usually exist as gases or liquids at room temperature, although some, such as water and ethanol, are liquids.
- Melting and boiling points
- These are relatively low for small-molecule substances because the weak forces between molecules are overcome, not the covalent bonds within molecules.
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