Even amateur artists soon discover that the exclusive use of primary colours can cause artistic fatigue. The secret is to mix the base colour with something else, to dilute the effect. The characteristic smell of fried onions diminishes as you walk past the street vendor.

Children can understand the effects of dilution in terms of the particles involved. As the spacing between the particles increases - be it colours or odours - the effects become harder to detect.

Take a strongly coloured material and dissolve in 100ml water. Suitable choices include food dyes, black ink and certain coloured laboratory chemicals - for example, potassium permanganate gives an intense purple colour even in a solution of 1% strength.

Ask children to devise a method to dilute the coloured solution in a systematic way and check if the colour is still visible. For example, taking 10ml of the original solution and diluting to 100ml gives a dilution of 10 times.

In order to check on the fading colour, the children will need some kind of comparison. This could be by placing the tube in front of an illuminated white surface or holding the tube next to one containing pure water.

An interesting variation is to use a fluorescent material. Some water-soluble red inks show this effect. Add a few drops of red ink to a tube of water. In a darkened corner, shine a torch beam from below the tube. The red ink shows up as a green glow - it fluoresces.

Dilute the ink as before and find how many dilutions are necessary before the fluorescence disappears.

To model the effects of dilution, use a transparent dish on top of an overhead projector. Add marbles to represent water particles and add 10 of a different colour as the dye. Place the dye particles in one group then agitate the dish to show how they spread out, or diffuse. For dilution, remove nine of the 10 dye particles to make clear the difference. Colours in liquids, and odours in gases - eg perfumes - can both diffuse.

Ray Oliver teaches science at St Albans Girls’ School,Hertfordshire