Igneous rocks (those which have solidified from molten rock) in the earth's crust show the same effect as the menthol; they only have big crystals if they have solidified slowly. Molten rock which has flowed out of a volcano cools far too rapidly to produce any crystals, so solid lava is a glass.

How it works:

1. Menthol is chosen for this exhibit because it melts at about 43°C. You melt some menthol on a microscope slide with warm air from a hairdryer and then freeze it again with cold air from a refrigerator in the base of the exhibit. The microscope sits in front of a video camera, so you can see what happens on a TV screen.

2. In any solid, the particles ( usually little groups of atoms called molecules) vibrate faster and faster as you heat them up. At some point they will have enough speed to break free of the electric forces that hold them tightly together. This is where a solid melts and the substance becomes a liquid. Menthol particles (molecules) can break free at 43° C; ice melts into water at 0°C. If you carry on heating up the liquid, you reach another stage when the particles break free entirely from one another and form a gas.

3. If a liquid cools down slowly, it will turn into a crystal. The particles (atoms or small groups of atoms) line up in orderly rows, columns and layers. As the particles lose energy in the liquid, they become attached to the nearest edge of the crystal and so make the edge move into what was the liquid. You can see the "edges" of crystals move across the slide in the exhibit as the liquid menthol freezes.

4. If the cooling is faster, the particles cannot form a large-scale orderly arrangement and a mass of little crystals will form instead. If you vary the cooling rate in the exhibit using the cold blower control, you can grow large and small crystals on the same slide.

5. The colours of the crystals on the screen are not their true colours, but are produced by polarising filters on either side of the crystals. Part of the spectrum of white light from the projector bulb gets filtered out and you see the remainder as a beautiful mixture colour. The crystals show different colours if their particles are lined up in different directions on the slide.

6. You will notice that crystals begin to grow in the same places time after time if you repeat the melting and cooling sequence. This is because any "change of state" like crystallising needs somewhere to start, so any speck of dust or roughness on the glass slide acts as a centre for growing crystals.

Did you know?

Sometimes a gas can convert straight into a solid - think of frost being produced from water vapour in the air on cold nights.

Silicon chips for computers are made by growing different layers of crystal on a wafer of silicon from vapours in an oven. (N.B. These are not the oven chips you buy in the supermarket.)

When liquid rock flows out of a volcano, it solidifies rapidly in a few hours. This makes a glass, in which the particles are still arranged in a higgledy-piggledy fashion, as they were in the liquid.

If it solidifies very slowly underground, it forms a crystalline rock, like granite, where the separate minerals form recognisable crystals.

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