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.
Redox reactions, rate of reaction (kinetics).
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).
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).