Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
4. Health and Safety
Why does hydrogen peroxide bubble up when you put it on your cut?
Why is it when you put hydrogen peroxide in your mouth it bubbles?
Hydrogen peroxide is H2O2. You can see that this is water (H2O) with an extra oxygen atom attached. Catalysts in your blood called peroxidases and catalases break it into water and oxygen.
Catalase is an enzyme (a protein the body makes that speeds up chemical reactions). It is very effective at speeding up the natural breakdown of hydrogen peroxide into oxygen and water. In each second, a molecule of catalase can break down 40 million molecules of hydrogen peroxide.
Your cells produce hydrogen peroxide as an undesirable side effect of breathing oxygen. Hydrogen peroxide is dangerous to cells, so they produce catalase to quickly break it down and escape damage. When you have damaged your skin, the cells that produce catalase are exposed. When you add hydrogen peroxide, the catalase in the cells breaks it down, and bubbles of oxygen form.
Hydrogen peroxide is used to damage germs that might find their way into damaged skin. If you use lots of hydrogen peroxide, the catalase can’t keep up, and the bacteria get damaged. Some skin cells also get damaged, which is why the peroxide stings a little, and peroxide is no longer recommended as a disinfectant for wounds now that better alternatives are available.
Other organisms also breathe oxygen, and so they need their own catalase enzymes. You can see them in action if you add some dried yeast to a cup of hydrogen peroxide. The peroxide starts to bubble vigorously as the oxygen is produced.