Dry ice is solid carbon dioxide. At room temperature, dry ice undergoes sublimation. Explain what happens to the particles of carbon dioxide during sublimation, and why the mass of carbon dioxide is conserved even though the substance appears to disappear.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Dry ice is used in theatres to create a low-lying mist effect. Stage crew notice that blocks of dry ice left in a warm room gradually shrink and vanish without leaving any liquid behind.
Model answer (4 marks)
Sublimation is the direct change of state from solid to vapour/gas without passing through the liquid phase.
The CO₂ particles gain enough energy (latent heat/thermal energy from the surroundings) to overcome the intermolecular forces that keep them fixed in the solid lattice.
They then spread apart and move freely with high kinetic energy, forming a gas.
Mass is conserved because the same number of CO₂ particles exists as a gas as existed in the solid; no particles are created or destroyed during a change of state, which is a physical change.
The CO₂ particles gain enough energy (latent heat/thermal energy from the surroundings) to overcome the intermolecular forces that keep them fixed in the solid lattice.
They then spread apart and move freely with high kinetic energy, forming a gas.
Mass is conserved because the same number of CO₂ particles exists as a gas as existed in the solid; no particles are created or destroyed during a change of state, which is a physical change.
Examiner tips
- Use the word ‘sublimation’ first; explain energy gain and force overcoming; describe particles becoming a gas; state mass conservation as a physical change.
- Show the sequence: solid → energy input → forces overcome → gas; link to conservation of mass.
Common mistakes
- Confusing sublimation with melting or evaporation; writing ‘liquid’ instead of ‘gas’; claiming mass is lost rather than conserved.
Mark scheme (4 marks)
- Sublimation is the direct change of state from solid to vapour/gas without passing through the liquid phase
- The particles gain enough energy (latent heat / thermal energy from surroundings) to overcome the intermolecular forces holding them in fixed positions
- The particles spread apart and move freely with high kinetic energy, forming a gas
- Mass is conserved because the same number of carbon dioxide particles exists as a gas as existed in the solid; no particles are created or destroyed during a change of state, which is a physical change
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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