Thermochromism

A material appears coloured because it absorbs, reflects or scatters particular wavelengths of visible light. Temperature can shift molecular equilibria, trigger phase transitions or rearrange a material's structure, changing those optical properties and therefore its observed colour. The microscopic mechanism varies across material families; thermochromism is a broad phenomenon rather than one single chemical reaction.

Reversible thermochromic materials can change colour repeatedly as temperature rises and falls, through physical or chemical changes that can cycle back. Irreversible materials undergo a lasting physical or chemical change and do not recover their original colour on cooling. Whether a material is reversible, and the temperatures at which its colour changes, depend on its composition and design.

Examples include thermochromic liquid crystals, inorganic compounds and organic dye systems. Some commercial inks use microencapsulated leuco-dye systems: a colour-forming dye, a developer and a solvent are enclosed together, and temperature changes alter the chemical environment and colour state. Liquid-crystal materials can produce colour changes associated with temperature-dependent molecular ordering. These distinct approaches have different response ranges and durability.

Thermochromic materials are used to indicate temperature, create colour-changing inks and products, and add responsive functions to textiles. In laboratory teaching, thermochromic ink can reveal heating patterns: a Royal Society of Chemistry education demonstration uses such ink to show the standing-wave pattern inside a microwave oven. Practical performance depends on transition temperature, reversibility, stability and the material's surroundings. For textiles, reviews identify challenges including durability during washing and maintaining sensitivity.