This green to red to green color change clock reaction is a perfect Christmas chemistry project. It’s a good example of a redox reaction and acid-base reaction, plus it’s a great opportunity to raise interest in chemistry.
Christmas Chemistry Demo Materials
The Christmas chemistry color change demonstration requires few materials, plus the chemicals are safe enough to rinse down the drain after use.
- Water (preferably distilled, but tap water is fine it the pH is nearly neutral)
- 15 grams glucose (C6H12O6)
- 7.5 grams sodium hydroxide (NaOH)
- Indigo carmine indicator
The Christmas Chemistry Project
- Prepare “solution A” by dissolving 15 g glucose in 750 ml water. Prepare “solution B” by dissolving 7.5 g sodium hydroxide in 250 ml water.
- Warm solution A so it’s about body temperature (36-37°C or 98-100°F).
- Add a pinch of indigo carmine to solution A. You just need enough to turn the solution visibly blue.
- Pour solution B into solution A. This changes the color from blue to green. If left to sit, the color will change from green to red/amber.
- Pour the red solution into an empty clear container from a height of at least 60 centimeters (2 feet). The height is needed to ensure enough oxygen dissolves in the solution to change its pH. The color of the liquid returns to green.
- Once again, the color will change to red/amber. The demonstration may be repeated many times.
How It Works
Indigo carmine or indigotine is 5,5′-indigodisulfonic acid sodium salt with the chemical formula C16H8N2Na2O8S2. The dry salt is purple. Dissolved in water (usually as a 2% aqueous solution), it’s blue at pH 11.4 and yellow at pH 13.0. Indigo carmine is also used as a redox indicator that turns yellow when it is reduced. The glucose changes the color of the dye.
The reaction is much like that of the blue bottle demonstration, where the alkaline sugar solution reduces the dye to a paler or colorless leuco-dye. Shaking a flask or pouring the liquid from a height dissolves oxygen in the solution, oxidizing the dye back into its colored form. The rate of the color change depends on the concentration of the reactants and the temperature. Variations use potassium hydroxide (KOH) rather than sodium hydroxide (NaOH) and dextrose rather than glucose. Other pH indicators work to produce different colors.
As with all chemistry demonstrations, wear proper gear. Goggles, gloves, and a lab coat are recommended. At the end of the demonstration, the solution may be washed down the drain with running water.
Indigo carmine is safe enough for use as a food colorant (E132 or FD&C Blue #2). Aside from its use as a safe pH indicator, it’s also used as an intravenous dye for mapping the urinary tract, to color medications, to detect dissolved ozone, and to detect amniotic leaks. However, it can be irritating if inhaled.
Sodium hydroxide is a strong base. Eye or skin exposure causes irritation and chemical burns. Avoid contact with this solution and rinse any exposed areas immediately with water.
More Color Change Chemistry Fun
The “Christmas chemistry” demonstration is one of many color change reactions. Others to try include the blue bottle reaction (blue to colorless), vanishing valentine or hot and cold heart (pink to colorless), Halloween reaction (orange to black), water into wine (colorless to red), and Gatorade and drain cleaner reaction (blue to yellow).
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