The oxygen cycle. The oxygen reservoir is the atmosphere. During the day plants absorb carbon dioxide and emit oxygen through photosynthesis. At night plants absorb oxygen and emit carbon dioxide. Oxygen is absorbed and released by oceans.
Animals respire, absorbing oxygen and exhaling carbon dioxide.
Preparaing oxygen Pure oxygen has a large number of uses in the modern world. It is used, forexample, in medicine, but also on amuch larger scale for making fuels burn more effectively, such as in aniron-making blast furnace or in an oxyacetylene welding and cutting lance. On a large industrial scale, atmospheric air is cooled. In the school laboratory oxygen is made byreacting two chemicals together or bycausing an oxygen-rich compound todecompose. It is common to use the apparatus shown on this page for small samples.