Explain why water (H₂O) is a liquid at room temperature but methane (CH₄) is a gas at room temperature, even though both are covalent molecules.

Edexcel GCSE Chemistry (1CH0) — 1.6 Covalent bonding · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Water has a boiling point of 100 °C, while methane has a boiling point of −161 °C. Both substances are made up of small covalent molecules.

Model answer (4 marks)

Both water and methane are small covalent molecules with weak intermolecular forces. Methane has only London dispersion forces, which are very weak, so little energy is required to overcome them and it is a gas at room temperature. Water, however, has hydrogen bonds between its molecules, which are much stronger than dispersion forces. Therefore more energy is needed to break these hydrogen bonds, so water remains a liquid at room temperature.

Examiner tips

  • Use the word ‘hydrogen bonds’ to show you know the type of force in water.
  • Explain that methane only has London dispersion forces, making them weaker.
  • Show the link between stronger forces and higher boiling point.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing hydrogen bonds with dipole–dipole forces.
  • Claiming methane has hydrogen bonds.
  • Using vague terms like ‘stronger forces’ without specifying hydrogen bonds.

Mark scheme (4 marks)

  1. Both water and methane are small covalent molecules with weak intermolecular forces between the molecules
  2. Little energy is needed to overcome the intermolecular forces in methane, so it is a gas at room temperature
  3. Water molecules have stronger intermolecular forces than methane molecules
  4. More energy is needed to overcome the stronger intermolecular forces in water, so it remains a liquid at room temperature

Key terms in this question

covalent molecules

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