Kinetic Theory of Gases Model
Properties of Matter

Model of expanding liquids and gases

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Students use marbles in a tray to model the expansion of liquids and gases.

Apparatus and Materials

  • Metal tray lined with cork mat
  • Marbles, coloured, 20 - 24, about 1 cm diameter

Health & Safety and Technical Notes

Beware of marbles on the floor.

Read our standard health & safety guidance

The metal tray should have near-vertical sides and a thin cork base. Each tray should contain 20 to 24 coloured marbles.


  1. Add a few marbles to the tray and incline the tray slightly so that the marbles are touching, representing a liquid.
  2. Agitate the tray.
  3. Remove some of the marbles so that the model represents a gas.
  4. Again agitate the tray.

Teaching Notes

  • When the tray representing the liquid is agitated, the marbles move around each other and the volume they occupy increases a little.
  • In liquids the expansion is easier to explain. Heating raises the speed of the molecules which are in constant, colliding turmoil. Molecules push each other further apart, on the average, against those fairly long-range attractive forces which still hold the liquid together.
  • In the case of gases the picture of expansion is clearer still. Heating a gas makes its molecules move faster and so they transfer more momentum at each impact. If the gas is in a container that allows expansion then the bigger pressure will drive the walls of the container outwards until the pressure is the same as before but the gas has a bigger volume. The molecules are further apart and, having longer to travel, they bombard the walls less frequently though each impact is more violent.
  • If the heated gas is in a container which does not allow expansion, then the pressure of the gas on the container walls increases.

This experiment was safety-tested in March 2006

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