Multiple chemical reactions occurring at the same time keep this solution clear, for a while, then it suddenly changes to a deep dark blue.
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.
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.
Be careful when working with the iodine – it stains, and it stains really well. Be very careful not to spill any of the solution.
Dispose all liquids down the drain with plenty of water.
Dig deeper into the science behind clock reactions in this paper from the Journal of Chemical Education.
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