Heating causes the color change of many substances. Often, this indicates thermal decomposition and the formation of new compounds, which is usually an irreversible process.
For some compounds, heating causes reversible color changes. These compounds are called thermochromic compounds.
This experiment demonstrates the following thermochromic compounds:
1) Copper(I) mercury iodide (Cu2[HgI4])
At room temperature, Cu2[HgI4] is a red solid compound, which undergoes a phase transition at ~55°C. The compound turns black, upon cooling it recovers the red color. The color change is caused by the transition from one crystal phase to another:
β-Cu2[HgI4] ↔ α-Cu2[HgI4]
Caution! Salts of mercury are toxic! Rubber gloves must be worn when handling them.
2) Silver mercury iodide (Ag2[HgI4])
Ag2[HgI4] is yellow at low temperatures and reddish above 50 °C:
β-Ag2[HgI4] ↔ α-Ag2[HgI4]
For both Cu2[HgI4] and Ag2[HgI4], the β and α crystal lattices of the compounds have different spatial placement of atoms.
Thermochromic compounds are used in everyday life as well. For example, some pans have a thermochromic spot that changes color when temperature reaches above a certain level.
Color changing mugs show their printed image only when a hot beverage is added. A thermochromic heat sensitive coating is sublimed upon heating over the entire outer surface of the mug. Just add hot water and at 40-50oC this coating becomes transparent to reveal the image underneath.