Make a Color Change Liquid Thermometer


Cobalt(II) chloride may appear either pink or blue in solution, depending on which ion is formed. (Chemicalinterest)

Cobalt(II) chloride may appear either pink or blue in solution, depending on which ion is formed. (Chemicalinterest)

This color change chemistry project will give you a liquid that fluctuates between pink and blue as the temperature changes. In essence, it’s a color change thermometer.

What You Need

  • beaker or attractive clear container
  • 3 g cobalt(II) chloride hexahydrate (also known as cobaltous chloride hydrate)
  • 500 ml alcohol

Prepare the Solution

  1. Mix 3 grams of cobalt(II) chloride hexahydrate in alcohol.
  2. The pink solution will turn blue as it is heated and will return to the pink color as the solution cools.
  3. 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 will be extremely sensitive to color changes near room temperature.

How It 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 will change the solution from pink to blue as will 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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