Hydrogen peroxide bubbles upon exposure to the enzyme catalase, which causes the peroxide to break apart. Hydrogen peroxide only bubbles on contact with open wounds.
Hydrogen peroxide, a common product in the cleaning of minor wounds, contains two atoms of hyrdrogen and two of oxygen. Upon exposure to an open wound, hydrogen peroxide interacts with the enzyme catalase, which is present in the cells of most living things and is important in protecting cells from damage. Catalase breaks peroxide apart to form water and oxygen; the oxygen rises to the surface and escapes as visible bubbles. Hydrogen peroxide does not bubble on unbroken skin because catalase only exists on the interior of cells.