Everything around us is made of tiny particles. The kinetic particle theory explains the three states of matter — solid, liquid and gas — by how these particles are arranged, how strongly they attract one another and how they move.
The three states
- Solid: particles are packed closely in a regular, fixed pattern. They vibrate about fixed positions, so a solid has a fixed shape and fixed volume.
- Liquid: particles are still close together but arranged randomly. They can slide over one another, so a liquid has a fixed volume but takes the shape of its container.
- Gas: particles are far apart and move quickly in random directions. A gas has no fixed shape or volume, spreads out to fill its container and is easily compressed.
Key idea
Heating gives particles more kinetic energy, so they move faster. Cooling removes energy and slows them down. The forces of attraction are strongest in solids and weakest in gases.
Changes of state
Adding or removing energy changes the state: melting (solid→liquid), freezing (liquid→solid), boiling/evaporation (liquid→gas), condensation (gas→liquid) and sublimation (solid→gas directly). During a change of state the temperature stays constant, because the energy supplied is used to overcome the forces of attraction between particles rather than to raise the temperature. This is why a pure substance melts and boils at fixed, sharp temperatures.
Remember
- Melting and boiling happen at fixed temperatures for a pure substance.
- Evaporation occurs only at the surface and at any temperature; boiling occurs throughout the liquid at its boiling point.
- Gases are easily compressed because of the large spaces between particles.