Why Does Hydrogen Peroxide Bubbles on a Wound

One bottle of hydrogen peroxide with a piece of liver in it and one bottle of plain hydrogen peroxided
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Have you ever wondered why hydrogen peroxide bubbles on a cut or wound, yet doesn't bubble on unbroken skin? Here's a look at the chemistry behind why hydrogen peroxide bubbles and what it means when it doesn't bubble.​

Why Hydrogen Peroxide Forms Bubbles

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 hydrogen peroxide 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don't see bubbles form when peroxide is applied to an infected wound or bloody cut, there is a chance your peroxide is no longer active.

Hydrogen Peroxide as a Disinfectant

The earliest use of hydrogen peroxide was as a bleach since oxidation is good at altering or destroying pigment molecules, however, peroxide has been used as a rinse and disinfectant since the 1920s. It helps disinfect wounds a few ways. First, since it's a solution in water, hydrogen peroxide helps rinse away dirt and damaged cells and loosen dried blood. The bubbles help lift away debris. Although the oxygen released by peroxide doesn't kill all types of bacteria, some are destroyed. Also, peroxide has bacteriostatic properties, which means it helps prevent bacteria from growing and dividing. It also acts as a sporicide, killing potentially infectious fungal spores.

However, hydrogen peroxide isn't an ideal disinfectant, because it also kills fibroblasts, which are a type of connective tissue your body uses to help repair wounds. So, hydrogen peroxide should not be used for long periods of time because it can inhibit healing. Most doctors and dermatologists advise against using peroxide to disinfect open wounds because it can slow the healing process.

Make Sure Hydrogen Peroxide Is Still Good

Eventually, hydrogen peroxide breaks down into oxygen and water. If you use this peroxide on a wound, you're basically using plain water. Fortunately, there is a simple test to see whether or not your bottle of peroxide is still good. Simply splash a small amount into a sink. Metals (like near the drain) catalyze the conversion to oxygen and water, so they also form bubbles like you would see on a wound. If bubbles form, the peroxide is effective. If you don't see bubbles, it's time to get a new bottle of hydrogen peroxide. To keep it lasting as long as possible, make sure it stays in its original dark container (light breaks down peroxide) and store it in a cool location.

Test It for Yourself

Human cells aren't the only kind of cells that release catalase when they are broken. Try pouring hydrogen peroxide on a whole potato. Compare this with the reaction you get when you pour peroxide on a cut potato slice.