2.3: Colour in Chemistry

Figure 2.21. Periodic table of elements. Image attribution: 2012rc, Periodic table large, on Wikimedia Commons, CC BY 3.0

The science of chemistry deals with chemical elements – which are the substances of matter (Figure 2.21). Chemistry also deals with the properties and behaviours of these substances when mixed and the development of methods to create new substances.

The relationship between chemistry and colour is complex. Chemical processes are used to create colours that we use every day, and colour is used in chemistry to measure and analyse substances and chemical processes. It’s useful to learn about this aspect of colour theory if you’re working with colour materials in creative and scientific fields.

This section covers selected topics that explain the relationship between colour and chemistry (you can also find additional information in section 

3.5 Colour systems: paint pigments and dyes)

Spectrophotometry and colorimetry

Figure 2.22. Beckman Ir-1 Spectrophotometer, c. 1941. Image attribution: Chemical Heritage Foundation, Beckman Ir-1 Spectrophotometer, ca. 1941, on Wikimedia Commons, CC BY-SA 3.0

Spectrophotometers and Colorimeters are both used to accurately measure colour, which is important in some scientific processes and also for matching colours for commercial production of products where colour accuracy is essential. They are both complex devices that are used in many areas of chemistry – and biochemistry in particular. Both use light and a solution (liquid) to measure the absorption of light

A Spectrophotometer (Figure 2.22) measures the reflection and intensity of light for all colours – including those colours that aren’t part of the visible spectrum. It measures the exact wavelengths of light – not just the primary colours, by passing a beam of light through a diffraction grating which splits the light into different wavelengths. A spectrophotometer is used to detect the purity of substances, to detect DNA and RNA concentrations in a sample, and lots of other biomedical analytics

A Colorimeter (Figure 2.23) measures how much of a particular colour from the visible spectrum is in a solution by shining light through coloured filters or shining LED lights through the liquid. The device then measures how much of each colour is absorbed. It measures the three additive primaries (RGB) of coloured light. It can be used for blood analysis, detecting soil and food nutrients, chemical reaction levels, monitoring bacteria growth in biological experiments, etc.

Learn more about both devices

Figure 2.23. Colorimeter. Image attribution: Sonal Shinde, Colorimeter, on Wikimedia Commons, CC BY-SA 4.0

Metamerism– colour perception and matching

Figure 2.24 Room interior with two light sources. Image by 5460160 via Pixabay, licensed under CC0.

If you work in fashion, interior design or any industry where colour matching is very important, it is useful to understand the concept of colour metamerism and how it can affect the colours you use, how you display objects, products, and creative works, and how you use lighting in different environments.

Have you ever decided to wear all black clothes that look the same colour in your bedroom, but when you go outside, you notice they are all slightly different? Some black garments are more reddish-black, others slightly more green or blueish-black?

Have you ever chosen a carpet or paint for your home that you thought would match your furniture, but once you have it in your home, it looks completely different? It might also appear to be different colours between day and night, or on sunny or overcast, cloudy days.

These are examples of Metamerism, or in fact, illuminant metameric failure. The light in your home can be different colours from room to room and also a different colour to the sunlight outside. Some colours look the same under certain light sources but different under other light sources, which can include incandescent, fluorescent, LED or sunlight.

We could also say that colour blindness is a form of metamerism – observer metameric failure – because people see colour differently depending on their biology and the functionality of their eyes – as outlined in the sections on colour blindness and vision difference in this resource

This also relates to the difference in colour on digital screens. You might design a website with a very specific colour palette, but you can’t guarantee that everyone who views your website will have a screen that is properly calibrated to accurately show the colours you selected. You can test whether your computer or mobile device is “colour blind” by following some of the steps on this website: Is Your Computer Color Blind?.

How does Metamerism work?

Metamerism is the term we use when colours match under one lighting condition but not another. Colours that do match in this way are called metamers. It’s based on the science of how different coloured substances absorb and reflect light, the amount of electromagnetic radiation, and which wavelengths of the visible spectrum are reflected or emitted.

A practical example of a colour metamer is using two different methods of creating orange paints that appear to be identical

You can create an orange paint by:

1. mixing red and yellow paint pigments to create what the eye perceives as orange
2. using a ready-made orange paint pigment that was made with different chemicals to the red and yellow pigments.

Your eyes will see the same colour even though the two oranges were created with completely different pigments that reflect slightly different light wavelengths.

When colours that should be the same don’t match under different lighting conditions and are perceived differently, this is metameric failure. Some colours are more prone to metameric failure such as whites, greys, beiges, blacks, pinks and mauves. This is where the science of colorimetry becomes important for testing colour consistency and the specific wavelengths of light reflected or emitted.

How to prevent metameric failures (colour mismatches)

If you are choosing fabrics for a fashion collection and you want them to match, you should collect swatches and samples of fabrics and accessories that will go together in an outfit or ensemble and lay them out under different lights to make sure they match and look good together.

For example, you might think you have the perfect matching zip, buttons, sewing thread and bias binding for a cotton dress, but these items are made from different materials (nylon, plastic and metal) to the cotton dress fabric. They might look the same in one type of light, but if you look at them under a different light source, they might reflect different amounts of light wavelengths and appear different to your eyes.

This is because the dyes and pigments colouring these materials may have slight chemical differences and therefore reflect light differently.

Figure 2.25 T-shirts in rainbow colours. Image attribution: Photo by Greg Rosenke on Unsplash

For the same reason, if you are decorating a house, it’s a good idea to try sample paint pots or get carpet and fabric samples and put them on the floor or walls to make sure you have the right colour for the environment and where the light falls. You also need to check if the colours match under daylight from the windows and under the artificial lights in your home.

With digital media, it’s impossible to know if your viewers have screens that are calibrated. It can help to test your website on as many different types of screens and, if possible, older technology, where screens might not work that well or may not have good brightness and contrast. This way, you will know if your colour designs will look good under different conditions that are out of your control. You could also encourage your audience to take the computer colour blindness test mentioned above.

Test yourself: is it illuminant or observer metameric failure?

A multiple choice quiz to test your understanding of metameric failure

(5 minutes or less)

If you need to learn more about this topic, you can read this section again: 2.2 Colour in chemistry

Luminescence

Figure 2.26 Fluorescent minerals. Image attribution: Fluorescent minerals (Hgrobe 06:16, 26 April 2006 (UTC)) – credit: Hannes Grobe/AWI, Fluorescent minerals hg, CC BY-SA 2.5, on Wikimedia Commons

Luminescence is the term we use to describe when substances spontaneously emit light. It’s also described as “cold radiation” because it can happen to substances that aren’t heated.

There are many different kinds of luminescence, and you may already know some of them because they are quite common. Here are a few examples of luminescence and how they work:

Photoluminescence– resulting from absorption of photons

Fluorescence: photons immediately radiate visible light when ultraviolet light waves hit a fluorescent substance. Examples include fluorescent paint pigments, textiles, fluorescent lights, and blacklight inks and dyes that glow when you shine an ultraviolet light on them (Figure 2.27).

Phosphorescence: when a phosphorescent substance is exposed to short wavelengths of light (high energy), it glows by absorbing the light and emitting it at a longer wavelength (low energy). Photons may radiate for a prolonged period of time after the light source is removed. Glow-in-the-dark toys and paints are good examples of this.

Figure 2.27. Fluorescent clothes that glow with UV blacklight. Image attribution: Black light theatre Prague, Black light theatre Prague HILT 13, CC BY-SA 4.0, on Wikimedia Commons

What is the difference between fluorescence and phosphorescence? Phosphorescent substances continue to emit photons for a longer period of time after the light source is removed, whereas fluorescent substances tend to emit photons for a shorter time.UV blacklight" 

Chemiluminescence – from chemical reactions

Glow sticks are a good example of chemiluminescence. When you crack a glow stick, two chemicals are mixed together that produce a reaction. This reaction excites electrons in the atoms and they emit photons. This reaction is only temporary and you can’t “recharge” a glow stick. You can only create this chemical reaction once.

Bioluminescence is a form of chemiluminescence. Marine creatures, fireflies, algae (Figure 2.28), bacteria and other types of organisms that glow in the dark create light through chemical reactions that occur in their bodies. It happens when a light-emitting molecule reacts with a certain type of enzyme that the animal produces.

Learn more about bioluminescence.

Watch a short video of a surfer on a bioluminescent wave here.

Electroluminescence – from passing an electric current through a substance

LEDs (Light Emitting Diodes) are semiconductor devices that emit light when an electric current is passed through them. The colour of LED lights depends on the amount of energy passed through the semiconductors, which then determines the energy of the photons. Different amounts of energy produce different light wavelengths and therefore different colours (Figure 2.29). White light LEDs are created by using multiple semiconductors or a layer of light-emitting phosphors on the semiconductor device. It’s a combination of fluorescence and electroluminescence. While it is possible to create white light from red, green and blue LEDs, this isn’t the best method for colour rendering as only a narrow band of light wavelengths is used. The first LEDs emitted low-intensity infrared light. Visible and ultraviolet light LEDs were developed later.

Figure 2.29. Red green and blue LEDs. Image attribution: R, G, and B LEDs [7]. by PiccoloNamek, Uploaded to English Wikipedia under GDFL-self (GNU Free Documentation Licence) by the author, on Wikimedia Commons

Radioluminescence – light produced by bombardment with ionising radiation

Radioluminescence happens when light is generated from a substance that is bombarded with ionising radiation, such as alpha particles, beta particles, or gamma rays. Dials on watches and clocks that glow in the dark are made from luminous paint containing the radioactive substance Radium (Figure 2.30).

Radium paint is a toxic substance and it created serious health problems for those who worked with it to create watch, clock and instrumentation dials. See Problematic colours in this resource for more information about the Radium Girls who worked in the factories that produced these radium dials.

Radium-based paints were replaced with Promethium and Tritium later in the 20th century as they have much lower radiation, making them safer to use.

Figure 2.30. Radium clock dial from the 1950s. Image attribution: Arma95, Radium Dial, CC BY-SA 3.0, on Wikimedia Commons

Mechanoluminescence – physical pressure applied to substances like crystals

Piezoelectricity: the devices that light gas stoves, grillers and some lighters use piezo ignition (Figure 2.31). This mechanoluminescence works when pressure is applied to a PZT crystal (lead zirconate titanate) with a small hammer, which produces a sudden high voltage and generates an electric spark that ignites the gas.

Figure 2.31. A Piezo ignition device. Image attribution: Petteri Aimonen, Piezo igniter, marked as public domain, on Wikimedia Commons

There are many other kinds of luminescence not included in this resource. Learn more from this Wikipedia page.

Test yourself: what type of luminescence is this?

A multiple choice quiz to test your understanding of types of luminescence

(5 minutes or less)

How did you go?

If you need to learn more about this topic, you can read this section again: 2.2 Colour in chemistry

Colour chemistry 1: animals that can change colour

Figure 2.32 Blue ringed octopus. Image by Rickard Zerpe on Wikimedia Commons, licensed under CC BY-SA 2.0.

There are many ways that colour is involved in chemical processes. Some of these processes occur naturally – as with certain animals that can change their colour by cell signalling (Figure 2.32). Some processes are used for medical diagnostic tests that help to keep us healthy – like COVID tests. Other processes are used to measure chemical substances and reactions in laboratories, for creative purposes like photography and making art materials, or simply for entertainment like fireworks.

The four pages in this section explain some examples of colour processes in chemistry with links to further information, and practical learning activities that are simple science experiments you can do at home.

Animals that can change their colour

Only animals classed as Ectotherms are able to change their colour. An ectotherm cannot generate its own body heat like mammals and birds. This group of animals includes reptiles, frogs, cephalopods (including species of squid, octopus, cuttlefish, and nautilus), insects, crustaceans and fish. Colour changes happen because of how light is absorbed and reflected by the chemical pigments in the cells of these creatures.

Reasons for the ability to colour change are mainly to camouflage and protect the animal from dangers such as predators, to assist with hunting prey, or to communicate warning and mating signals to other creatures. Colour change can also help an animal regulate its body temperature. For example: a lizard may be able to change to a darker colour that absorbs more lightwaves from the sun to warm itself.

There are different types of colour change in ectotherm animals, and these can be triggered by different processes in chromatophores (cells that can produce colour). Some of these processes are physiological – the changes in chromatophores are controlled by muscles and pigment sacs. Others are caused by crystal structures in the cells which can reflect different light wavelengths. 

Watch this video to learn about the complex way chameleons and other animals can change their colour (4:28 minutes)

Media atribution: Chameleons are Masters of Nanotechnology by Reactions on YouTube

Learn more from this online resource.

Colour chemistry 2: photography and chromatography

Figure 2.33. Daguerrotype photographic image by Louis Daguerre, 1837. Image attribution: Louis Daguerre creator QS:P170,Q131405, Daguerreotype Daguerre Atelier 1837, marked as public domain, on Wikimedia Commons

Photography

The section on Optics in this resource explains how cameras work to project an image into the device using lenses and a camera obscura. In digital devices, the image is captured and saved as a digital image file, but originally, in traditional photography, the image was captured and fixed onto a light-sensitive film, which could then be used to reproduce multiple images.

Converting the image that appears in the camera to something that is printed on paper requires multiple chemical processes. An introduction to chemical photography is explained here…

Black and White photography

Black & white photography was invented before colour photography. It involves the use of light-sensitive silver halide crystals that form an emulsion on celluloid or plastic film with gelatin (originally, glass plates were used). The energy from the light waves that enter the camera and hit the film causes a chemical reaction with the silver halide. Creating silver halide crystals for photographic film is a complex chemical process making them photosensitive to all visible wavelengths of light

The more light that comes into the camera and hits the light-sensitive film, the greater the reaction in the silver halide crystals, which creates silver ions. The grains of silver halide create an image by reacting to the different amounts of light hitting different parts of the film and creating different quantities of silver ions. However, you can’t see the image on the film until the film is ‘developed’ because it’s still light-sensitive and the image is not yet stable.

Developing photographic film is another process where chemicals are used to turn the silver ions into silver metal – more light produces more silver and a darker, opaque area on the film. Less light produces a more transparent area on the film. This process creates what we call a negative image – light parts of the image appear dark and dark parts appear light. Once the film is developed, the chemical processes must be stopped and fixed so that the silver halide crystals are removed and only the silver metal is left. Once developed, you can take the film out into the light and see the negative image.

So how do we create a positive image from the negative film? Another chemical process is used with light-sensitive paper, which also contains silver halide crystals held on the paper with a gelatinous emulsion – like the photographic film.

A white light is shone through the negative film onto the paper, activating the silver halide to create more silver ions on the paper. You can’t see the image on the paper until it is developed in a bath of chemicals that turn the silver ions into silver metal – which oxidises and shows up as dark areas on the paper. The process is then stopped and fixed – just like the film. This is how we get a positive image on a photograph – the clear areas on the negative let more light through and create darker areas on the paper. The dark, opaque areas on the film let less light through and therefore create lighter areas on the photographic paper.

Learn more about the history of photography and Daguerre(Figure 2.33) who was one of the first photographers to create stable photographic images.

Colour photography

Colour photography is more complex than black and white. It is a subtractive colour process. Colour film uses silver halide just like black and white photography, but the colour film has three layers of emulsion, each with a different light-sensitive dye mixed with silver halide that is sensitised so that each layer only captures either red, green or blue light. To create a colour negative image, the dyes form the complementary or opposite colour:

• Red-sensitive layers have cyan-coloured dye.
• Green-sensitive layers have magenta-coloured dye.
• Blue-sensitive layers have yellow-coloured dye.

The film is developed, and the silver halide is removed in this process, leaving only the coloured layers on the film. Printing to colour photographic paper is a similar process to printing black and white, turning a negative colour image into a positive colour image, but the paper has three layers of emulsion, just like the colour film, sensitive to red, green and blue light, which creates a chemical reaction turning the dyes into cyan, magenta and yellow dyes (Figure 2.34).

Note: there are also special colour films that create a positive image instead of a negative one, and these were used in slide projectors – a technology not often seen today since the advent of digital photography and projections.

Figure 2.34. Early colour photograph of Agen, France by Louis Ducos du Hauron, 1877. Image attribution: Louis Ducos du Hauron (1837 – 1920), Duhauron1877, marked as public domain, on Wikimedia Commons

Learn more about the history of colour photography.

Chromatography

Chromatography is a chemical process that is used for separating the different parts of a mixture – a liquid or gas substance that has multiple ingredients. The name means ‘colour writing’ from the Greek words chroma and graphe. This process was originally used to separate different coloured pigments that were all mixed together – a technique also used to create coloured dyes for making textiles and carpets.

Chromatography today isn’t specifically about separating colours. It’s used to separate all kinds of substances.

The way it works is by dissolving the mixture in some kind of solvent which could be gas or liquid – the mobile phase – and then using another material which could be a liquid or solid – the stationary phase – to separate the parts of the mixture. Because the different substances in the mixture travel along the stationary phase material at different speeds, they get separated from each other and form different bands.

If you’re trying to separate colours from a liquid mixture by using paper, for example, you would see different stripes of colour appearing as some colours move faster along the paper and so move closer to the top, and others move more slowly, so remain closer to the bottom – like a rainbow of different coloured stripes. Figure 2.35 shows an example of this using paper and a liquid solution to show the chromatography of chlorophyll.

Figure 2.35. Chromatography of chlorophyl. Image attribution: Flo~commonswiki, Chromatography of chlorophyll – Step 7, CC BY-SA 2.5 on Wikimedia Commons

A very simple way to see this process work is to use black ink (the mixture), water (the solvent) and absorbent paper (the stationary phase material) to separate the colours from the ink mixture. See the activity details below – you can try this experiment at home.

Learn more about chromatography:

• Ask a Scientist: What is Chromatography?
• Explain That Stuff: Chromatography

Activity: paper chromatography experiment

You can make your own science experiment at home with a few simple materials to see how chromatography works in practice.

Watch this video for details and try your own experiments:

Media attribution: Paper Chromatography | Fun Experiments and Clear Explanations, Science Buddies on YouTube

Colour chemistry 3: universal indicator and diagnostic tests

Figure 2.36. Test tubes containing solutions of pH 1–10 colored with an indicator. Image attribution: Alvy16, PH scale 3, CC BY 4.0

Universal indicator

We can identify the acidity levels (pH value) of many substances with colour testing.

What is pH?

pH, originally meaning “potential of Hydrogen”, is a numbering system used for measuring the acidity of substances dissolved in a water solution. A low pH value has a higher acid content. A high pH value indicates a higher alkaline or basic substance. Water has a neutral pH value of 7, which is why it’s a good solution to test the pH of other substances dissolved in it. For example, lemon juice can have a pH of around 2 – which is very acidic. Lemons have a high quantity of citric acid in them, which is why they have a low pH value. The detergent you use to wash your dishes is alkaline or basic, so it might have a pH value of around 8.

Learn more from this book Acids and Alkalis by Denise Walker.

To test the pH level of any solution, a universal indicator is used. This can be in the form of a paper tape that you dip into your solution, or it could be a liquid that is added to a solution. The universal indicator contains a set of chemicals that change colour depending on the amount of acid or base substances in the solution (Figure 2.37). They work by producing extra protons depending on how acidic or alkaline the solution is, which in turn emit photons with different wavelengths.

Figure 2.37. Chemistry of Universal Indicator by James Kennedy Monash. Image attribution: Chemistry of Universal Indicator by James Kennedy Monash, Creative Commons licence Attribution-Non-commercial-ShareAlike-4.0, select image to enlarge.

Indicators in Nature

Hydrangeas are flowering plants that can grow flowers in a range of colours, including white, pink, purple and blue. Did you know that Hydrangea flowers can change their colour depending on the acidity of the soil? You can turn your pink hydrangea flowers blue by adding an acidic chemical like aluminium sulphate to your plant’s water (Figure 2.38). The roots absorb this chemical, and the flower petals act as a natural indicator, turning from pink to blue if the soil pH level is more acidic than alkaline (basic).

Figure 2.38. Blue hydrangea flowers. Image attribution: photograph by Lisa Cianci, Creative Commons licence Attribution-Non-commercial-ShareAlike-4.0

You can also use red cabbage as an indicator by making a solution with chopped-up leaves and water. Red cabbage contains a water-soluble pigment called anthocyanin, which turns red if mixed with an acid substance and blue/green if mixed with an alkaline/base substance.

There are other plants that have anthocyanin and can also be used in the same way. Can you find three other examples?

Learn more about acids and bases.

You can buy universal indicator online and do your own experiments with substances around your home to see which are acids and which are bases, or you can make your own indicator with red cabbage. You can start by testing foods, drinks and cleaning products around your home. Is your tap water a pH of 7? If not, it might have other chemicals in it. See the activity below for instructions on making your own red cabbage indicator.

Activity: make a red cabbage indicator

You can make your own indicator at home with a red cabbage and a few simple materials to test the pH of different substances.

Watch this video for details and try your own experiments:

Media attribution: Test asides with home-made pH indicator, BBC Earth Lab on YouTube

Diagnostic medical tests

Have you ever wondered how a pregnancy test or a COVID Rapid Antigen Test (RAT) works? How do the coloured lines appear on the test strip?

A pregnancy test uses chemicals that change colour when they come into contact with certain enzymes that attach to antibodies. When you are pregnant, your body produces a hormone called human chorionic gonadotropin (hCG). This hormone is excreted by the kidneys into urine – so putting urine on the test strip will start a chemical process that tests for the hCG hormone. The test strip contains an enzyme that will attach itself to the hCG hormone in the urine. As the urine moves along the test strip, there are two lines that can change colour – one will change colour and become visible if it comes into contact with the enzyme that is attached to the hCG – the test line. Another line will change colour and become visible if the enzymes not attached to hCG come into contact with it – the control line.

Two lines are needed to ensure the test has worked correctly because if there was only one line, it could show a false-negative result if the test didn’t work properly.

Figure 2.39. How do pregnancy test work? Image attribution: Compound Interest – How do pregnancy tests work? Select image to enlarge.

Watch this video that explains how a COVID rapid antigen test works:

Media attribution: Chemist Breaks Down How At-Home Covid Tests Work | WIRED by Wired on YouTube

Colour chemistry 4: fireworks

Figure 2.40. New Year’s Eve Fireworks on Sydney Harbour. Image attribution: Rob Chandler, New Year’s Eve on Sydney Harbour, CC BY-SA 2.0 on Wikimedia Commons

How do they make different coloured fireworks?

Fireworks were recorded as originating from China in the 8th Century CE. It is said that explosive black powder was accidentally invented by alchemists who mixed potassium nitrate (also called saltpetre), sulfur and honey together while trying to create the elixir of life. The mixture exploded when heated – this is called an exothermic chemical reaction. Recipes for this explosive mixture eventually made their way across Asia and the Arabic world to Europe, which is where fireworks as we know them today were first created.

The explosive part of fireworks is gunpowder (black powder) which is now made from saltpetre, sulfur and charcoal. The reason fireworks can appear to have a range of colours is because different chemical compounds are used, which are mostly metals that burn very brightly, and these are coated in gunpowder. Exploding gunpowder burns the chemical compounds, which causes electrons in the compounds to get excited and release excess energy as photons (light). These photons have different wavelengths in the visible spectrum depending on the energy of each chemical’s electrons, and this is how we see fireworks as different colours (Figure 2.41).

Somecolours, like blue fireworks, are harder to make because the chemical compounds are not stable. Purple is also difficult because it’s made from a mixture of red and blue compounds.

Fireworks colours and their chemical compounds:

• Red – Strontium salts
  Strontium Nitrate, Strontium Carbonate and Strontium Sulfate
• Orange – Calcium salts
  Calcium Carbonate, Calcium Chloride and Calcium Sulfate
• Yellow – Sodium salts
  Sodium Nitrate, Sodium Oxalate and Cryolite
• Green – Barium salts
  Barium Nitrate, Barium Carbonate, Barium Chloride and Barium Chlorate
• Blue – Copper salts
  Copper (I) Chloride, Copper Carbonate and Copper Oxide
• Purple – Combination of Red and Blue
  Strontium and Copper compounds
• Silver – white hot metals
  Magnesium and Aluminium
• White – burning metal
  Magnesium, Aluminium and Titanium

Figure 2.41. Image attribution: this image shared from www.compoundchem.com, Creative Commons Attribution-NonCommercial-NoDerivatives licence, © Compound Interest 2015. Select image to enlarge.

Test yourself: colour chemistry

A multiple choice quiz to test your understanding of colour in chemistry

(5 minutes or less)