Explain why hydrogen chloride (HCl) is a gas at room temperature, whereas sodium chloride (NaCl) is a solid at room temperature. Refer to the types of bonding and structure in each substance in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen chloride and sodium chloride both contain chlorine, but they have very different properties at room temperature.
Model answer (4 marks)
HCl is a covalent compound; the H and Cl atoms share a pair of electrons to form a simple molecule. The molecules are held together only by weak London dispersion forces, so they can move freely and exist as a gas at room temperature.
NaCl is an ionic compound; Na⁺ and Cl⁻ ions are held together in a giant lattice by strong electrostatic (ionic) bonds. The lattice requires a large amount of energy to break, so NaCl remains a solid at room temperature.
NaCl is an ionic compound; Na⁺ and Cl⁻ ions are held together in a giant lattice by strong electrostatic (ionic) bonds. The lattice requires a large amount of energy to break, so NaCl remains a solid at room temperature.
Examiner tips
- Use the word "covalent" for HCl and "ionic" for NaCl. Mention weak London forces for HCl and strong ionic lattice for NaCl. Explain that the energy needed to break bonds determines state at room temperature.
- Include the key terms: covalent, ionic, lattice, London dispersion, electrostatic forces.
Common mistakes
- Confusing hydrogen chloride with hydrochloric acid (the aqueous solution). Forgetting to mention the type of intermolecular forces in HCl. Saying NaCl is a covalent compound instead of ionic.
Mark scheme (4 marks)
- HCl contains covalent bonding (a shared pair of electrons between hydrogen and chlorine)
- HCl exists as simple molecules with only weak intermolecular forces between them
- NaCl has ionic bonding / is an ionic compound with a giant ionic lattice structure
- The strong electrostatic forces / ionic bonds in NaCl require much more energy to break than the weak intermolecular forces in HCl, so NaCl is solid and HCl is a gas at room temperature
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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