1. Education
You can opt-out at any time. Please refer to our privacy policy for contact information.

Discuss in my forum

Halloween Reaction or Old Nassau Reaction

Orange and Black Clock Reaction

By

The Old Nassau reaction is an oscillating clock that goes between orange and black.

The Old Nassau reaction is an oscillating clock that goes between orange and black.

Siri Stafford, Getty Images
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 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 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+

     

  3. When the concentration of iodide ions becomes sufficent 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)

     

  4. If I- and IO3- ions remain, then an iodide-iodate reaction takes place:

    IO3- + 5 I- + 6 H+ --> 3 I2 + 3 H2O

     

  5. The resulting statch-iodine complex is black to blue-black:

    I2 + starch --> a blue/black complex

©2014 About.com. All rights reserved.