Why Does Hydrogen Peroxide Bubble on a Cut?

Learn the Chemistry Behind the Fizz

Medical supplies on a table

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Have you ever wondered why hydrogen peroxide bubbles on a cut or wound, yet it doesn't bubble on unbroken skin? Here's a look at the chemistry behind what makes hydrogen peroxide fizz—and what it means when it doesn't.​

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's not found on the surface of your skin. That's why pouring peroxide on unbroken skin won't cause bubbles to form. Do keep in mind that since it is so reactive, hydrogen peroxide has a shelf-life—especially once the container it's in has been opened. If you don't see bubbles form when peroxide is applied to an infected wound or bloody cut, there's a chance your peroxide has exceeded its shelf-life and is no longer active.

Hydrogen Peroxide as a Disinfectant

Since oxidation is a good way to alter or destroy pigment molecules, the earliest use of hydrogen peroxide was as a bleaching agent. However, peroxide has been used as a rinse and disinfectant since the 1920s. Hydrogen peroxide works to disinfect wounds in several ways: First, since it's a solution in water, it helps rinse away dirt and damaged cells and loosen dried blood, while the bubbles help lift away debris. Although the oxygen released by peroxide doesn't kill all types of bacteria, some are destroyed. Peroxide also has bacteriostatic properties, meaning it helps prevent bacteria from growing and dividing, and 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 the body uses to help repair wounds. Since it inhibits healing, hydrogen peroxide should not be used for prolonged periods of time. In fact, most doctors and dermatologists advise against using it to disinfect open wounds for this very reason.

Make Sure Hydrogen Peroxide Is Still Good

Eventually, hydrogen peroxide breaks down into oxygen and water. Once it has, if you use it on a wound, you're basically using plain water. Fortunately, there's a simple test to see whether or not your peroxide is still good. Simply splash a small amount into a sink. Metals (like those near the drain) catalyze the conversion of oxygen and water, so they also form bubbles as you'd 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. To ensure hydrogen peroxide lasts as long as possible, keep it in its original dark container (light breaks down peroxide) and store it in a cool location.

Test It Yourself

Human cells aren't the only ones that release catalase when they are compromised. Try pouring hydrogen peroxide on a whole potato. Next, compare that reaction to the one you get when you pour peroxide on a slice of cut potato. You can also test the reactions of other substances, like how alcohol burns on skin or wounds.