Halloween Reaction Materials
- soluble starch
- sodium metabisulphite (Na2S2O5)
- mercury(II) chloride
- potassium iodate (KIO3)
- 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
- Mix 50 ml solution A with 50 ml of solution B.
- Pour this mixture into 50 ml of solution C.
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.
- Sodium metabisulfite and water react to form sodium hydrogen sulfite:
Na2S2O5 + H2O --> 2 NaHSO3
- Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
IO3- + 3 HSO3- --> I- + 3 SO42- + 3 H+
- 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)
- If I- and IO3- ions remain, then an iodide-iodate reaction takes place:
IO3- + 5 I- + 6 H+ --> 3 I2 + 3 H2O
- The resulting statch-iodine complex is black to blue-black:
I2 + starch --> a blue/black complex