This demonstration is known as the Old Nassau clock reaction or Halloween clock reaction. As you can see from the video, there are three rows of beakers with different compounds dissolved in each row. In the first row of beakers sodium hydrogensulfite and starch solution are placed. Beakers in the second row contain mercury(II) chloride solution, while potassium iodate solution is placed in the third row of beakers. Contents of the third row beakers were added to the first row of beakers. After that the contents of the second row of beakers were simultaneously added to the mixture in the first row. Several seconds later the mixture will turn an opaque orange/red color and this change is firstly observed in the first beaker, and than successfully in other three. After a further few seconds the mixture suddenly turns blue-black. The second color change (orange to black) is not normally expected by the audience and comes as a real surprise.
The reaction in this experiment takes place in several steps that are responsible for this clock mechanism. Initially, iodate ions are reduced by hydrosulfite ions to produce iodide ions:
Once the concentration of iodide ions is high enough to exceed the solubility product of mercury(II) iodide, opaque orange HgI2 precipitate is formed:
When all mercury(II) ions are precipitated in the reaction with iodide ions, the excess of iodides reacts with iodate ions and elemental iodine is formed:
Iodine formed in this reaction reacts with starch indicator to give a blue colored complex:
As the concentration of potassium iodate decreases from the first to the last beaker, so does the rate of the reaction. Orange and blue colors are firstly detected in the first beaker, followed with the color appearance in the remaining ones.
Because of the system complexity altering the concentrations and amounts of reactants cannot be predicted with simple proportion.