Kinetic Particle Model of Matter: Question 6
Syllabus 2.1.1
A pure solid substance is heated steadily until it just melts. Which statement correctly describes what happens to its particles during melting?
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Worked solution
Step 1: Recall the solid state before melting
In a solid, particles are held in fixed positions in a regular, close-packed arrangement by strong forces of attraction, and can only vibrate about those fixed positions.
Step 2: Consider what heating does
Heating transfers energy to the particles, which vibrate more and more vigorously. Once melting begins, enough energy has been supplied for particles to partly (not completely) overcome the forces holding them in their fixed positions.
Step 3: Describe the liquid that results
Once melted, the particles are free to move around and slide past one another, but they remain close together, still touching their neighbours. This is the liquid state, very different from a gas, where particles spread out with large empty spaces between them.
Step 4: Match to the options
This description matches option A.
Why the other options are wrong
| Option | Problem |
|---|---|
| B | Heating supplies energy; it does not remove it or tighten the fixed arrangement. |
| C | This describes boiling/evaporation (liquid to gas), not melting. |
| D | Melting does not stop particle vibration or create a more compact arrangement; the arrangement becomes less ordered, not more. |
Final answers
- During melting, particles gain enough energy to partly overcome the forces holding them in fixed positions, becoming free to move around each other while remaining close together, option A.