Brownian Motion      Evaporation      Expansion      Diffusion    Pressure in a gas

The particle theory (sometimes called kinetic theory) states that all matter is made up of tiny particles that are constantly moving.

in a Solid:kt_anim1
The particles are held closely together in a regular pattern and can only just vibrate
in a Liquid:kt_anim2
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.
in a Gas:kt_anim3
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


To see a simulation showing the three states click the image below

Friction causes an increase in temperature  because it gives the particles on the surface more energy, making them vibrate faster.
In the simulation below when the chemistry book is slid back and forth you will see the particles starting to vibrate faster.
Notice they are vibrating slightly before the experiment starts.Click on the image above to see the simulation

Brownian motion.
This movement of particles is very hard to prove because the particles are too small to see.

However, if smoke (or any other tiny particles) are viewed through a microscope we will see the particles appear to vibrate as they are buffeted by the air particles (or water molecules) around them.

This seemingly random movement of particles suspended in air or water is referred to as Brownian Motion (named after the Scottish botanist Robert Brown who was looking at pollen grains suspended in water and was the first to observe the effect)

Brownian Motion

The random motion of a pollen grain in water.
Remember we would not normally be able to see the water molecules, just the pollen grain.


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.
We describe this process as evaporation.

How to increase the rate of evaporation

  • Increase the temperature
    If we warm the liquid the particles will have more energy and move faster.
    More of them will be able to break fee from the surface so the liquid will evaporate faster.
  • Put the liquid where there is some wind
    This will blow away the particles that have left the surface helping the liquid evaporate.
  • Increase the surface area
    By pouring the liquid into a wider dish we will increase the number of particles at the surface so more will be able to leave.
  • Increase the temperature until it starts to boil
    Normally the particles are only leaving the liquid at its surface, but when the liquid boils the particles have enough energy to leave the separate in all parts of the liquid, causing bubbles of water vapour which rise to the surface.


When a substance is heated the particles move faster and push themselves a little further apart.
This makes the substance expand (get larger) when heated (or contract when cooled)
Gases expand more than solids or liquids.

The unusual properties of water
Water is a little unusual. It contacts as usual down to about 4ºC but then starts to expand.
The result is that water expands as it turns into ice (and causes ice to have a lower density than water).

This has very important implications in ecology.
During cold weather only the surface of ponds and rivers freeze and turn to ice.
This provides a safe environment for fish to live below the ice.

If water did not have these unusual properties the whole of the pond would freeze and kill the animals and plants living there.


The moving particles are responsible for causing substances to slowly mix (or diffuse) together.
This mixing together is called diffusion.

Diffusion happens the most rapidly in a gas because the particles are far apart and able to mix easily together.
Example: some perfume is sprayed into the corner of a room.
It would not be long before the perfume could be smelt all over the room

Diffusion happens slowly in liquids because the particles are close together.
It might take several hours for the drop of purple dye to mix into the water (remember, you are not allowed to stir it!)
Example: coloured specks of sugar (100’s and 1000’s) are sprinkled onto the surface of some triffle.
By the next day you can see the colour starting to spread outwards into the custard (or cream)

Diffusion is nearly non-existent in solids but gases can very slowly diffuse into a metal.

If a drop of purple dye is placed in water the movement of the particles causes the purple dye molecules to spread outwards.

Pressure in a gas

Gas particles colliding against the walls of its container push outwards causing pressure

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.