Colour-changes with Temperature
Chemicals that change colour with temperature are thermochromic
A simple explanation of the way thermochromic (colour-changing) chemicals work, and how they are used.
You may have seen novelty goods that change colour with heat: mood rings, T-shirts, bath toys, coffee mugs, you name it, they’ve done it. Then there are more serious applications: child thermometers, fridge thermometers, battery indicators. They are all done with special chemicals which change colour with heat, called thermochromic chemicals.
How do they work?
There are two types of chemical used in this area. One has a familiar name: Liquid Crystal. The other has a not-so-familiar name: Leuco Dyes. Each works in a slightly different way but the aim is the same. When the temperature changes something must happen to the chemical so that the light passing through it changes.
Light Spectrum
White light is made up of the full spectrum of colours, the colours of the rainbow. When something has a colour, it is reflecting only the parts of white light that make that colour. The rest of the light is absorbed by the object.
Crystals
Liquid Crystals are liquids that have crystalline properties. Crystals are clear, often coloured solids. They are clear because they are minerals in which the atoms which make up the solid are very regularly arranged which means that light can pass through. When they are coloured it is because the arrangement within the crystal is refracting (bending) the light so that only particular parts of the light get through.
Liquid Crystals
A liquid crystal is usually made up of large molecules which, although liquid and so mobile, also line themselves up. Often this is because they have strong charge differences between one part of the molecule and another. So the negative parts of the molecules will be attracted to the positive parts of other molecules and they will line up. When a liquid crystal is heated it begins to change its structure; the molecules move faster and line up in different ways. Thermochromic Liquid Crystals go through a structure at a certain temperature in which the light is refracted in such a way that colour is produced. This colour will change through the entire range of the spectrum. This happens because as the temperature increases, the layers of molecules will gradually move further apart. This causes the refraction of the light to change, hence changing the colour of the light that gets through the crystal.
Leuco Dyes
Leuco dyes work in a different way. They are large organic molecules whose structure is based on several hexagonal rings of carbon atoms with extra groups bonded onto them. These types of compounds can absorb certain wavelengths of colour when the structure becomes conjugated. This means that the carbon atoms share one of their “spare” electrons over the whole ring system creating a similar structure to that of graphite. The conjugated system can absorb some colours in light, making the dye coloured. Leuco dyes have two states. In one the structure is not conjugated and so gives no colour. In the other it is conjugated and so gives a colour. In thermochromic dyes the Leuco compounds are mixed with a solvent which is normally solid. With a slight increase in temperature its melts and dissolves the dye and a weak acid, which combine to change the dye to a conjugated structure, giving it a colour. When the dye cools again it returns to a non-conjugated structure and the colour disappears.
Uses
Liquid crystal thermochromic dyes are used in applications which require more accuracy and range such as thermometers. Leuco dyes are used in other applications like novelty items and warning signs such as “too hot”.
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