Hydrogen peroxide is a chemical compound with the formula . It is the simplest peroxide (a compound with an oxygen-oxygen single bond) and in its pure form is a colorless liquid, slightly more viscous than water. For safety reasons it is normally used as an aqueous solution, also colorless. Hydrogen peroxide is a strong oxidizer and is used as a bleaching agent and disinfectant. Concentrated hydrogen peroxide, or 'high-test peroxide,' is a reactive oxygen species and has been used as a propellant in rocketry. Organisms naturally produce trace quantities of hydrogen peroxide, most notably by a respiratory burst as part of the immune response.