Section 1 of 4Particles in Each State
The arrangement and movement of particles decide whether a substance is a solid, liquid or gas.
Solid
- Particles are closely packed in a regular arrangement and vibrate about fixed positions, so a solid keeps its shape and volume.
Liquid
- Particles are close together in a random arrangement and can flow over one another, so a liquid has a fixed volume but no fixed shape.
Gas
- Particles are far apart and move randomly at high speed, so a gas fills any container and can be compressed easily.
Intermolecular forces
- The forces between particles are the same strength for a given substance in every state;
- what changes is the particles' energy relative to these forces.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Density | High | Medium | Low |
| Arrangement | Regular | Random | Random |
| Movement | Vibrate about fixed positions | Flow over one another | Move quickly in all directions |
| Kinetic energy | Lowest | Higher | Highest |
| Definite shape? | Yes | No | No |
| Definite volume? | Yes | Yes | No |
Section 2 of 4Energy and Changes of State
1
Particles in a solid have the least kinetic energy, so they stay in fixed positions held by the forces between them.
2
Adding energy to a solid increases the vibrations until particles can break free of their positions, which is melting.
3
Liquid particles have enough energy to partly overcome the forces, so they can slide past one another.
4
Gas particles have the most energy, enough to overcome the forces completely, so they move freely and spread out.
Section 3 of 4Using the Particle Model
Using the Particle Model
Explains shape
- Solids keep their shape because their particles are fixed in place, while liquids flow to fill the bottom of their container.
Explains density
- A gas is much less dense than a solid because, in the same volume, it contains far fewer particles spread far apart.
Explains compression
- Gases are easy to compress because the large gaps between particles can be reduced by pushing the particles together.
Limitation
- The model treats particles as simple spheres with no detail, so it is a simplification of real matter.
Section 4 of 4Exam Checklist & Tips
4.1.1 – Key Points for the Exam
- Kinetic theory A model that describes matter as particles whose arrangement and movement depend on their energy.
- Solid A state of matter with particles in fixed positions, giving a definite shape and volume.
- Liquid A state of matter with particles that can flow, giving a fixed volume but no fixed shape.
- Gas A state of matter with particles far apart, which fills any container it is placed in.
- Intermolecular forces The attractive forces that act between the particles of a substance.
Common Mistakes
- Saying particles themselves expand or melt when a substance is heated;
- the spacing and movement of the particles change.
- Claiming that particles in a solid do not move; they vibrate about fixed positions.
- Saying intermolecular forces disappear in a liquid;
- the particles simply have enough energy to move past one another.
- Drawing gas particles touching each other rather than widely spaced.