Solid carbon dioxide (dry ice) turns directly into a gas at −78 °C without first becoming a liquid. This process is called sublimation. Explain, in terms of particles, why energy must be supplied for sublimation to occur, and why the resulting gas occupies a much larger volume than the solid.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.1 States of matter · Explain · 4 marks · View as Markdown

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

Solid carbon dioxide (dry ice) sublimes at −78 °C at atmospheric pressure, changing directly from a solid to a gas.

Model answer (4 marks)

In a solid the particles are fixed in a lattice and are held together by strong forces of attraction. To change to a gas the particles must be pulled apart, so energy must be supplied to overcome these attractive forces. In the gas the particles are far apart, move rapidly and randomly in all directions, and therefore the gas occupies the entire available volume, which is much larger than the solid.

Examiner tips

  • Use the word "energy" and "forces of attraction" to show understanding of the need for energy.
  • Explain that particles are far apart and move rapidly to justify the larger volume.
  • Mention the change from a fixed lattice to a random distribution.

Mark scheme (4 marks)

  1. In the solid, particles are held in fixed positions by strong forces of attraction (between particles).
  2. Energy is needed to overcome these forces of attraction between the particles.
  3. In the gas, the particles are much more spread out / far apart compared to the solid.
  4. In the gas, particles move rapidly and randomly in all directions, so the gas fills the available space / volume.

Key terms in this question

sublimation · particles · gas · solid

Related

More States of matter questions

▶ Try answering this question with AI marking (free) →