Kinetic molecular theory is a particle model that explains the physical properties of solids, liquids, and gases through particle arrangement, motion, and energy. It is assessed at both IB Chemistry SL and HL in Structure 1.1.
How the model works
All matter consists of particles that are continually moving. Their kinetic energy is the energy associated with this motion, and temperature in kelvin is a measure of their average kinetic energy.
When a substance is heated, energy is transferred from the surroundings to its particles. Their average kinetic energy usually increases, so they move or vibrate more rapidly. During a change of state, however, the transferred energy is used to overcome attractions between particles rather than increase temperature.
| State | Particle arrangement and movement | Resulting properties |
|---|---|---|
| Solid | Particles are closely packed and vibrate around fixed positions | Fixed shape and volume; difficult to compress |
| Liquid | Particles remain close together but move past one another | Fixed volume but no fixed shape; flows |
| Gas | Particles are widely separated and move rapidly in random directions | No fixed shape or volume; easily compressed |
Gas pressure results from particles colliding with the walls of their container. At a higher temperature, particles have greater average kinetic energy, so collisions are generally more frequent and forceful. This can increase pressure if the volume remains constant.
A common misconception is that all particles at one temperature have the same kinetic energy. They actually have a distribution of energies; temperature represents the average kinetic energy. Another mistake is saying that solid particles do not move: they vibrate about fixed positions.
Exam technique
For an IB explain question, connect the macroscopic observation to particle behaviour. For example: heating a gas increases average kinetic energy, causing more frequent and forceful wall collisions, so pressure increases at constant volume. Avoid merely stating that particles “speed up” without linking this to the observed property.
