The ‘Old Nassau’ or Halloween clock reaction

Orange liquid in a glass laboratory beaker against a green-blue background

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Illustrate dramatic colour changes as a result of redox and precipitation reactions in this vivid demonstration. Includes kit list and safety instructions.

In this experiment, based on the ‘Old Nassau’ or ‘Halloween’ clock reaction, students observe as a mixture containing iodate(V) ions, hydrogensulfate(IV) (hydrogensulfite) ions, mercury(II) ions and starch suddenly turns orange after a few seconds, forming a precipitate of mercury(II) iodide.

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