Elephant's toothpaste

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Elephant toothpaste reaction

Elephant's toothpaste is a foamy substance caused by the rapid decomposition of hydrogen peroxide.[1][2][3] This is often used for classroom demonstrations[1][2][3] because it requires only a small number of ingredients and makes a "volcano of foam". This sometimes is known as the "Marshmallow Experiment", but is unrelated to the psychological Stanford marshmallow experiment.

Explanation

Description

Concentrated (>30%)[4] hydrogen peroxide is first mixed with liquid soap. Then a catalyst, often potassium iodide or catalase from baker's yeast, is added to make the hydrogen peroxide decompose 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 coloring is also added before the catalyst.

Chemical explanation

This experiment shows the catalyzed decomposition of hydrogen peroxide. Hydrogen peroxide (H2O2) decomposes into water and oxygen gas, but normally the reaction is too slow to be easily perceived or measured:[1]

${\displaystyle {\ce {2H2O2->{2H2O}+O2{\uparrow }}}}$

The iodide ion from potassium iodide acts as a catalyst—it speeds up the reaction without being consumed in the reaction process.[1][2][3] The iodide ion changes the mechanism by which the reaction occurs:

${\displaystyle {\begin{array}{llllll}{\ce {H2O2}}&+\ {\ce {I-}}&{\ce {->H2O}}&+\ {\ce {IO-}}\\{\ce {H2O2}}&+\ {\ce {IO-}}&{\ce {->H2O}}&+\ {\ce {O2}}{\uparrow }&+\ {\ce {I-}}\\\hline {\ce {2H2O2}}&&{\ce {->2H2O}}&+\ {\ce {O2}}{\uparrow }&&\Delta rH^{\circ }=-196{\ce {kJ/mol}}\end{array}}}$

The reaction is exothermic; the foam produced is hot.[1][2] A glowing splint can be used to show that the gas produced is oxygen.[5]