The iodide clock reaction is one of the classic chemical reactions you are shown in school/university. Two colourless solutions are mixed together and, in a given amount of time, the solution suddenly turns dark blue – almost black. There are several variations of the clock reaction but this video concentrates on the iodate version.
The chemistry is rather difficult as there are multiple reactions going on. The first is in one of the clear solutions where bisulfate is generated when sodium sulfite (Na2SO3) and citrus acid (C6H8O7) are mixed together in water. When it is mixed with a solution of starch and an iodate ion (IO3−) source, it reacts to produce iodide (I−). If there is an excess of iodate (IO3) then it can oxidise the iodide (I−) (which was produced in the previous step) to form iodine (I2). This iodine (I2) is then immediately reduced back to iodide (I−) (the same stuff in the second step) when there is an excess of bisulphate (HSO3−). When the bisulphate runs out, there is nothing to reduce the iodine (I2) back to iodide (I−) or to react with the iodate, so the iodine builds up and can form triiodide (I3−) with iodide (I−).
IO3− + 3 HSO3− → I− + 3 HSO4−
IO3− + 5 I− + 6 H+ → 3 I2 + 3 H2O
I2 + HSO3− + H2O → 2 I− + HSO4− + 2 H+
I2 + I− ⇌ I3−