Section 1 of 6What the model explains (HT only)
States and energy
- States and energy In the particle model, a substance is made of particles whose arrangement and movement depend on how much energy they have and on the forces between them.
- Changing state means giving the particles enough energy to overcome, or take away enough energy to let them be held by, those forces.
Strength of forces
- Strength of forces The stronger the forces between particles, the more energy is needed to melt or boil the substance,
- so it has higher melting and boiling points.
- A substance with weak forces between its particles changes state at low temperatures.
Section 2 of 6What happens as a substance is heated
1Solid warms
- Solid warms Thermal energy is transferred to the particles and becomes kinetic energy, so they vibrate more strongly about fixed positions.
2Melting
- Melting At the melting point the vibrations are strong enough to break the bonds or forces holding particles in position.
- The solid becomes a liquid and the temperature stays constant while this happens.
3Liquid warms and evaporates
- Liquid warms and evaporates Particles move faster.
- Some at the surface gain enough energy to overcome the forces holding them and escape as a gas, which is evaporation.
4Boiling
- Boiling At the boiling point particles throughout the liquid have enough energy to break free, so bubbles of gas form.
- The temperature is again constant on the heating curve.
Section 3 of 6Limitations of the model (HT only)
Particles drawn as solid spheres
- Particles drawn as solid spheres The model shows every particle as a small, hard, solid sphere.
- Real atoms, ions and molecules are not always spherical, and many molecules have complicated shapes.
Particles not truly inelastic
- Particles not truly inelastic Particles in the model behave like inelastic balls.
- In reality the particles can change shape slightly and store energy when they collide.
No forces shown
- No forces shown Simple diagrams give no indication of the forces of attraction between particles, yet these forces decide the state and the melting and boiling points.
Wrong scale
- Wrong scale The spheres are drawn large and close together,
- so the diagram does not show how small particles are compared with the empty space between them, particularly in a gas.
Section 4 of 6Reading a heating curve
| Part of curve | What is happening | Temperature |
|---|---|---|
| Sloping section (solid, liquid or gas) | Energy supplied raises the average kinetic energy of the particles. | Rising |
| Flat section at melting point | Energy is used to overcome forces between particles as the solid melts. | Constant |
| Flat section at boiling point | Energy is used to separate the particles completely as the liquid boils. | Constant |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Particle model | A simple model in which matter is made of tiny particles whose arrangement and motion explain states of matter. |
| Kinetic energy | The energy a particle has because of its movement or vibration. |
| Intermolecular forces | Attractive forces between molecules, which are overcome when a simple molecular substance melts or boils. |
| Evaporation | Particles escaping from the surface of a liquid at a temperature below its boiling point. |
| Limitation | A way in which a model fails to match what is really happening. |
Section 6 of 6Exam technique
1
Thinking that the flat parts of a heating curve mean no energy is being supplied — energy is still transferred, but it changes the state instead of the temperature.
2
Saying that the particles themselves melt or expand when a substance melts — the particles stay the same, only their arrangement and motion change.
3
Forgetting to name a specific limitation, such as that forces between particles are not shown, and writing only 'it is not accurate'.
4
Mixing up breaking intermolecular forces with breaking covalent bonds when a simple molecular substance changes state.