# 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.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.1 Introduction to the particulate nature of matter · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. In the solid, particles are held in fixed positions by relatively strong intermolecular forces, with only vibrational motion.
2. On heating, particles gain sufficient (kinetic/thermal) energy to overcome intermolecular forces and escape directly into the gas phase.
3. 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).
4. In the gas phase, particles move rapidly and independently with negligible intermolecular forces, and are widely separated compared to the solid.

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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