The three states
The kinetic particle model says that all matter is made of tiny particles that are always moving. The state of matter depends on how the particles are arranged and how much energy they have:
| State | Arrangement | Motion |
| Solid | close, regular, fixed | vibrate about fixed positions |
| Liquid | close, irregular | move around each other |
| Gas | far apart, random | fast, random, in all directions |
Key idea
Temperature is a measure of the average kinetic energy of the particles. Heating a substance gives its particles more kinetic energy, so they move or vibrate faster.
Changes of state and gas pressure
Adding energy can melt a solid or boil a liquid; removing energy reverses these. In a gas, particles collide with the container walls and each collision exerts a tiny force. The total effect of many collisions is the gas pressure. Heating a fixed volume of gas makes particles hit the walls harder and more often, so the pressure rises.
Worked example
A sealed rigid can of gas is heated. Explain what happens to its pressure.
Heating increases the average kinetic energy of the particles, so they move faster. They collide with the walls more frequently and with greater force. Since the volume is fixed, the pressure increases.
Remember
- Gases are easily compressed; solids and liquids are not.
- Higher temperature means higher average kinetic energy.
- Gas pressure is caused by particle collisions with the walls.