If the two gases hydrogen and chlorine are mixed in the dark, nothing happens. If placed in direct sunlight, the effect is instant and dramatic.
The gases explode with sufficient violence to break the glass container.
This improbable choice of reaction was modified to measure the strength of diffused sunlight. The rate of production of the product gas, called hydrogen chloride, gave a measure of the intensity of the light.
Children can prepare light-sensitive materials quite easily, following the example of those who originally developed photography. Silver salts are changed by light and were used widely in the production of black and white negatives, and pictures for newspapers.
Ask your pupils why solutions of silver salts are always stored in brown glass or opaque bottles.
Using two large test-tubes, place silver nitrate solution in one, and potassium bromide in the other. When mixed, an instantaneous change is observed. The two colourless solutions produce a cloudy mixture containing silver bromide. This material was used in black and white photography.
If the silver bromide is left in direct sunlight, the colour will change through purple to the black of powdered metallic silver.
There is an interesting alternative technique. Filter the mixture containing silver bromide and spread the filter paper out on the table.
Cover part of the light-sensitive salt, leaving the rest exposed to the light (for example, use a star-shaped cut-out of cardboard or aluminium foil). You will need an intense light source for the next stage. This could be a bright spotlight or a piece of burning magnesium ribbon, such as an old-fashioned flashgun.
After exposure to the light, remove the card mask. The silver bromide exposed to light will have gone grey-black but the protected material will still be pale yellow.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire