This video clip can be used to introduce Thermochromic materials.
Thermochromic materials are typically microencapsulated, with the microcapsules having a diameter typically between 3 to 5 µm, which is significantly larger than traditional pigment particles. The active mixture of compounds often contains leuco dyes in a low-melting point solvent that is a solid at room temperature. These dyes have two forms, one coloured and one colourless. When the solvent is solid, the dye is in its coloured form. When the temperature is increased and the solvent melts, the dye changes to its colourless form. By mixing a microencapsulated thermochromic pigment with a thermoplastic polymer, versatile products can be made that change colour when the temperature changes, for example, a bath toy that changes colour to warn when the water is too hot. Thermocolour film changes colour from black through red, green and to blue as the temperature increases. Students can use this film to investigate the thermal conductivity of different materials. Initially, students could compare the effect of placing a beaker of hot water on thermocolour film which has been placed on a metal lid with film which has been placed on a plastic lid. They should find that a coloured halo spreads out rapidly on the metal lid, but slowly on the plastic lid.