peroxide (pərŏkˈsĪd) [key], chemical compound containing two oxygen atoms, each of which is bonded to the other and to a radical or some element other than oxygen; e.g., in hydrogen peroxide, H2O2, the atoms are joined together in the chainlike structure HOOH. Peroxides are powerful oxidizing agents. They are unstable, releasing oxygen when heated. Peroxides may be formed directly by reaction of an element or compound with oxygen. In dry, carbon-dioxide-free air, sodium or barium metal reacts to form its peroxide. In moist air, zinc metal is oxidized and hydrogen peroxide is formed. When a metal peroxide is treated with a dilute acid, a solution of hydrogen peroxide and a metal salt is formed. Ethers can react with oxygen from the air to form peroxides. This creates a special hazard, since the peroxides are often so unstable that they decompose explosively if heated.