My son has a small toy car made of a die-cast metal. When it is immersed in icy water, the body paint turns purple. When put into warm water, it turns blue. If half-dipped, it will show both colours. He has also worked out that if he breathes heavily on it, it will warm up and turn blue this way. How does it work?
• This is thermochromism – the ability of a substance to change colour when its temperature changes. A mood ring, which alters colour as the wearer’s body temperature changes, is an excellent example of this, but it is more usefully incorporated into items such as baby bottles, which change colour when the milk inside is cool enough to drink.
Thermochromism can be based on liquid crystals but, in this case, a has probably been added to the paint. This pigment switches between two states, depending on its temperature. In one state, it is transparent; in the other, it absorbs light at particular wavelengths. The absorbed colour is subtracted from the light reflected, giving the object a different colour.
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Mike Follows, Willenhall, West Midlands, UK