Explain, using the particle model, what happens to the particles when a solid substance is heated and melts, and state one limitation of the particle model in describing this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student heats a solid substance and observes it changing to a liquid at its melting point. The student uses the particle model to explain what they see.
Model answer (5 marks)
In a solid the particles are arranged in a fixed, regular pattern and are held in fixed positions by strong forces between them.
When the solid is heated the particles gain kinetic (thermal) energy.
The increased energy weakens the forces holding the particles together, so the particles can move more freely.
The substance changes from solid to liquid – a physical change – so its chemical composition remains unchanged.
A limitation of the particle model is that it represents all particles as identical solid spheres and does not show the forces between them, so it cannot explain why different substances have different melting points.
When the solid is heated the particles gain kinetic (thermal) energy.
The increased energy weakens the forces holding the particles together, so the particles can move more freely.
The substance changes from solid to liquid – a physical change – so its chemical composition remains unchanged.
A limitation of the particle model is that it represents all particles as identical solid spheres and does not show the forces between them, so it cannot explain why different substances have different melting points.
Examiner tips
- Use the phrase "fixed, regular pattern" for solids
- Show the energy gain and weakening of forces
- Mention that the change is physical and composition unchanged
- State a clear limitation such as lack of force representation
Common mistakes
- Using vague terms like "particles move" without linking to energy gain
- Forgetting to note that the change is physical, not chemical
- Giving a limitation that is too general, e.g. "particles are too simple" without reference to forces
Mark scheme (5 marks)
- In a solid, particles are held in fixed positions / arranged in a regular pattern
- When heated, the particles gain (kinetic/thermal) energy
- The forces between the particles are overcome / weakened, allowing particles to move more freely
- This change of state (melting) is a physical change, so the chemical composition of the substance does not change
- One limitation of the particle model: the model does not show forces between particles / all particles are represented as identical solid spheres / the model does not explain why some substances have higher melting points than others
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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