Explain why hydrogen chloride (HCl) has a low melting point.

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).

Hydrogen chloride is a gas at room temperature with a melting point of –114 °C. It consists of simple molecules held together by covalent bonds.

Model answer (4 marks)

HCl is a simple molecule, so it has no permanent dipole–dipole or ionic forces.
The molecules are held together only by weak London dispersion forces.
These forces are small, so only a little energy is required to break them.
The strong covalent H–Cl bond inside each molecule is not broken during melting.

Examiner tips

Use the word ‘simple molecule’ to show understanding of structure. Mention London dispersion forces explicitly. Explain that only a small amount of energy is needed. State that the covalent bond remains intact during melting.

Common mistakes

Confusing intermolecular forces with the covalent bond. Forgetting to mention that HCl is a simple molecule. Using vague terms like ‘weak forces’ without specifying London dispersion.

Mark scheme (4 marks)

HCl consists of simple molecules
There are weak intermolecular forces between the molecules
Only a small amount of energy is needed to overcome these intermolecular forces
The covalent bond within the molecule is strong / is not broken on melting

Key terms in this question

melting point