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Spark Science: Iodine Clock Reaction (Science Magic!)

September 20, 2018
Posted By: TELUS Spark

Iodine Clock Reaction Experiment

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Let’s talk chemistry!

 What happens when you mix together two colourless liquids? 


Hydrogen peroxide may irritate some skins. Wear gloves to avoid reactions and iodine stains!

  • Distilled Water
  • Hydrogen Peroxide (3%)
  • Tincture of Iodide (2%)
  • 1000mg Vitamin C tablets
  • Liquid Starch
  • Clear cups
  • Measuring spoons

Vitamin C Solution

Make a Vitamin C solution by taking one 1000mg tablet of Vitamin C and crush it up into a fine powder

Put the crushed Vitamin C into a clear cup

Pour 60mL of distilled water into the cup

Set aside


Solution A

Take ½ a teaspoon of the Vitamin C solution and put it in a separate cup

Pour 60 mL of distilled water into the cup

Pour 5mL of tincture of Iodide to the mixture

  • The liquid should be colourless
  • If not, add more from the Vitamin C mixture


Solution B

Pour 1 tablespoon of hydrogen peroxide into a new cup

Add in 60mL of distilled water

Add 1/2 tsp of liquid starch


Now we’re ready to mix! Both solutions should be clear.

Pour solution A into solution B and watch the iodine clock reaction take place in front of your eyes!


Safe Disposal

Dilute all solutions with water before draining it down the sink

What’s happening?

When the two colourless solutions are mixed, 2 chemical reactions are happening simultaneously called oxidation and reduction. 

Oxidation is the process of losing electrons and gaining oxygen.

Reduction is the process of gaining electrons and losing oxygen.

Let’s focus on the iodine! Iodine is quite unique because it can be in two different oxidation states. Iodine can either have a negative charge or no electric charge. When combined with different chemicals, iodine has the ability to bounce back and forth between the two. Two forms of iodine are created – the elemental form and ion form.

Before mixing the two solutions together, the iodine has a negative charge. Once mixed, both reactions start to take place and there is a tug of war battle. Iodide ions react with the hydrogen peroxide to produce elemental iodine. Elemental iodine makes the solution a dark colour! The liquid starch wants to react with the elemental iodine, however the Vitamin C  d e l a y s  this process and this reaction is a lot faster, which prevents elemental iodine to combine with the starch. Eventually, the Vitamin C will be used up, allowing elemental iodine to react with the liquid starch. The excess presence of Vitamin C with iodine ions is the reason why the solution remains clear.


Some iodine reactions can be slow! Can you manipulate the speed of the reaction?

Hint: Try experimenting with the temperature of the liquids.