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Iodine clock Reaction

Magical chemical reaction: A colourless solution suddenly turns dark blue !!

Iodine clock reaction is a very famous chemical reaction and very spectacular: A colourless solution suddenly without apparently anything happening turns very dark blue sometimes it even becomes black.

It looks like magic! It is quite complicated but it’s not difficult, because there is many different chemical reactions happening at the same time, and that is why the blue colour appear suddenly. This colour is caused by the interaction between the ion named triiodide (I3-) formed by three iodine atoms with a negative charge and the starch present in the solution.

Iodine clock Reaction
Lewis Structure of the triiodide ion I3

In the solution we have starch and if the triiodide ion is obtained, the change of the colour occurs because these two substances (starch and triiodide ion) can interact together to give the dark blue colour. But in order to have the I3 ion in the solution, the iodine atom (I2) and the iodide ion (I) should be present together, so they can interact to form the triiodide ion ( I3).

I2 + I –> I3

There are different ways to do the clock reaction, the way that will be explained here is done with the iodate ion IO3 plus bisulfite HSO3- which react together to form the iodide ion (I-).

Iodine clock Reaction

So that is how the iodide ion (I) is obtained in the solution. However as soon as this iodide ion is in the solution some of the iodate that are left react with the iodide to make iodine (I2).

Iodine clock Reaction
Formation of iodine (I2)

So in the solution there is Iodine and iodide which should transform to triiodide ion I3 which interact with the starch and give the blue colour. It is important to arrange the concentration of reactants in order to make sure that the bisulfate used to produce iodide ion will run out before the iodate, so the bisulfate should be the limiting reactant, if not this will affect the colour change.

The beauty of this reaction is that the change in the colour is too fast, it can not be recognized by the eye, so anyone can see the colour changing, one moment it is colourless the next moment it’s dark blue.