The Chemistry of Colour

by Kate Onissiphorou

The chemistry of colour covers the way in which different frequencies and wavelengths of visible light are absorbed or reflected by molecules known as pigments. These pigments have specific compositions and configurations that correspond to their respective colours. 

From the varying colours of autumn leaves to the dyes used in the clothing industry, our world is full of rich and diverse colours. But exactly how are these colours formed, and why? Read on to learn more about the chemistry and science of colour.

The science of colours

In simple terms, a substance can either be colourless or it can have a colour(s). Solid objects are typically coloured, while many gases tend to be transparent (colourless). The precise colour (or lack of colour) is determined using a couple of different methods, depending on the substance involved.

The colour of simple inorganic substances, such as gemstones for example, is determined by the presence of certain elements. For example, rubies are red due to the presence of chromium, while emeralds are green because they contain chromium, vanadium, and iron.

In many cases, the colour (or its absence) is determined by the presence of a certain atom or molecule that has a corresponding chromophore(s), which are structures in molecules that can either absorb or reflect visible light. 

The colours of some objects can also be determined by the way microscopic components are arranged. This is true for both butterfly wings and peacock feathers. In these examples, the overlapping microscopic structures reflect light in a way that blocks other wavelengths or frequencies of light, resulting in a variety of shiny hues being displayed at different angles.

Close up of colourful peacock feathers

How chemistry colours our world

Whether you’re watching a film on your smartphone or admiring a painting at the museum, the chemistry of colours has an important role to play. On an LCD screen, for example, each pixel is a cell filled with twisted nematic liquid crystals that can change polarity depending on the electric voltage passing through it. This change also alters the colour and tone of the light produced by each pixel.  

Unlike the liquid crystals on flat-screen TVs or smartphones, pigments in paintings aren’t dynamic and do not change instantaneously. Although some pigments may alter over time due to chemical degradation, the chromophores in the pigment molecules remain relatively stable.

Wooden dice spelling the words beta carotene, with a plate of carrots in the backgroundColours can also be found in the food we eat. Some of these colours, such as the yellow-orange colour of beta carotene, occur naturally, while others, like Brilliant Blue FCF, are added artificially.

Chemical structure of beta-carotene
The chemical structure of beta-carotene

Colour wheel chemistry

The colour wheel in chemistry is related to spectrophotometry (the measurement of light absorption), with each colour directly corresponding to its respective wavelength. However, the colours we see refer to the reflected wavelength of light as opposed to the absorbed wavelength.

Generally, the range of colour regions and their corresponding wavelengths can be tabulated as follows:

Colour Region Wavelength in Nanometer
RED 750 − 610
ORANGE 610 − 590
YELLOW 590 − 570
GREEN 570 − 500
BLUE 500 − 450
VIOLET 450 − 370

 

The type of substance determines the colour or colours it absorbs, while its concentration affects how much light is absorbed. The colour that we perceive is the complementary or opposite colour of the absorbed light.

For example, if we perceive that an object or substance is red, the absorbed light is actually blue-green. You can refer to the colour wheel to determine the complementary colours. The colour wheel

Determining the colour and amount of light that’s absorbed can be useful when analysing composition and concentration. Chemical indicators can be then used to determine the acidity or alkalinity of a substance. For example, phenolphthalein is colourless in acidic solutions but turns magenta in basic solutions, as shown in the illustration below:

Graphic showing what happens to the chemical structure of phenolphtalein when an acid or an alkali is addedA level chemistry: colour changes

If you did A level chemistry, you’ll probably have learned that a change in the colour of a substance is an indicator of chemical change. This is true for chemical indicators used in titration experiments, for example. 

It’s also generally true for many chemical reactions, be it a decomposition reaction, combination reaction, single displacement reaction, double displacement reaction, or combustion. During a chemical reaction, atomic bonds are either created or broken (or both), resulting in a change in the structure and composition of the substances involved.

Colour-changing experiments in chemistry

If you want to put what you’ve learnt into practice, here are some examples of experiments in chemistry that demonstrate change in colours during reactions:

Copper oxide and sulphuric acid

Copper oxide is a black solid (powder) while sulphuric acid is a colourless liquid. When the two are mixed together they produce copper sulphate and water, and the once colourless solution changes to a cyan-blue colour.

Red cabbage juice

Red cabbage juice can be used as an indicator for determining whether a solution is acidic or basic. The juice colour changes from red to green when you add a basic solution. It reverts to red when you add an acidic solution and becomes blue when the solution is neutralised.

Blue bottle experiment

The blue bottle experiment is a classic demonstration of chemical change as indicated by the colour change. To perform this experiment, you’ll need to combine glucose, methylene blue, and sodium hydroxide in a flask. When you shake the flask and let it settle, the solution changes from blue to clear. If the flask is swirled around again, the liquid will revert back to blue.

Summary

The colours that we see are the result of the absorption of light of certain wavelengths by a substance. The chemistry of colour is essential to different industries such as food preparation and the clothing sector. It’s also important in chemical analysis as an indicator of chemical change. 

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