Hydrogen peroxide (H2O2) is the simplest peroxide (a compound with an oxygen-oxygen single bond). It is also a strong oxidizer and used in tooth bleaching. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. Due to its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent. The oxidizing capacity of hydrogen peroxide is so strong that it is considered a highly reactive oxygen species.