Pure ozone is an unstable, faintly bluish gas with a characteristic fresh, penetrating odor.
The gas has a density of 2.144 grams per liter at STP . It is the most chemically active form of oxygen. It is formed in the ozone layer of the stratosphere by the action of solar ultraviolet light on oxygen. Although it is present in this layer only to an extent of about 10 parts per million, ozone is important because its formation prevents most ultraviolet and other high-energy radiation, which is harmful to life, from penetrating to the earth's surface. Later, in the presence of a catalyst, the atomic and diatomic oxygen reunite to form ozone.