Thermochromic Paint


















A thermochromic paint is prepared using Ag2[HgI4]. A painted paper is laminated. When the temperature is increased, the color of drawings changes from yellow to red.

Two 100 cm3 beakers, filter funnel, filter paper, glass rod, yellow paper for drawing, paint brush, hair dryer.

Mercury(II) iodide, an aqueous solution of potassium iodide, an aqueous solution of silver nitrate, dissolving starch.


Preparation of the aqueous solutions:
An aqueous solution of KI: 7.16 g KI is dissolved in 25 cm3 H2O
An aqueous solution of AgNO3: 7.35 g AgNO3 is dissolved in 30 cm3 H2O
Starch “solution” (actually a colloid): 10 g of dissolved starch is stirred into a small amount of cold water and then added to 50 cm3 of boiling water.

Fig. 1. Before heating

Fig. 2. After heating

Preparation of thermochromic paint:

  • Place 10 g of HgI2 in a beaker with 25 cm3 aqueous solution of KI.
  • Stirring with a glass rod is applied until HgI2 is dissolved.
  • Separate the solid residue with filtration and use the filtrate.
  • Add 30 cm3 of an aqueous solution of AgNO3 to the filtrate in small amounts, with constant stirring.
  • A pulpy product, yellow in color, should be obtained.
  • Wash the precipitate with water several times.
  • After that, while stirring constantly, add a small portions of the starch “solution”.
  • Use this thermochromic paint to draw  the Macedonian flag on a piece of yellow paper that represents the background.
  • Laminate the paper with drawing with a plastic foil.
  • Use hair dryer to heat the drawing and demonstrate the thermochromic properties or you can use cold and hot water as shown on the video clip.

Observation and Discussion

The salt HgI2 is a solid substance with an intense red color. The following reaction takes place when a solution of KI is added to HgI2:

2KI(aq) + HgI2(s)  =  K2[HgI4](aq).

Adding a solution of AgNO3 to the filtrate of K2[HgI4] precipitates a yellow precipiatae of Ag2[HgI4](s):
K2[HgI4](aq) +AgNO3(aq)  =  Ag2[HgI4](s).

The product is a complex salt silver tetraiodomercurate(II).

The complex salt Ag2[HgI4] is a thermochromic substance that changes color from yellow to red at a temperature higher than 50 ºC. When cooling the substance the yellow color returns. Adding a starch “solution” to Ag2[HgI4] actually serves as a glue between the thermochromic color and the paper onto which it is  applied.      
The part of the paper that is colored with thermochromic paint changes from yellow to red when the temperature increases.

Safety Tips
The mercury and its salts are highly poisonous substances. Wearing protecting gloves is required when handling these compounds.   
If there is a lack of HgI2 in the school laboratory, it can be prepared by incorporating Hg and I2, or by mixing an aqueous solutions in a stochiometric molar ratio of potassium iodide and mercury(II) soluble salt.