Particles and changes of state
Explain melting and evaporation using a simple particle model.
Before you begin
- Solids, liquids and gases
- Reading a thermometer
Connect particle arrangement to state
In a solid, particles vibrate about fixed positions. In a liquid, neighbouring particles can move past one another. In a gas, particles are widely separated and move freely between collisions. A particle diagram is a model: its dots represent particles, not tiny visible pieces of the material. The spaces between gas particles are not filled with another substance in a pure gas.
Explain a physical change
When a substance melts or boils, its particles remain the same chemical substance while their arrangement and motion change. Energy is transferred to overcome attractions. Evaporation happens at a liquid's surface and can occur below its boiling point. In a closed system, mass is conserved during a change of state even though volume may change.
Worked example
Water disappears from an uncovered shallow dish. Explain where it went.
Show the worked solution
- Some surface water molecules escape into the surrounding air.
- The water is now water vapour, still made of water molecules.
- The dish is an open system, so its measured mass decreases as water leaves.
The water evaporated into the air; it was not destroyed.
Try it yourself
What happens to water molecules when ice melts?
Common mistakes
- Evaporation and boiling are not identical processes.
What can you explain now?
Why does a sealed container keep the same total mass when ice inside it melts?
Compare with the explanation
No matter enters or leaves the container.
The water changes state but the same particles remain within the sealed system. Their arrangement changes, not the total amount of matter.
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