One of the most well known types of chromism is thermochromism, when a material changes color in response to temperature changes. However, it is important to distinguish thermochromism from incandescence, or thermal radiation. When materials are heated, their atoms gain energy, and some of that energy can be emitted as light—red hot metal, for example, or lava. This is not the same mechanism that works in thermochromic materials, which tend to change color at much lower temperatures than required for incandescence.
One mechanism of thermochromism can be found in liquid crystals. Temperature can effect the spacing between crystals, as well as promote phase changes, and thus how light interacts with them. Some dyes known as leucodyes are also thermochromic. These dyes have much less variation in color change than liquid crystals, typically only switching between "cold" and "hot" colors, but can have a wide variety of colors for each "setting". Liquid crystal also tend to be more accurate as temperature sensors than dyes are.
Many inorganic materials are also thermochromic, but without the strong color changes apparent in liquid crystals and dyes. Many thermochromic color changes of inorganic materials are a result of phase changes, or charge-transfer bands.
Thermochromism is responsible for the color change of mood rings (liquid crystals), baby bottles or cups or mugs that change color when heated (dyes), and thermal paper that sometimes used for receipts (dyes). Other applications include propane tank level indicators (liquid crystals), thermometers (liquid crystals), and color changing t-shirts (dyes).
Sources/Further Reading: (Image 1—Science Teacher Stuff) (Images 2 and 3—2022 Article) (Explain that stuff) (Wikipedia) (ChemEurope) (2023 article)
