kinetics rate of reaction Redox reactions
Age Group
15 16 17
Expected time
3 periods
The clock reaction.PDF
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The clock reaction

What’s the mystery? Two translucent liquids are mixed. At first, nothing happens: the resulting solution is still translucent. Suddenly, with no warning, the solution turns blue-black all at once.
Domain(s) Chemistry
Subdomain keywords Redox reactions, rate of reaction (kinetics).
Age group 15 to 17 years old.
Expected time for the mystery Approximate time for teacher preparation: up to one hour for preparation of the solutions and trials to ensure that the reaction takes place as planned.Approximate time in classroom: up to six periods of 45 mins each (one period to engage and explore, one to explain, and three to plan, perform, and present the creative activity that incorporates the colour change).
Safety/supervision Iodine (I2) is produced in the reactions. The reaction vessels should be tapped. At the end of the reaction, the iodine produced should be neutralised with Na2S2O3 solution.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and list of materials
  • Potassium iodide, KI 0.1M (solution A).
  • Hydrogen peroxide, 3% H2O2 (aq) in an acidic environment + starch (solution B).
  • Sodium thiosulphate, Na2S2O3•5H2O ~0.05M (solution C).
  • Neutralising solution: sodium thiosulphate, Na2S2O3•5H2O ~0.05M (the solution is the same as solution C. It is written separately to remind that an additional amount for neutralisation is necessary).
  • 3 × 10ml graduated cylinders
  • 3 large test tubes
  • Suitable test tube rack
  • 3 rubber stoppers for the test tubes
  • 3 plastic pipettes
Learning objectives Introduction of interesting Redox reactions.Introduction of reaction rates, the effects of fast and slow reactions, and their uses.
Resources Videos of the clock reaction: