Properties of small molecules — AQA GCSE Combined Science
Test yourself on Properties of small molecules with AQA GCSE practice questions.
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Properties of small molecules explained
Small-molecule substances contain discrete molecules held together by covalent bonds, but the forces between separate molecules are weak.
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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.
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. 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.
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 by weak intermolecular forces, while the atoms inside each molecule are joined by strong covalent bonds. As the molecules get larger, with more atoms and more electrons, the intermolecular forces become stronger. More energy is therefore needed to overcome these forces when the substance melts or boils, so melting and boiling points rise. For example, methane (CH₄) is a gas at room temperature, but larger alkanes such as octane (C₈H₁₈) are liquids. The covalent bonds themselves 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 because their molecules are neutral overall. Each molecule contains covalent bonds in which electrons are shared between atoms, but the molecule as a whole has no net electric charge. Since there are no charged particles that are free to move, no current can flow. This applies whether the substance is solid, liquid or gaseous. For example, methane (CH₄) and water (H₂O) are simple molecular substances and do not conduct electricity, unlike ionic compounds when molten or dissolved, or metals, which contain free 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.
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.
Your focus
- Describe the structure of small-molecule substances in terms of discrete molecules.
- Explain why small-molecule substances have relatively low melting points and boiling points.
- Relate the physical state of small-molecule substances at room temperature to the strength of intermolecular forces.
Show all 15 objectives
- Distinguish between intermolecular forces and covalent bonds in small-molecule substances.
- Explain that melting and boiling overcome intermolecular forces, not covalent bonds.
- Apply this understanding to explain the physical properties of small-molecule substances.
- Describe how intermolecular forces change as the size of small molecules increases.
- Explain why larger molecules have higher melting and boiling points than smaller molecules.
- Distinguish between overcoming intermolecular forces and breaking covalent bonds during melting and boiling.
- Explain why simple molecular substances do not conduct electricity.
- Relate the absence of free charged particles to the inability to carry an electric current.
- Compare simple molecular substances with ionic and metallic conductors in terms of charged particles.
- Describe the difference between strong covalent bonds within molecules and weak intermolecular forces between molecules.
- Explain how weak intermolecular forces lead to low melting and boiling points in molecular substances.
- Apply the idea of weak intermolecular forces to explain why molecular substances are often gases or liquids at room temperature, and explain their lack of electrical conductivity due to the absence of mobile charged particles.
Properties of small molecules exam tips
Marking 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.
- 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.
- 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).
- Intermolecular forces act between separate small molecules, not between the atoms within a molecule.
- The strength of intermolecular forces increases as the size of the molecules increases.
- Larger molecules need more energy to overcome their intermolecular forces during melting or boiling.
- Melting and boiling points therefore increase as molecular size increases.
- Covalent bonds within each molecule remain intact when a simple molecular substance melts or boils.
- A suitable comparison, such as methane being a gas and octane being a liquid at room temperature, supports the trend.
- Simple molecular substances contain molecules that are electrically neutral overall.
- Covalent bonds involve shared electrons, but this sharing does not give the molecule a net charge.
- Electrical conduction requires charged particles that are free to move.
- Because simple molecular substances have no free charged particles, they do not conduct electricity.
- The absence of conduction applies in solid, liquid and gaseous states.
- A contrast with ionic or metallic conductors can be used to show why simple molecular substances behave differently.
- 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.
Examiner 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.
- 💡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.
- 💡Use the phrase intermolecular forces when explaining melting and boiling, and keep it distinct from covalent bonds.
- 💡Link molecular size to the energy needed to overcome the forces, then state the effect on melting and boiling points.
- 💡Give a named example, such as methane compared with a larger alkane, to make the trend concrete.
- 💡State clearly that conduction needs charged particles that are free to move.
- 💡Explain that simple molecular substances have no overall charge and no free charged particles.
- 💡Use a named example, such as methane or water, and contrast it with an ionic or metallic conductor.
- 💡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.
Common Mistakes
- Error: thinking that covalent bonds break when a small-molecule substance boils. Correction: only weak intermolecular forces are overcome; covalent bonds remain intact.
- Error: assuming all small-molecule substances are gases. Correction: some are liquids or low-melting solids, such as bromine and iodine.
- Error: confusing intermolecular forces with covalent bonds. Correction: intermolecular forces act between molecules; covalent bonds act within molecules.
- Error: saying that covalent bonds break when a small-molecule substance boils. Correction: only intermolecular forces are overcome; covalent bonds remain intact.
- Error: using 'intermolecular forces' and 'covalent bonds' interchangeably. Correction: intermolecular forces act between molecules; covalent bonds act within molecules.
- Error: thinking that melting involves breaking bonds within molecules. Correction: melting overcomes forces between molecules, not bonds within them.
- Saying that covalent bonds break when a simple molecular substance melts or boils; correct this by stating that only the weak intermolecular forces are overcome.
- Confusing intermolecular forces with covalent bonds; correct this by describing covalent bonds as strong forces within molecules and intermolecular forces as weaker attractions between molecules.
- Claiming that all small molecules have low melting points regardless of size; correct this by explaining that the trend depends on molecular size, so larger molecules have higher melting and boiling points.
- Thinking that shared electrons in covalent bonds can carry charge through the substance; correct this by stating that the electrons are localised within the molecule and the molecule is neutral overall.
- Assuming that a simple molecular substance will conduct when melted; correct this by explaining that melting overcomes intermolecular forces but does not produce free charged particles.
- Confusing simple molecular substances with ionic compounds; correct this by noting that ionic compounds can conduct when molten or dissolved because their ions are free to move.
- Saying that boiling breaks covalent bonds: correct this by stating that boiling overcomes weak intermolecular forces while covalent bonds stay intact.
- Confusing intermolecular forces with covalent bonds: correct this 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: correct this by explaining that no free charged particles are present, so they do not conduct.