Have you ever seen a mood ring that seemingly changes colors in response to your mood? Or, maybe you have seen T-shirts, toys, and even thermometers that change colors in response to temperature changes. All of these color changes, even the mood ring, change colors due to special chemicals, called thermochromic chemicals.
Two types of thermochromic chemicals are Liquid Crystals and Leuco Dyes. Each change colors in a different way. The color of anything is due to how it reflects light.
White light is made up of the full spectrum of colors. This means that it contains all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. When something has color, it reflects parts of white light that make that color. The rest of the light is absorbed by the object. If all of the spectrum colors are reflected, the object’s color is white. If all of the spectrum colors are absorbed, the object’s color is black.
Crystals are solids made of atoms (smallest part of an element) that are very regularly arranged. If the arrangement of the atoms allows light to pass through, the crystal will be clear. If the crystal has color, then the arrangement of the atoms causes some of the light spectrum to be reflected.
Liquid crystal are generally large molecules (combinations of two or more atoms forming a single substance) arranged in a regular pattern. This happens because the molecules are dipolar, meaning it has a positive and a negative side. The positive part of one molecule is attracted to the negative part of another molecule resulting in the molecules lining up. Even though these chemicals are in liquid form, they have characteristics of a crystal, which is a solid. Thus the name–Liquid Crystals.
When a liquid crystal is heated the molecules that form it move faster and line up in different ways. As the temperature increases, the layers of molecules will gradually move further apart. This causes changes in the part of the light spectrum that is refracted. Liquid chemicals can make color changes through the entire range of the spectrum. Liquid chemicals can be used to indicate minute temperature changes, such as is needed for thermometers.
Mood rings have nothing to do with mood changes. Instead, the room contains a liquid crystal that changes in response to changes in the temperature of your skin.
Leuco Dye generally do not make accurate thermometers like liquid crystals. Instead, they are better used for indicating general temperature ranges of too hot or too cool. They are also used for novelty applications, such as T-shirts that change colors when someone touches it, toys or even fun color changing pencils.
Leuco dyes (LD) are not like other dyes. Instead, they are solid microcapsules that when cool reflect a specific spectrum color. When heated the microscapsules melt and its content causes changes in the leuco dye that affects how it absorbs light. Leuco dyes are unique in that they are colored when cool and clear when hot. For example, a blue Leuco dye combined with a regular yellow paint would change from green when cool (blue + yellow) to yellow when warm (clear + yellow).
Leuco dyes which have the property of changing from a color state to a clear state when a simulus is appled, and then returning to a colored state when the stimulus is removed are called reversible leuco dyes. The stimuli for the color change depends on the dye. Stimuli includes heat and light. Those that are heat sensitive are considered thermochromic. Those that are light sensitive are considered photochromic.