Hydrogen peroxide is chemical similar to water. Water has the chemical formula H2O, while hydrogen peroxide is H2O2. Under the right conditions, hydrogen peroxide breaks down into water (H2O) and oxygen gas. The oxygen is what makes the bubbles. One of the chemicals that can break down hydrogen peroxide is an enzyme called catalase. Catalase is found in human cells. When you get a cut or scrape, some of your cells are damaged, allowing the catalase to escape. That decomposes the hydrogen peroxide, releasing the oxygen bubbles. So the bubbles do not tell you that there are any germs or infection, only that you have damaged cells.

Catalase is found in other places too, including raw potatoes and yeast. Either will cause hydrogen peroxide to bubble just as it does on a cut.