DEMO 005


This is a classic surprise reaction. Two colorless solutions are mixed and nothing appears to happen for nearly one minute. The solution suddenly turns dark blue. The demo can be used at many levels ranging from very introductory "gee whiz" to a p-chem discussion of competing reactions.


100 mL of Solution A (0.20 M H2SO4, 0.088 M H2O2)
100 mL of Solution B (0.0016 M
Na2S2O3, 0.052 M KI, starch)
500 mL flask or beaker


After appropriate introduction, mix the two solutions and swirl or stir. If appropriate, show consternation that nothing is happening. Your behavior depends on the degree of showmanship you wish to display. If you time the reaction, you can show the effect of temperature by mixing cold or warm solutions. The effect of concentration on rate can be shown by using solutions of greater or smaller concentration.


In the iodine clock reaction, there are really two processes happening simultaneously. The first is a slow reaction producing iodine:

H2O2(aq) + 2 I-(aq) I2(s) + 2 OH-(aq)

However, the iodine is never seen because of the second very fast process in which it is immediately reduced back to colorless iodide ion:

I2(s) + 2 S2O32-(aq) 2 I-(aq) + S4O62-(aq)

Thus, iodine is slowly formed and then instantly converted back to iodide ion until all the thiosulfate ion (S2O32-) is used up. At that point the iodine concentration shoots up and the intense blue color of the starch-iodine complex appears. The time required to reach this point depends on the rates of the two reactions, and consequently on the concentrations of all the reactants. Anything that accelerates the first reaction will shorten the time. This increasing the concentration of iodide, hydrogen peroxide, or acid (it neutralizes the hydroxide ion) will make T shorter. On the other hand, increasing the thiosulfate concentration will have the opposite effect; it will take longer for the iodine color to appear.


The reaction produces iodine which can stain skin and fabric. It can be destroyed by reaction with sodium thiosulfate solution. Drink neither Solution A or B.