The programme is divided into three sequences:

• 1.Brownian Motion
• 2.Diffusion
• 3.Changes of State

The programme could be used at any stage of the teaching of the motion of particles in various states and would make an excellent summary or revision aid. It could be shown in full or in discrete sections as appropriate.

Brownian Motion

00.17 Properties of the three states of matter

Solids, liquids and gases represent three states of matter. They have characteristic properties.

01.05 An introduction to particle theory

A brief description of the arrangement and motion of particles in the three states of matter.

01.36 Evidence for the particle theory - Brownian motion

The behaviour of smoke particles observed through a microscope provides evidence for the random motion of particles in the air. This effect was discovered in the 1820s by Robert Brown, who observed the comparable motion of pollen grains suspended in water.

Diffusion

04.32 The diffusion of gases in air

Diffusion is illustrated by the behaviour of bromine vapour in an air-filled gas jar. It can be simply explained in terms of the particle model of gases.

05.47 Gaseous diffusion in a vacuum

Bromine vapour diffuses much faster in a vacuum, when there are no particle collisions between the air and bromine.

06.32 Comparing the rates of diffusion of gases

Gases diffuse at different rates. Nitrogen dioxide molecules diffuse faster than bromine molecules which are heavier.

We can also compare the rates of diffusion of colourless gases such as ammonia and hydrogen chloride. When these gases meet, they form a white smoke of ammonium chloride.

08.31 Diffusion in liquids

Diffusion also occurs in liquids. The rate of diffusion of one liquid in another increases as the temperature rises.

Changes of State

09.34 Energy transfer during melting

We can use simple particle theory to interpret the change of state which occurs during melting. A simple demonstration shows that this change of state is accompanied by an energy transfer.

10.38 Energy transfer during evaporation

The particle theory also explains what happens during evaporation. Here again energy is absorbed by the substance as

it undergoes a change of state. A simple demonstration shows that some liquids take more energy from their surroundings than others when they evaporate.

12.30 Condensation

Cooling a gas produces a liquid. If air is cooled using liquid nitrogen, we get liquid air. Cooling carbon dioxide gas with the same coolant produces solid carbon dioxide. This changes directly from solid to gas when it warms up.