Properties of small molecules — AQA GCSE Chemistry
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Properties of small molecules explained
Properties of small molecules covers 6 specification points, including Substances that consist of small molecules are… and These substances have only weak forces between….
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Open a point below for its explanation and revision guidance.
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. 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.
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. 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.
It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.
When a simple molecular substance melts or boils, energy is transferred to the substance. That energy is used to overcome the weak intermolecular forces between the molecules, allowing the molecules to move further apart or past one another. The strong covalent bonds within each molecule are not broken, so the molecules themselves stay intact. For example, when ice melts, the water molecules separate from the ordered lattice but each H₂O molecule remains covalently bonded; when water boils, molecules escape into the gas phase without decomposing. This distinction is central to explaining why simple molecular substances have low melting and boiling points despite containing strong covalent bonds. Always state clearly which forces are overcome and which bonds remain intact.
The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
Small covalent molecules are held together in a liquid or solid by intermolecular forces acting between whole molecules. These forces are much weaker than the covalent bonds inside each molecule. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome them when a substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, whereas larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds within each molecule are not broken during melting or boiling; only the forces between molecules are overcome.
These substances do not conduct electricity because the molecules do not have an overall electric charge.
Simple molecular substances do not conduct electricity in any state because their molecules are neutral overall. A molecule such as methane (CH₄) contains covalent bonds, but the shared electrons are held between the atoms and are not free to move through the substance. There are no charged particles, such as ions or delocalised electrons, that can carry charge. Even though individual atoms in a bond may carry small partial charges, the molecule as a whole has no overall electric charge. This is why substances like water, carbon dioxide and iodine do not conduct electricity, unlike ionic compounds when molten or dissolved, or metals and graphite which contain delocalised electrons.
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.
Molecular substances have low melting and boiling points because the intermolecular forces between their molecules are weak compared with the covalent bonds inside each molecule. When a simple molecular substance melts or boils, energy is used to overcome the weak intermolecular forces, not to break the strong covalent bonds. This explains why substances such as iodine or carbon dioxide have low melting and boiling points despite containing strong covalent bonds. It also explains why molecular substances are usually gases or liquids at room temperature. Their inability to conduct electricity is due to the absence of mobile charge carriers, not weak forces. The key idea is the contrast: weak forces between molecules, strong bonds within molecules.
Your focus
- Identify common small-molecule substances and their states at room temperature.
- Explain why small-molecule substances have relatively low melting and boiling points.
- Distinguish between covalent bonds within molecules and intermolecular forces between molecules.
Show all 18 objectives
- Define intermolecular forces as forces between molecules.
- Describe intermolecular forces in simple molecular substances as weak.
- Contrast weak intermolecular forces with strong covalent bonds within molecules.
- Explain that melting and boiling overcome intermolecular forces rather than covalent bonds.
- Describe how molecules remain intact during a state change.
- Use a named example to distinguish between intermolecular forces and covalent bonds.
- Describe how intermolecular forces change with molecular size.
- Explain why larger molecules have higher melting and boiling points.
- Distinguish between intermolecular forces and covalent bonds when describing melting and boiling.
- State why simple molecular substances do not conduct electricity.
- Identify the charged particles needed for electrical conduction.
- Compare simple molecular substances with ionic and metallic conductors.
- Explain bulk properties of molecular substances using the strength of intermolecular forces.
- Contrast intermolecular forces with covalent bonds in terms of relative strength.
- Apply the idea to explain melting point, boiling point and state at room temperature.
Properties of small molecules exam tips
Marking 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.
- Distinguish between the strong covalent bonds within molecules and the weak intermolecular forces between molecules.
- 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.
- State that melting or boiling involves overcoming intermolecular forces between molecules.
- State that covalent bonds within molecules are not broken during melting or boiling.
- Explain that molecules remain intact and only become further apart or move past one another.
- Use a named example, such as water or methane, to show that the molecules survive the state change.
- Link the energy transferred during melting or boiling to overcoming weak intermolecular forces rather than breaking strong covalent bonds.
- Intermolecular forces act between separate molecules, not within them.
- The strength of intermolecular forces increases as molecular size increases.
- Larger molecules therefore need more energy to overcome these forces.
- Melting and boiling points increase with molecular size.
- Covalent bonds within molecules remain intact during melting and boiling.
- A suitable comparison, such as methane and octane, can illustrate the trend.
- Simple molecular substances contain neutral molecules with no overall charge.
- There are no free ions or delocalised electrons to carry charge.
- Covalent bonds hold atoms together within molecules but do not provide mobile charge carriers.
- Conduction requires charged particles that are free to move.
- This explains why simple molecular substances do not conduct electricity in the solid, liquid or gaseous state.
- Intermolecular forces are weak compared with covalent bonds.
- Melting and boiling overcome intermolecular forces, not covalent bonds.
- Weak intermolecular forces explain low melting and boiling points.
- Strong covalent bonds remain intact when a molecular substance changes state.
- Bulk properties such as state at room temperature can be explained using this contrast.
Examiner 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.
- 💡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.
- 💡Use the phrase overcome intermolecular forces and state explicitly that covalent bonds are not broken.
- 💡Name a substance and describe what happens to its molecules during the state change.
- 💡Avoid saying bonds are broken unless you mean covalent bonds, and make clear which type of force or bond you are discussing.
- 💡State clearly that it is the intermolecular forces that are overcome, not the covalent bonds.
- 💡Link molecular size to the strength of intermolecular forces before explaining the effect on boiling point.
- 💡Use a named example, such as methane versus octane, to support the trend.
- 💡Refer explicitly to the absence of free ions or delocalised electrons as the reason for no conduction.
- 💡Avoid saying molecules have no charge at all; state that they have no overall electric charge.
- 💡Contrast with a conducting substance, such as a molten ionic compound, to show understanding.
- 💡Use the phrase weak intermolecular forces compared with strong covalent bonds to structure your answer.
- 💡Explain bulk properties by referring to energy needed to overcome intermolecular forces.
- 💡Check that your answer distinguishes between forces between molecules and bonds within molecules.
Common Mistakes
- Claiming that small molecules have low boiling points because the covalent bonds are weak; correct this 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; correct this by focusing on intermolecular forces rather than molecular size.
- Assuming all small-molecule substances are gases; correct this by noting that some, such as water and ethanol, are liquids at room temperature.
- Saying that covalent bonds between molecules are weak; correct this by stating that intermolecular forces between molecules are weak while covalent bonds within molecules are strong.
- Describing intermolecular forces as bonds; correct this by calling them forces of attraction between molecules.
- Stating that molecules break apart when a simple molecular substance boils; correct this by explaining that molecules separate from one another but covalent bonds remain intact.
- Saying that covalent bonds break when a simple molecular substance boils; correct this by stating that intermolecular forces are overcome and covalent bonds remain intact.
- Stating that molecules decompose on melting; correct this by explaining that the molecules separate but keep their covalent bonds.
- Confusing the energy needed to overcome intermolecular forces with the much larger energy needed to break covalent bonds; correct this by distinguishing the two clearly.
- Saying that covalent bonds break when a molecular substance boils; the correction is that intermolecular forces are overcome, while covalent bonds stay intact.
- Confusing intermolecular forces with covalent bonds; the correction is that intermolecular forces are between molecules and are much weaker.
- Assuming all small molecules have low boiling points regardless of size; the correction is that larger molecules have stronger intermolecular forces and higher boiling points.
- Thinking that covalent bonds allow conduction because electrons are shared; the correction is that shared electrons are localised and not free to move through the substance.
- Confusing simple molecular substances with ionic compounds; the correction is that ionic compounds conduct when molten or dissolved because their ions are free to move.
- Believing that all non-metals cannot conduct; the correction is that graphite, a non-metal, conducts because it has delocalised electrons.
- Stating that covalent bonds break when a molecular substance boils; the correction is that only intermolecular forces are overcome.
- Saying intermolecular forces are strong because covalent bonds are strong; the correction is that the two are different and intermolecular forces are much weaker.
- Describing melting as breaking molecules apart; the correction is that molecules separate from each other but remain intact.