The kinetic theory states that all matter is made up of tiny particles that are constantly moving.

The particles are held closely together in a regular pattern and can only just vibrate
The particles are still close together but are now able to slide over one another and mix. The motion is more random than in a solid.
The particles are far apart and able to move freely in straight lines.
They have a lot more kinetic energy than the particles in a solid or liquid.


If a substance is warmed the particles will vibrate FASTER.

If the particles are smaller they will move faster

The link below demonstrates these points nicely:

Animation to show the movement of particles in a gas
Click on the link above. It will open a new web page that shows the animation.
The little moving dots represent the tiny particles, usually molecules, that the gas is made from.
Try adjusting the sliders to show the effect of changing various properties of the gas.

The kinetic theory of gases can explain many observations

The particles at the surface of a liquid have enough energy to break free and fly off into the air.
this makes it evaporate.
If the liquid is warm the particles are moving faster so they fly off quicker. This makes it evaporate faster.

When a substance is heated the particles move faster and push themselves a little further apart.
This makes the substance expand (get larger)

The moving particles are responsible for causing substances to slowly mix together.
this mixing together is called diffusion

Pressure in a gas: Gas particles moving cause the pressure in a gas.
When a balloon is inflated the moving gas particles push against the rubber.
If the balloon is warmed the particles move faster and push harder on the rubber.
This is why the balloon expands.