Ozone, a pale blue gas and an allotrope of oxygen, is much more unstable than oxygen. Ozone is formed from oxygen in the earth's atmosphere under the action of ultraviolet light and electric discharges. In standard conditions, ozone gas condenses at low temperatures into dark blue liquid, and violet black solid if the temperature continues to drop. High concentrations of ozone are found in the ozone layer in the stratosphere, where it absorbs most of the ultraviolet radiation from the sun to prevent it from reaching the earth's surface.
Ozone is widely applied for its strong oxidation and sterilization ability. Ozone applications can be divided into sterilization, decolorization, deodorization and oxidative decomposition, etc. Its application fields include environmental sterilization, water treatment, food processing and preservation, chemical oxidation, etc. Ozone can be generated with methods including dielectric-barrier discharge, ultraviolet radiation, electrolysis, radiochemistry, etc.