How do colour-changing pencils work?

Image © 2017 Let’s Talk Science

Colour-changing objects are a ton of fun to play with, but have you ever wondered how they work? When an object changes colour as a result of a change in temperature, it is called thermochromic (from the Greek words thermos meaning “heat,” and chroma meaning “colour”).

Thermochromic substances tend to use either Thermochromatic Liquid Crystals (TLCs) or leuco dyes to create their changeable colours. TLCs are used for such things as mood rings, aquarium thermometers, and rechargeable battery testers. TLCs are able to give a fairly accurate measure of temperature. However, because of their sensitivity to heat as well as other substances, they are tricky to work with. To learn about the science behind TLCs, check out the article Feeling Moody? on CurioCity.

Novelty items which do not require precise temperature sensitivity, such as colour-changing pencils and hidden-image cups and clothing, use leuco dyes. Cold-activated leuco dyes change colour when the temperature decreases (e.g., stadium cups) and heat-activated leuco dyes change colour when the temperature increases (e.g., coffee mugs). The colour-changing pencils in the image above use head-activated leuco dyes.

Did you know? The first synthetic organic dye, mauveine, was discovered by accident in 1856.

Unlike TLCs, which can indicate a range of temperatures by turning different colours, leuco dyes can only appear in two states - a coloured state (non-leuco form) and a transparent state (leuco form). At normal room temperature, the dye is in its coloured state. When it is heated, the dye changes to its transparent state. This allows any colours or patterns printed on an item under the leuco dye layer to become visible. When the object cools back down to room temperature, the dye changes back to its coloured form. Since the dye can go back and forth between the two colour states, it is called a reversible process (see Figure 1).

Leuco form (left) and coloured form (right) of crystal violet lactone (Public Domain)

Did you know? The phenolphthalein that is used in acid-base titrations is a leuco dye.

So how is a leuco dye able to change between these two colour states? The answer lies in a chemical system enclosed within tiny capsules (microcapsules). The microcapsules found in leuco dyes contain three substances:

1. A colourant
2. An organic acid
3. A solvent (usually an ester or alcohol to activate the acid)

At room temperature, the solvent is a solid and the particles of colourant and acid are tightly packed together. As a result, the colourant and acid share electrons. When light reflects off of the colourant in this solid form, a visible colour can be seen. When the temperature rises, the solvent becomes a liquid. This makes the particles of acid and colourant move away from each other so that they can no longer share electrons. At this point the colourant cannot reflect light. We see this as a lack of colour (see Figure 2).

Leuco form (left) and coloured form (right) of crystal violet lactone (Let's Talk Science, 2017)

So thanks to some neat chemistry we can make our world even more colourful and fun! What would you create with a leuco dye? You can purchase powdered leuco dyes from Solar Color Dust.com and create your own fun colour-changing items! See below for some DIY colour-changing craft project videos.

DIY projects

References

How do Mood Pencils Work?
Quality Logo Products

Thermochromic color-changing materials (2017)
Chris Woodford

Chameleon Dyes (2012)
Janice Vancleave

Thermochromic colors in textiles
S. Periyasamy, Gaurav Khanna

Dyestuff
Dyestuff