The color of a thermochromic system depends on its temperature. The colors of leuco dye-based systems can also be influenced by adding acids or bases to the thermochromic reactions. These can be used to create colorful demonstrations of acid-base chemistry. Thermochromism found in color changing cups can also be used to visualize heat flow, and therefore thermodynamic principles, associated with stretching and contracting elastomers.
Dean Campbell tries to use at least one demo for every class to illustrate concepts described in his chemistry courses. In this post, he includes short descriptions of the demonstrations and props he has used while teaching his collegiate General Chemistry II courses.
Diffusion of HCl(g) from concentrated solutions of HCl can be used to illustrate some chemistry related to the train accident in Ohio.
Thermal paper such as that used for point of sale receipts typically functions by darkening when exposed to heat. The pigment system used in this paper also darkens when exposed to solvents of intermediate polarity and acids. This enables thermal paper to be used as an inexpensive sort of indicator paper for a variety of demonstrations.