Halloween clock reaction

Solutions in different beakers change their colors from colorless to orange and then become dark-blue one by one

Scientific name: Clock reaction between bisulfite, iodate and mercury(II) ions gives orange mercury iodide precipitate and after this - the dark-blue complex of iodine and starch


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Mercury compounds are highly toxic! Filter off the mercury(II) iodide precipitate. If some of the mercury(II) chloride solution was unused, add excess sodium hydroxide to precipitate mercury hydroxide. Filter off the mercury hydroxide precipitate.

All mercury compounds should be placed in sealed containers for professional disposal . Wear eye protection and lab gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

Na2S2O5 + H2O → 2NaHSO3

IO3- + 3HSO3- → I2 + 3SO42- + 3H+

Hg2+ + 2I- → HgI2

IO3- + 5I- + 6H+ → 3I2 + 3H2O

I2 + starch → a blue-black complex

Step-by-step instruction

  1. Prepare solutions of sodium metabisulfite and starch in the first set of beakers. Approximately 4 g of starch and 13 g of sodium metabisulfite per 1 l of water.
  2. Prepare solutions of mercury(II) chloride in the second set of beakers. Approximatelly 3 g of mercury(II) chloride per 1 l of water.
  3. Prepare water solution of potassium iodide (in various concentrations) in the third set of beakers.
  4. Add the contens of the third set of beakers to the first one.
  5. Then contents of the second set of beakers should be simultaneously added to the mixture in the first set of beakers.

Scientific background

Firstly the iodate(V) ions oxidize the hydrogensulphite ions:

IO3- + 3HSO3- → I2 + 3SO42- + 3H+

The hardly soluble mercury iodide is formed from Hg2+ and I-.

As soon as concentration of the HgI2 exceed its solubility, the solid orange mercury(II) iodide precipitates.

After all mercury ions form a precipitate, excess of the I- ions stays.

There is a reaction of comproportionation between the iodide and the iodate ions in the formed solution. Molecular iodine is formed.

Then dark-blue complex I2 and starch is formed.


Published on 10 March 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst