Mercury(II) Iodide Thermochromism

Solid substance changes its color from orange-red to pale yellow under heating.

Scientific name: Mercury (II) Iodide undergoes phase transition from alpha to beta crystalline form.


Mercury(II) Iodide and Thermochromism

by myst32YT

Sodium iodide mercury chloride

by ynimrod


Wear eye protection goggles and gloves. Mercury compounds are extremely toxic. Avoid body tissue contact with it.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

HgCl2 + 2KI → HgI2 + 2KCl

HgI2 (red-orange alpha crystalline form) -[heating]→ HgI2 (pale yellow beta crystalline form)

Step-by-step instruction

  1. Fill the test tube with mercury (II) iodide. Mercury (II) iodide could be obtained by mixing mercury chloride solution with potassium iodide solution. They form the precipitate of mercury (II) iodide which could be separated from the solution by filtration.
  2. Put the test tube with mercury (II) iodide on a stand.
  3. Heat the test tube with a gas burner.
  4. The substance changes color from orange–red to pale yellow.

Scientific background

At 126 °C mercury (II) iodide undergoes phase transition between red-orange alpha and pale yellow beta crystalline forms. Such an ability of a substance to change its color with temperature change is called thermochromism.


Published on 23 April 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst