Occurrence

Occurrence: - Ozone is present in sufficient amounts in the atmosphere at higher altitudes i.e. 20km above earth’s surface. It is formed from oxygen present there, under influence of ultraviolet rays from sun. Ozone can absorb UV light to reform oxygen. Thus the ozone layer acts as a screen and prevents the harmful radiations from reaching the surface of earth. However, dust particles and chlorogluoro carbons decompose it. In small amounts, it occurs near the sea side or big lakes where it is formed by slow evaporation of water.

Laboratory preparation: - ozone can be obtained by passing silent electric discharge through dry, pure and cold oxygen. It is believed that some of the oxygen molecules dissociate and then atomic oxygen combines with oxygen molecules to form ozone.  

Since formation of ozone is an endothermic reaction, it required high energy which is obtained by silent electric discharge.

The mixture obtained is called ozonised oxygen. The apparatus used for this purpose is known as ozoniser. The commonly used ozonisers are (i) Siemen’s ozoniser  (ii) Brodie’s ozoniser.

Pure ozone from ozonised oxygen: - The ozonised oxygen is cooled by passing a spiral tube cooled by liquid air where it condenses as its condensation temperature is – 112.^4oC. The liquid ozone thus obtained may contain some dissolved oxygen, which is fractionally distilled to get pure ozone.