Halloween clock reaction. Clock reactions are reactions that show a sudden change after a set period of time, rather than occurring immediately on mixing, as is the case with most reactions. Here, three colourless solutions containing iodate (V) ions, hydrogen sulphite ions, mercury(II) ions and starch are mixed. After a few seconds the solution suddenly turns orange with a precipitate of mercury (II) iodide. A few seconds later it suddenly turns black, as a starch-iodine complex forms. The reaction is also known as the Old Nassau reaction after a nickname of Princeton University, where it was discovered, whose colours are orange and black.