Explain why ammonia (NH₃) has a low boiling point and does not conduct electricity when it melts.

Cambridge International IGCSE Chemistry (0620) — 2.5 Simple molecules and covalent bonds · Explain · 4 marks · View as Markdown

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

Ammonia is a gas at room temperature and is used as a refrigerant. It is a simple covalent molecule with a boiling point of –33 °C.

Model answer (4 marks)

Ammonia is a simple covalent molecule, so it is a molecular substance.
The only forces between NH₃ molecules are weak dipole–dipole interactions and London dispersion forces.
Because these forces are weak, only a small amount of energy is required to overcome them, giving a low boiling point.
In all states NH₃ contains no free electrons or ions, so it cannot conduct electricity.

Examiner tips

Use the word ‘molecular’ to show it is a simple covalent substance. Mention the specific weak forces. Explain that low energy to break forces = low boiling point. State the absence of ions/electrons for conductivity.

Common mistakes

Confusing hydrogen bonding with dipole–dipole forces. Claiming ammonia conducts electricity. Forgetting to mention the lack of free charge carriers.

Mark scheme (4 marks)

Ammonia consists of molecules / is a simple molecular substance
There are weak intermolecular forces between the molecules
Little energy is needed to overcome these weak intermolecular forces, so the boiling point is low
There are no free/mobile electrons or ions present (in any state), so it cannot conduct electricity

Key terms in this question

boiling point