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Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but itoften co-exists on Earth with its solid state, ice, and gaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.[1][2] Under nomenclature used toname chemical compounds, Dihydrogen monoxide is the scientific name for water, though it is almost never used.[3]
Water covers 70.9% of the Earth's surface,[4] and is vital for all known forms oflife.[5] On Earth, 96.5% of the planet's water is found in oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.[6][7] Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in iceand groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies andmanufactured products.[6]
Water on Earth moves continually through the hydrological cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching thesea. Evaporation and transpiration contribute to the precipitation over land.
Safe drinking water is essential to humans and other lifeforms. Access to safe drinking water has improved over the lastdecades in almost every part of the world, but approximately one billion people still lack access to safe water and over 2.5 billion lack access to adequate sanitation.[8] There is a clearcorrelation between access to safe water and GDP per capita.[9] However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.[10] A...