The Iodine Clock is one of the more famous chemical reactions because it’s fairly easy to set up and can be performed without any seriously dangerous substances. You might have seen the Iodine Clock before, but not like this. Now you can check out the reaction in slow motion.The Iodine Clock is so-called because it can be precisely tuned to go off after a certain time by varying the concentration of the reactants. So what you see happening is two colorless liquids being combined in a beaker, and for a moment, nothing happens. Then the solution suddenly snaps to dark blue or black in a split second. It’s an impressive trick, and also useful experiment to demonstrate chemical kinetics.There are a few ways to do the Iodine Clock, all of which give the same results. However, all methods rely on Iodine to cause a colored reaction with starch. The example in the video uses the Iodate method. There are several reactions happening when you combine the iodate with the starch, but you can watch the video for all the details. The important part is that the reactions will proceed and consume all the color-producing Iodine until one of the reactants runs out (in this case it’s bisulfate).When one of the reactants runs out, that’s an example of kinetics. You can actually calculate how much time you want to elapse before the reaction breaks down (but make sure your Iodine source isn’t the limiting reactant!). When that does happen, the Iodine builds up and causes a dramatic color change as it reacts with the starch in solution.In the slow motion video we can actually see that the liquid is not changing color all at once, despite what out sluggish human eyes tell us. The change is coming on in several areas at once as spindly tendrils of inky blue. This happens because when the solutions are combined the concentration is not uniform across the entire container — there are localized areas where the reaction breaks down and Iodine builds up faster. Not only is it cool science, it’s a cool video too.