Iodine Clock Reaction

(Chemistry for ages 5+)


Chemical reactions happen all the time and everywhere. Sometimes they cause visual changes like changing color or shape of the reactants. The video above shows a neat color-changing reaction called the Iodine Clock Reaction. Here’s what you’ll need to do this at home:



Safety goggles
Clear plastic cups
Iodine (typically found in the first aid section)
Vitamin C tablets (be sure they are not gummy or chewable as these tablets have extra stuff that can change the reaction)
Orange juice (or other juice high in Vitamin C)
Hydrogen peroxide
Starch (liquid or powder)
Warm water
Adult supervision (Adult supervision at all times please)



  1. Start by putting on your safety goggles. Anytime you are working with chemicals that might splash, it is important to protect your eyes.
  2. Crush up a Vitamin C tablet. You can do this by placing the tablet on a flat surface and using the rounded edge of a spoon to crush it. You can also place the tablet in a plastic bag and crush it using a mallet, rolling pin, or even stomping on it with your foot.
  3. Put the crushed-up tablet in one of your plastic cups and add ½ cup of warm water. Stir until the tablet is dissolved. This makes your first solution, which can be called “Solution A.”
  4. Next, add 1 tablespoon of iodine to Solution A. Be very careful handling iodine because it can stain your clothes. Stir it together. What do you notice about the color of the solution? Is it what you expected?
  5. In another plastic cup, add ½ cup of warm water, 3 tablespoons of hydrogen peroxide, and some starch. If you are using powdered starch, add a scoop (a heaping teaspoon or so), but if you are using liquid starch, add ½ to 1 teaspoon. Stir well, though the starch will not fully dissolve, so just make sure everything is mixed together. This is Solution B.
  6. Now pour Solution B into Solution A. Notice anything? Pour the mixed solutions back into the empty cup that had Solution B. Pour the mixed solutions back and forth between the cups until you notice a significant change.
  7. Starting with clean cups (they can be rinsed out and reused or you can use new cups), make Solution A using ½ cup of orange juice (or whatever juice you have that is high in Vitamin C). Add 1 tablespoon of iodine and stir it into the juice. What do you notice about the color?
  8. Repeat step 5 to make a fresh batch of Solution B.
  9. Again, pour Solution B into Solution A. Do you notice any color changes after this first mixture? Pour the mixed solutions into your empty cup. Do you see the color changing? Pour the solutions back and forth until the color stops changing. What was different about the color changes between using the Vitamin C tablet and the juice in Solution A?



In mixing Solution A, we see the first color-changing reaction because Vitamin C reacts with the iodine to create iodide which is colorless. That’s why the brownish-yellow color of the iodine disappears. When you mix the solutions together, the iodide reacts with starch and it turns a dark bluish-black color.

The color change takes awhile to happen because the Vitamin C first has to be all used up, and then 3 molecules of iodide must bind with 1 molecule of starch to lead to the color change. When juice is used in Solution A, there is much less Vitamin C, so the reaction happens a bit faster.