This color change chemistry project gives you a liquid that fluctuates between pink and blue as the temperature changes. In essence, it’s a color change liquid thermometer.
What You Need
The color change liquid thermometer is a solution of cobalt(II) chloride in alcohol:
- beaker, test tube, or attractive clear container
- 3 g cobalt(II) chloride hexahydrate (also known as cobaltous chloride hydrate)
- 500 ml alcohol
Note that cobalt(II) chloride also changes color in response to humidity or hydration. Anhydrous cobalt(II) chloride is blue, while the hexahydrate is red.
Prepare the Solution
- Mix 3 grams of cobalt(II) chloride hexahydrate in alcohol.
- The pink solution turns blue as it is heated and returns to the pink color as the solution cools.
- Alternatively, heat the solution until it it is just slightly warmer than room temperature. Add water dropwise until the blue solution turns pink. This solution is extremely sensitive to color changes near room temperature.
How the Color Change Liquid Thermometer Works
Aqueous solutions of hydrated cobalt chloride are pink, but formation of [CoCl4]2- produces a blue color. Addition of hydrochloric acid to an aqueous cobalt(II) chloride solution changes the solution from pink to blue as does temperature change of the alcohol solution. This presumably occurs because the amount of water attached to the cobalt ion changes as you vary the temperature of the alcohol solution.
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References
- Finkel, Paul G.; Stecher, B.M. (1960). The Merck Index of Chemicals and Drugs (7th ed.). Merck & Co.
- Solomon, M. E. (February 1945). “The use of cobalt salts as indicators of humidity and moisture”. Annals of Applied Biology. 32 (1): 75–85. doi:10.1111/j.1744-7348.1945.tb06763.x
- Wells, A. F. (1984). Structural Inorganic Chemistry (5th ed.). Oxford: Clarendon Press. ISBN 0-19-855370-6.