Iodine Clock Reaction




The reaction used in this experiment is a classic example of a chemical clock. The term, chemical clock, refers to a reaction where the concentration of one of the chemical components undergoes an abrupt change in concentration after a time period and leads to a sudden change in color. The reaction in this experiment is termed an iodine clock reaction, because it is the molecular iodine (I2) that undergoes the sudden concentration change. When the iodine concentration increases, it reacts with the starch in the solution to form a complex, turning it a deep purple.

Items Needed:


  • 1,000 milligrams Vitamin C
  • 1 teaspoon iodine
  • 1 tablespoon hydrogen peroxide
  • 1/2 teaspoon corn starch
  • 6 oz warm water
  • 3 cups



  1. Label cups with numbers 1, 2 and 3.
  2. Pour 2 ounces warm water into each cup.
  3. Into Cup 1, add 1000 milligrams Vitamin C.
  4. Into Cup 2, add 1 teaspoon iodine.
  5. Into Cup 3, add 1 tablespoon hydrogen peroxide and 1/2 teaspoon corn starch.
  6. Mix 1 teaspoon of solution of Cup 1 into Cup 2.
  7. Set Cup 1 aside.
  8. Pour contents of Cup 2 into Cup 3 and mix back and forth for approximately one minute.
  9. Contents will turn into a deep purple color.



As iodine was introduced, the solution turned a deep purple as expected.

Submitted by

Peter Raisen

3rd Period Chemistry