Unit 4 · Lesson 4.2.2
Changes of State
Describe the particle arrangement in solids, liquids and gases, explain melting, boiling and evaporation, and relate heating to internal energy.
States of Matter
Solid
- Particles are tightly packed in a regular pattern and vibrate about fixed positions, giving a fixed shape and volume.
Liquid
- Particles are close together but arranged randomly and slide over each other, giving a fixed volume but no fixed shape.
Gas
- Particles are far apart and move quickly in random directions, so a gas fills any container and can be compressed.
Density
- Solids and liquids are dense because particles touch, whereas gases have low density because particles are far apart.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular pattern | Random, close | Random, far apart |
| Movement | Vibrate in place | Move around each other | Fast in all directions |
| Energy of particles | Lowest | Greater | Greatest |
Changes of State
1
Heating a solid raises the average kinetic energy of its particles until it reaches its melting point.
2
At the melting point the energy supplied goes into the potential store, breaking the bonds that hold the particles in position.
3
The liquid is then heated until the boiling point, where energy again increases particle separation rather than temperature.
4
Boiling frees particles throughout the liquid, forming bubbles of gas that rise to the surface.
5
Cooling reverses the changes: condensing and freezing release energy to the surroundings.
Internal Energy and Evaporation
Internal Energy and Evaporation
Internal energy
- The internal energy of a substance is the total kinetic and potential energy of its particles.
Temperature rise
- Heating can increase kinetic energy, so the particles move faster and the temperature goes up.
Change of state
- Heating at the melting or boiling point increases potential energy instead, so the temperature stays constant.
Evaporation
- It happens at any temperature at the surface, when the fastest particles escape, so the remaining liquid cools.
Boiling versus evaporation
- Boiling occurs at one temperature with bubbles throughout the liquid, whereas evaporation does not form bubbles.
Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Melting: The change from solid to liquid at the melting point.
- Boiling: The change from liquid to gas throughout the liquid at the boiling point.
- Evaporation: The change from liquid to gas at the surface at any temperature.
- Internal energy: The sum of the kinetic and potential energies of the particles of a system.
- Intermolecular bonds: Attractive forces between particles that hold a solid or liquid together.
Common Mistakes
- Saying particles in a solid do not move; they vibrate about fixed positions.
- Saying the temperature rises during melting; it stays constant while energy breaks the bonds.
- Stating that particles themselves expand or melt when a substance does.
- Mixing up evaporation and boiling; evaporation happens only at the surface and at any temperature.