The idea
In a solid, particles vibrate around fixed positions. In a liquid, they remain close together but can move past one another. Gas particles are far apart and move freely between collisions. During melting or boiling of a pure substance at constant pressure, temperature stays constant while energy changes the interactions between particles rather than increasing average kinetic energy.
Worked example
Explain why ice can absorb energy while remaining at its melting temperature.
- Energy is transferred to the ice.
- During melting, energy is used to overcome interactions that maintain the solid structure.
- Average kinetic energy does not increase during the phase change, so temperature remains constant until melting is complete.
A mistake to avoid
Particles do not become larger when a substance expands. Changes in separation and arrangement explain expansion.
Check your understanding
Does boiling water break the covalent O–H bonds inside water molecules?
Show the explanation
No. Boiling separates water molecules from each other; the molecules remain H₂O.
Explore the lesson
This guide connects to How Do Particles Actually Behave? in the IB Chemistry course. The first two published lessons per subject are free previews; further interactive lessons require an account.