It is officially recognized that in 1774, oxygen (O2) was discovered by the English chemist Joseph Priestley . As a result of an experiment conducted in a closed vessel with mercury oxide, under the influence of sunlight directed by the lens, its decomposition occurred: 2HgO → O2 ↑ + 2Hg. This gaseous substance is characterized by a density under normal conditions of 0.00142897 g / cm³, a molar volume of 14.0 cm³ / mol, a melting point of minus 218.2 ° C and a boiling point of minus 182.81 ° C. The molar mass is 15.9994 g / mol. The main characteristic of oxygen is its ability to oxidize various substances. Being an active non-metal, O2 interacts with all metals with the formation of basic and amphoteric oxides, as well as with all non-metals (except halogens), resulting in acidic or non-salt forming oxides.
Oxygen is part of more than one and a half thousand substances, since it is the most common chemical element on Earth. It is part of various chemical compounds (there are more than one and a half thousand). In the solid crust, the O2 content is 47.4%. In sea and fresh waters , its share in the bound state is 88.8% of the mass. In the atmosphere, oxygen is in a free state, its volume fraction is approximately 21%, and the mass fraction is 23.1%. It is an essential component of organic substances that are present in every living cell. By volume in them it occupies 25%, and by weight 65%. The oxygen cycle in nature is due to its chemical activity.
The cycle is a series of changes in the substance, as a result of which it returns to the starting point, and the entire path is repeated. The oxygen cycle is a biogeochemical movement. Through it, O2 passes through the biotic sum of all ecosystems (biosphere or zone of life on Earth) and abiotic (lithosphere, atmosphere and hydrosphere) environments. The oxygen cycle describes its movement in the hydrosphere (the mass of water located underground and above its surface), the atmosphere (air), the biosphere (the global sum of all ecosystems) and the lithosphere (earth's crust). Violations of this cycle in the hydrosphere can lead to the development of hypoxic (low O2) zones in large lakes and the ocean. The main driving factor is photosynthesis.
Ecological systems (ecosystems) have many biogeochemical cycles that work in their composition. For example, the water cycle, the oxygen cycle, the nitrogen cycle, carbon, etc. All chemical elements travel a path that is part of biogeochemical cycles. They are an integral part of living organisms, but also move through abiotic ecosystems. These are water (hydrosphere), the earth's crust (lithosphere) and air (atmosphere). Living organisms fill the Earth's shell, called the biosphere. All nutrients, such as carbon, nitrogen, oxygen, phosphorus and sulfur, are used by them and are part of a closed system, so they are processed, not lost and not replenished constantly, as, for example, in an open system.
The largest reservoir of O2 (99.5%) is the Earth's crust and mantle, where it is contained in silicate and oxide minerals. The oxygen cycle ensured the ingress of only a small fraction in the form of free O2 into the biosphere (0.01%) and into the atmosphere (0.36%). The main source of atmospheric free O2 is photosynthesis. Its products are organic substances and free oxygen formed from carbon dioxide and water: 6CO2 + 6H2O + energy → C6H12O6 + 6O2.
Terrestrial plants, as well as phytoplankton of the oceans, are responsible for the oxygen cycle in the biosphere. The tiny marine cyanobacteria (blue-green algae) Prochlorococcus, 0.6 microns in size, were discovered in 1986. They account for more than half of the products of photosynthesis in the open ocean. An additional source of free atmospheric oxygen is the phenomenon of photolysis (a chemical reaction that occurs under the action of photons). As a result, atmospheric water and nitrous oxide dissociate into constituent atoms, hydrogen (H) and nitrogen (N) are removed from space, and O2 remains in the atmosphere: 2H2O + energy → 4H + O2 and 2N2O + energy → 4N + O2. Free atmospheric oxygen is consumed by living organisms in the processes of respiration and decay. The lithosphere uses free O2 as a result of chemical weathering and surface reactions. For example, it is spent on the formation of iron oxides (rust): 4FeO + O2 → 2Fe2O3 or oxides of other metals and nonmetals.
The oxygen cycle also includes a cycle between the biosphere and the lithosphere. Marine organisms in the biosphere serve as sources of calcium carbonate (CaCO3), which is rich in O2. When the body dies, its shell is carried out in the shallow water of the seabed, where it is located for a long time and forms limestone (sedimentary rock of the earth's crust). The weathering processes initiated by the biosphere can also extract free oxygen from the lithosphere. Plants and animals extract nutrients from sedimentary rocks and release oxygen.