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Hydrogen peroxide (H2O2) decomposes into water and oxygen gas, but normally the reaction is so slow as to be imperceptible.
2H2O22H2O + O2(g)
What happens when you pour hydrogen peroxide onto a cut? It bubbles! That's because there is something in your bodily fluids that catalyzes the reaction. A catalyst is a substance that speeds up a reaction, without being consumed itself.
In this demonstration we use a 30% hydrogen peroxide solution, which is much more concentrated than the 3% solution you can buy in the store. The production of oxygen gas is made more noticeable by adding some dish soap, which makes the foam. The reaction is catalyzed by iodide ion (I-) from potassium iodide. The iodide ion changes the mechanism, or pathway, by which the reaction occurs. In the first step of a two step process, iodide ion reacts with hydrogen peroxide to form water and the hypoiodite ion (IO-). This hypoiodite ion reacts with another molecule of hydrogen peroxide, giving water, oxygen, and iodide ion. Notice that the iodide ion is regenerated, allowing it to go back and react with another molecule of hydrogen peroxide. This can continue for thousands of cycles. Note, too, that if one adds these two steps together, the I- and OI- cancel out, giving the same net reaction as above. The iodide is neither produced nor consumed in the reaction, but it changes the reaction mechanism, causing it to go faster. The rapid production of bubbles of oxygen gas, along with the dish soap, quickly creates a large quantity of foam.

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