Explain why a substance that sublimes, such as iodine, transitions directly from solid to gas without passing through a liquid phase under standard atmospheric pressure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iodine (I₂) is a solid at room temperature and standard atmospheric pressure. When gently heated, it sublimes, producing a purple vapour, without forming a liquid.
Model answer (4 marks)
In the solid, iodine molecules are held in a crystal lattice by strong London dispersion forces, so they vibrate in fixed positions. When heated, the molecules gain kinetic energy until the vapour pressure of the solid equals atmospheric pressure. Because this occurs before the melting point, the solid bypasses the liquid phase and the molecules escape directly into the gas phase. In the gas phase the iodine molecules move rapidly and independently, with negligible intermolecular forces and are widely separated compared with the solid.
Examiner tips
- Use the term ‘vapour pressure’ and ‘triple point’ to show understanding of phase diagrams.
- Explain that the solid’s vapour pressure reaches 1 atm before the melting point.
- Mention the strong London dispersion forces that keep the solid together.
- Show the transition from vibrational motion to rapid, independent gas motion.
Common mistakes
- Confusing sublimation with evaporation or melting.
- Failing to mention vapour pressure or the triple point.
- Using vague terms like ‘strong forces’ without specifying London dispersion.
Mark scheme (4 marks)
- In the solid, particles are held in fixed positions by relatively strong intermolecular forces, with only vibrational motion.
- On heating, particles gain sufficient (kinetic/thermal) energy to overcome intermolecular forces and escape directly into the gas phase.
- Sublimation occurs because the vapour pressure of the solid reaches atmospheric pressure before the melting point is reached (i.e. the triple-point pressure exceeds atmospheric pressure).
- In the gas phase, particles move rapidly and independently with negligible intermolecular forces, and are widely separated compared to the solid.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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