Hydrogen peroxide bubbles when it is poured onto a cut or a wound because of an enzyme called catalase, which catalyzes the breakdown of hydrogen peroxide into its constituent molecules of water and oxygen. The bubbles are formed when oxygen gas is released.
Catalase is present in most cells of the human body, but it is not found on the surface of the skin. When skin in broken, cells are ruptured, exposing catalase. Catalase is also found in many bacteria, so peroxide is especially prone to bubbling when exposed to a wound that has become infected. Hydrogen peroxide is an effective disinfectant, but it can lead to scarring because it destroys newly formed skin cells.