The Iodine Clock Reaction

Multiple chemical reactions occurring at the same time keep this solution clear, for a while, then it suddenly changes to a deep dark blue.

Experimenting at Home

You can try an at home version of this experiment using a few things you may have in your bathroom medicine cabinet. In may ways this experiment feels almost like magic. Two colorless liquids are mixed together and after a few moments the mixture turns a dark blue color. There are actually a couple of simple chemical reactions going on at the same time to make this “clock reaction” occur. This version of the classic “iodine clock reaction” uses safe household chemicals most people have on hand at home.

What you need:

  • distilled water (tap water will work OK as well)
  • a couple plastic cups
  • 1000 mg vitamin C tablets
  • tincture of iodine (2%)
  • hydrogen peroxide (3%)
  • liquid laundry starch

What to do:

  • Make a vitamin C solution by crushing a 1000 mg vitamin C tablet and dissolving it in 2 oz of water. Label this as “vitamin C stock solution”.
  • Combine 1 tsp of the vitamin C stock solution with 1 tsp of iodine and 2 oz of water. Label this “solution A”.
  • Prepare “solution B” by adding 2 oz of water to 3 tsp of hydrogen peroxide and 1/2 tsp of liquid starch solution.
  • Pour solution A into solution B, and pour the resulting solution back into the empty cup to mix them thoroughly. Keep pouring the liquid back and fourth between the cups.

What’s going on?

There are actually two chemical reactions going on at the same time when you combine the solutions. During these reactions two forms of iodine created – the elemental form and the ion form.

In Reaction # 1 iodide ions react with hydrogen peroxide to produce iodine element which is blue in the presence of starch. BUT, before that can actually happen, the Vitamin C quickly reacts and consumes the elemental iodine.

The net result, at least for part of the time is that the solution remains colorless with excess of iodide ions being present. Now after a short time as the reactions keep proceeding in this fashion, the Vitamin C gets gradually used up. Once the Vitamin C is used up, the solution turns blue, because now the iodine element and starch are present.

Safety Precautions

Be careful when working with the iodine – it stains, and it stains really well. Be very careful not to spill any of the solution.

Waste Disposal

Dispose all liquids down the drain with plenty of water.

Learn More

Dig deeper into the science behind clock reactions in this paper from the Journal of Chemical Education.

About the author

Carl Nelson is the Chief Scientist at Imagination Station in Toledo, Ohio. He holds a Masters Degree in Experimental Physics from Michigan State University and has been having fun exploring (exploding?) science in Science Centers for the past 20 years.


13 Responses to “The Iodine Clock Reaction”
  1. cynthia says:

    Wow.. Thank you so much. We have been wanting to do more science fun in our childcare and the egg osmosis experiment went well. Cannot wait to do this!

  2. Anna says:

    thanks for the trick bt it didn’t work, is it bcoz i used corn starch instead of liquid laundry starch how do you make lquid laundry starch at home…….plz help i need this info by thursday.

  3. Carl says:

    Hi Anna,

    If you don’t have liquid laundry soap available, you can create a starch solution with cornstarch (or potato starch).

    To create a starch solution, begin by stirring 5g (1 to 2 tsp) of cornstarch with 30 mL (2 tbsp) of cold water until a uniform thin suspension is formed. This suspension is poured slowly into 500 mL (2 cups) of vigorously boiling water with stirring so that the boiling does not stop. The solution is allowed to cool before use.

    Give this a try and let me know how it works out.


  4. Ashley says:

    I had to cut the amount of vitamin c down from 1 tsp to a fourth of a teaspoon. I also had to add Magnesium Citrate to solution A. I tried this experiment several times in hot, cold and neutral temperatures. It worked one time with the original solution and failed every time after that until adding Magnesium Citrate.

  5. Jeff says:

    Hello Carl,

    Thanks so much for posting this information online! I am a magician hoping to use this effect in my show. I purchased all of the necessary ingredients however, the reaction (blue change) is taking 12 minutes to happen! Could it be a result of buying decolorized iodine instead of tincture? Please let me know your thoughts as i look forward to performing this amazing demonstration. Regards, Jeff

  6. lachlan says:

    can u drink it? how did stuart do it in this video? please email me

  7. Peter says:

    Can this be done with povidone iodine

  8. Carl says:

    I have not tried using povidone iodine for this reaction. I believe it typically has a higher concentration of available iodine. So if you try it I would reduce the amount appropriately. However, I’m not sure if the polyvinylpyrrolidone (Povidone) will somehow interfere with the reaction. Let me know how, or if it works for you!

  9. Justin says:

    Yes, Povidone iodine does work. I used it for a demonstration in my school. I have not compared the difference between povidone iodine and 2% iodine tincture.

  10. Carl says:

    Thanks for that info Justin!

  11. Carl says:

    Hi Lachlan,
    You can not drink the version of the clock reaction we shared. I’m not sure what solution Stuart used for his magic presentation in the video link you provided.

  12. Franie says:

    How can you turn it back into a colorless solution?


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