Elephant toothpaste is a foamy substance caused by the rapid decomposition of hydrogen peroxide. This is often used for classroom demonstrations because it requires only a small amount of ingredients and makes a "volcano of foam". This is also known as the "Marshmallow Experiment" in some cases, but should not be confused with the psychological Marshmallow experiment.
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Concentrated hydrogen peroxide is first mixed with liquid soap. Then a catalyst is added to make the hydrogen peroxide break down very quickly. Hydrogen peroxide breaks down into oxygen and water. As a small amount of hydrogen peroxide generates a large volume of oxygen, the oxygen quickly pushes out of the container. The soapy water traps the oxygen, creating bubbles, and turns into foam. Often some food colouring is added before the catalyst, which makes the resulting column of foam look even more like toothpaste.
This experiment shows the decomposition of hydrogen peroxide catalyzed by potassium iodide. Hydrogen peroxide (H2O2) decomposes into water and oxygen gas, but normally the reaction is too slow to be easily perceived or measured:
The iodide ions from potassium iodide acts as a catalyst—it speeds up the reaction without being consumed in the reaction process. The iodide ion changes the mechanism, by which the reaction occurs:
| H2O2 | + | I− | → | H2O | + | IO− | |||
| H2O2 | + | IO− | → | H2O | + | O2 | + | I− | |
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| 2H2O2 | → | 2H2O(l) | + | O2(g) | ΔrH° = −196 kJ/mol | ||||
Oxygen quickly leaves the system causing strong foaming of the detergent, so the foam erupts out. Steam rising from the foam shows that the reaction is exothermic (gives off heat). Minimum quantities of iodine are being liberated and can be detected by its colour if the experiment is done without dyes.[1]