Elephant's Toothpaste

30% hydrogen peroxide is added to a glass cylinder containing a concentrated aqueous mixture of potassium iodide and dishwashing soap. The iodide ion catalyzes the decomposition of hydrogen peroxide, generating oxygen gas which causes the soap to foam up and shoot up out of the cylinder.

Curriculum Notes 

This demonstration is usually used to illustrate catalysis in a kinetics unit. It could also be used as an example of a disproportionation reaction. Allow about 10 minutes for this demo.

Lead Time 
One day of lead time is required for this project.
Discussion 

This reaction is particularly effective not only because it is amusing, but any student who has put household 3% hydrogen peroxide on a cut or scrape is familiar with the catalytic decomposition of the substance:

2 H2O2(aq) --> 2 H2O(l) + O2(g)

 

In the case of cuts and scrapes, the catalyst is an enzyme called catalase which is present in our tissue. The antiseptic quality of the hydrogen peroxide is due to fact that the high concentration of oxygen which results from the decomposition process is toxic to the anaerobic bacteria that are responsible for infection. 

In this demonstration it is the iodide ion which catalyzes the decomposition.

 

It should be noted that oxygen is both oxidized and reduced during this reaction, going from a -1 oxidation state in the peroxide to -2 in water and 0 in elemental oxygen. This is an example of disproportionation.

 

The decomposition of hydrogen peroxide is fairly exothermic (-106 kJ/mol), which accounts for the "steaming" of the foam column.

 

There is an orangish-maroon discoloration of the foam that is characteristic of elemental iodine. This leads me to believe that some of the iodide ion is being oxidized in a side-reaction. Perhaps:

 

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

Materials 
  • large glass cylinder
  • small stoppered flask containing 50 ml of 30% hydrogen peroxide solution
  • small stoppered flask containing KI/soap solution
  • large basin to catch overflow
  • neoprene gloves
Procedure 

Place the glass cylinder in the basin. Pour the KI/soap solution into the cylinder. Wearing the gloves, carefully but quickly add the hydrogen peroxide solution to the cylinder. A steaming column of suds spurts out of the top of the cylinder.

 

 

Safety Precautions 

30% hydrogen peroxide solution is a strong oxidizer. Splash goggles and neoprene gloves should be worn during this demonstration. Thefoam is hot and probably contains quite a bit of unreacted hydrogen peroxide. Do not touch it.

Prep. Notes 

To prepare the KI/soap solution, dissolve 5 g of KI in a minimal amount of water in a 50 mL Erlenmeyer flask. Add about 10 mL of dishwashing liquid and swirl to mix. Be careful handling the 30% hydrogen peroxide. Be careful during clean-up, too; there could be a lot of unreacted hydrogen peroxide in the foam.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group