Iodine Clock Reaction


Sometimes, science seems too magical to be real. This demonstration, the iodine clock reaction, is definitely no exception. In this science project, you will watch an almost clear liquid turn dark blue, almost black, in an instant. This takes some preparation, and perhaps a trip to the pharmacy for some materials, but the work will pay off!

*Get an Adult* Iodine stains almost everything it touches, and it can be dangerous if ingested. Hydrogen peroxide can cause skin and eye irritation. Wear safety goggles throughout the whole experiment. Read the caution labels on each container.


*If you don’t have liquid starch, you can use 1/2 teaspoon of corn starch or potato starch. This will make the liquids more cloudy, and the reaction will happen more slowly but will produce similarly impressive results.


  1. Put on your safety goggles.
  2. Place one 1000 mg vitamin C tablet in a plastic bag and crush it with a rolling pin or the back of a large spoon. Try to make it into as much of a fine powder as possible.
  3. Put all the vitamin C powder in a plastic cup, and add 2 ounces (60 ml) of warm water. Then, stir for about 30 seconds. Don’t worry if the water is a bit cloudy.
  4. Put 1 teaspoon (5 ml) of the vitamin C solution from step 3 into a new plastic cup. Add 2 oz (60 ml) of warm water and 1 teaspoon (5 ml) of the iodine to this cup. Watch how the dark iodine turns clear and colorless. Put aside the vitamin C solution from step 3.
  5. In the last plastic cup, mix 2 oz (60 ml) of warm water, 1 tablespoon (15 ml) of hydrogen peroxide, and 1/2 teaspoon (2.5 ml) of liquid starch.
  6. Gather some friends and family and pour all of the liquid from step 4 into the liquid from step 5. Pour all of the liquid back and forth a few times. Then, set the cup down and wait. Between a few seconds and a few minutes, the liquid will suddenly turn dark blue, almost black, color.

Try This:

  • Try making the water colder or warmer. Does the water’s temperature affect how quickly the liquid turns blue?
  • Add more vitamin C solution to the cup in step 4. Does the amount added affect how quickly the liquid turns blue?
  • If you stir the liquid more, does this affect how fast the it turns blue?


This project is an example of a chemical reaction called the “Iodine Clock Reaction.” It’s called a “clock” reaction because you can change the time it takes for the liquid to turn dark blue. When you mix the two solutions, there is a chemical battle between the starch and the vitamin C. The starch is trying to turn the iodine blue, while the vitamin C is trying to keep it from turning blue. Eventually, the vitamin C loses, which results in a dark blue liquid.





Image Attribute:
By Voicu Dragoș (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

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