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An inorganic compound of hydrogen and oxygen that is an essential component of all living organisms. Water is made up of one oxygen atom and two hydrogen atoms bound by a covalent bond at an angle of 104.45°. As a result, water has complex chemical and physical properties with many anomalies, for example, that as a liquid it has the highest density at 4 °C or that the solid phase is lighter than the liquid. As a liquid, it covers 71 percent of the Earth’s surface, but it is also found in solid (ice) and gaseous (steam) phases. Water is constantly in motion on Earth due to the water cycle. If the normal hydrogen in a water molecule is replaced by deuterium, an isotope with one neutron and one proton, we speak of heavy water.