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What cause hydrogen peroxide to bubble up when apply to a scrapped knee?

Hydrogen peroxide bubbles when it comes into contact with an enzyme called catalase. Most cells in the body contain catalase, so when the tissue is damaged, the enzyme is released and becomes available to react with the peroxide.

Catalase allows hydrogen peroxide (H2O2) to be broken down into water (H2O) and oxygen (O2). Like other enzymes, catalase is not used up in the reaction, but is recycled to catalyze more reactions. Catalase supports up to 200,000 reactions per second.

The bubbles you see when you pour oxygen on a cut are bubbles of oxygen gas. Blood, cells, and some bacteria (e.g., staphylococcus) contain catalase, but it is not found on the surface of your skin so pouring peroxide on unbroken skin will not cause bubbles to form. Also, because it is so reactive, hydrogen peroxide has a shelf life once it has been opened, so if you don’t see bubbles form when peroxide is applied to an infected wound or bloody cut, there is a chance your peroxide is no longer active.