T565: Effects of Temperature - Thermochromic Compound
Introduction
A partially evacuated, sealed glass tube containing copper (I) tetraiodomercurate (II) is immersed in a beaker of boiling water. The compound changes from a bright red to dark brown at 80oC. When returned to room temperature, the red color reappears.
Equations
At room temperature, the Cu+ and Hg2+ ions are in separate layers, alternating with I- ions and each other. At high temperature, the metal ions move at random into vacant sites.
To Conduct Demonstration
- Heat water in a tall 1000mL beaker on a hot plate.
- Place the sealed tube of Cu2HgI4 in the beaker.
- If desired, measure the temperature of the water bath to determine the approximate temperature at which the color change takes place.
- Remove the tube and allow it to cool. The red color will return. Repeat as many times as desired.
- Alternate Procedure instead of steps one and two:
- Place tube of Cu2HgI4 in a liter tall form beaker of room temperature water and heat, noting the temperature that the compound begins and finishes turning color (70-80 degC).
Demo Time: 10 – 15 minutes if placing tube in boiling water.
40 minutes If heating the water with the tube in it.
Safety and Disposal
Wear heat-protective gloves when handling the beaker and tube.
Chemical in the sealed tube is a mercury compound. Be prepared to contain it and do mercury clean ip if the tube were to break!
References
Shakhashiri, B.Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; 1983.
W.L. Jolly; Encounters in Experimental Chemistry, 1972.
Acknowledgment
Arlan Norman, Fall 1987.