Everyday Science

What makes hydrogen peroxide foam when you pour it on a cut?
Answered by HowStuffWorks
  • HowStuffWorks


  1. The hydrogen peroxide that you buy at the store is normally a mixture of 3 percent hydrogen peroxide and 97 percent water. It's useful for cleaning cuts, although you might have noticed that it foams when it touches your wound. The foam is a reaction between the enzyme catalase in your blood and cells and the hydrogen peroxide. When H2O2 (hydrogen peroxide's molecular composition) hits catalase, the catalase separates it into water, or H2O, and oxygen, or O2. So that funny foam you see is really tiny bubbles of oxygen forming in the chemical reaction.

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