The kinetic particle model explains the properties of solids, liquids and gases by describing how particles are arranged and how they move.
Particles in the three states
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed | Irregular, close together | Random, widely spaced |
| Motion | Vibrate about fixed positions | Move around each other | Move rapidly in random directions |
| Spacing | Very small | Small | Large (about 10x the particle diameter) |
| Forces between particles | Strong | Moderate | Very weak (negligible except during collisions) |
Density and compressibility
- Solids and liquids have high density because particles are close together.
- Gases have low density because particles are far apart.
- Gases are easily compressed (particles can be pushed closer). Solids and liquids are virtually incompressible.
Temperature and particle energy
Temperature is related to the average kinetic energy of the particles. Higher temperature means particles move faster (in liquids and gases) or vibrate more vigorously (in solids).
At absolute zero (0 K, or C), particles have minimum kinetic energy.
Worked example
Explain why a gas fills its container but a solid does not.
Gas particles move rapidly in random directions with negligible forces between them, so they spread out to fill all available space. Solid particles vibrate about fixed positions held by strong forces, so a solid maintains a fixed shape and volume.
Common errors and how to correct them
Saying “particles expand” when heated. Particles do not change size. They move faster and the spaces between them increase, so the material expands.
Stating gas particles “have no energy” at low temperatures. Even at low temperatures, particles have some kinetic energy (unless at absolute zero). Lower temperature means less kinetic energy, not zero.
How to apply this in an exam
When describing a state of matter, always address three things: arrangement, motion, and the forces between particles. Use the specific terms (vibrate, move around each other, move rapidly in random directions).
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