Halloween Reaction or Old Nassau Reaction

Orange and Black Clock Reaction

Beaker with orange liquid
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The Old Nassau or Halloween reaction is a clock reaction in which the color of a chemical solution changes from orange to black. Here's how you can do this reaction as a chemistry demonstration and a look at the chemical reactions that are involved.

Halloween Chemical Reaction Materials

  • Water
  • Soluble starch
  • Sodium metabisulphite (Na2S2O5)
  • Mercury(II) chloride
  • Potassium iodate (KIO3)

Prepare the Solutions

  • Solution A: Mix 4 g soluble starch in a couple milliliters of water. Stir the starch paste into 500 ml boiling water. Allow the mixture to cool to room temperature. Add 13.7 g of sodium metabisulphite. Add water to make 1 liter of solution.
  • Solution B: Dissolve 3 g mercury(II) chloride in water. Add water to make 1 liter of solution.
  • Solution C: Dissolve 15 g potassium iodate in water. Add water to make 1 liter of solution.

Perform the Halloween Chemistry Demonstration

  1. Mix 50 ml solution A with 50 ml of solution B.
  2. Pour this mixture into 50 ml of solution C.

The color of the mixture will change to an opaque orange color after a few seconds as the mercury iodide precipitates. After another few seconds, the mixture will turn blue-black as the starch-iodine complex forms.

If you dilute the solutions by a factor of two then it takes longer for the color changes to occur. If you use a smaller volume of solution B the reaction will proceed more rapidly.

Chemical Reactions

  1. Sodium metabisulfite and water react to form sodium hydrogen sulfite:
    Na2S2O5 + H2O → 2 NaHSO3
  2. Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
    IO3- + 3 HSO3- → I- + 3 SO42- + 3 H+
  1. When the concentration of iodide ions becomes sufficient for the solubility product of the HgI2 to exceed 4.5 x 10-29 mol3 dm-9, then orange mercury(II) iodide precipitates until the Hg2+ ions are consumed (assuming an excess of I- ions):
    Hg2+ + 2 I- → HgI2 (orange or yellow)
  2. If I- and IO3- ions remain, then an iodide-iodate reaction takes place:
    IO3- + 5 I- + 6 H+ → 3 I2 + 3 H2O
  1. The resulting statch-iodine complex is black to blue-black:
    I2 + starch → a blue/black complex