Thursday 3 May 2012

IODINE CLOCK experiment

Iodine Clock

This is a super fun experiment but you do need to ensure that a responsible adult is about when carrying it out! 

When you are finished experimenting, remember to carefully pour all liquids down the drain with plenty of water and wash your hands.

Recycle the cups for another experiment after washing them well (I wouldn't recommend drinking out of them afterwards) or dispose of them in the bin.

What you will need for this experiment:

·         Iodine tincture - if you want to know how to make you own click here
·         Hydrogen peroxide
·         1000mg vitamin C tablet
·         Warm water
·         Liquid starch (corn flour will work but it takes longer for the reaction)
·         Safety goggles
·         A plastic bag
·         3 clear cups
·         A large spoon
·         A small measuring jug or, alternatively, measuring spoons

STEP by STEP instructions:

Put your safety goggles on.


Put the vitamin C tablet in a bag and get to work crushing it into a fine powder with the back of a large spoon.   Put all the powder into the first cup and add 60ml of warm water.  Stir for about a minute.  We’ll call this solution A.


Now for solution B: get another clear cup and first put 5ml of solution A into it and then add to that 60ml of warm water and 5ml of Iodine. 

Before you put the iodine in you may want to ask for a prediction of what will happen when you add the iodine to the mixture.

Did you notice when you do add the iodine the brown mixture turns clear?  Anyway this is solution B sorted!

In the last cup (we’ll call it – you’ve guessed it – solution C) mix together 60ml of warm water, 15ml of hydrogen peroxide and 2.5ml of liquid starch.
Right, we are prepped and ready to go! Gather as many people together as you can!

Get solution B and solution C.  Pour all of solution B into solution C pour in back a forth a few times from cup to cup.  Place the cup down and observe.  YOU WILL HAVE TO BE PATIENT – it could be anywhere between a few seconds and a few minutes, the liquid will suddenly turn dark blue!

What are we seeing?

What we are seeing is a chemical reaction known as ‘The Iodine Clock Reaction’. 

It is called a clock reaction because using higher concentrations of iodine makes the reaction act quicker.  It’s basically a battle of chemistry between the starch (which is trying to change the iodine to blue) and the vitamin C with is keeping it from turning blue!  Cool science or what!

This reaction is a redox reaction.  An oxidation and reduction reaction because ions are exchanged during the reaction.

Only iodine element in the presence of iodide ion will give the characteristic blue black colour.
Try changing the amount of iodine you add to this experiment.  Instead of 5ml try 10ml.  Do you think the reaction will work quicker or slower?

Once your solution has turned dark blue add 5ml of your vitamin C solution and stir it in well.  What happens?  It goes back to being clear, or if like us you use orange coloured vitamin C it turns back to being orange. 

You might like to see how many times you can do this before it remains dark blue.


  1. Lisa fab experiment, where do you get your iodine please as have run out of places to look x

    1. You can make your own, here's how:

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  3. I think you made a little mistake in your post: a redox reaction is defined as an exchange of electrons, not ions. The exchange of ions it would be a substitution reaction :)