Hydrogen peroxide is a chemical compound with the formula H2O2. In its pure form, it is a very pale blue liquid, slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic. Concentrated hydrogen peroxide, or “high-test peroxide”, is a reactive oxygen species and has been used as a propellant in rocketry. Its chemistry is dominated by the O–O bond.
Hydrogen peroxide is the simplest peroxide (a compound with an oxygen–oxygen single bond). It slowly decomposes in the presence of light. Hydrogen peroxide is typically stored with a stabilizer in a weakly acidic solution in a dark coloured bottle. Hydrogen peroxide is found in biological systems including the human body. Enzymes that use or decompose hydrogen peroxide are classified as peroxidases.