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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 bycovalent bonds. Water is a liquid at ambient conditions, but itoften co-exists on Earth with its solid state, ice, and gaseous state (water vapor orsteam). Water also exists in a liquid crystal state near hydrophilic surfaces.[1][2]
Water covers 70.9% ofthe Earth's surface,[3] and is vital for all known forms of life.[4] 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, and 0.001% in the air asvapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.[5][6] Only 2.5% of the Earth'swater is freshwater, and 98.8% of that water is in ice and 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 and manufactured products.[5]
Water on Earth moves continually through the hydrologicalcycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, andrunoff, usually reaching the sea. Evaporation and transpiration contribute to the precipitation over land.
Safe drinking water is essential to humans and other lifeforms. Accessto safe drinking water has improved over the last decades 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 toadequate sanitation.[7] There is a clear correlation between access to safe water and GDP per capita.[8] However, some observers have estimated that by 2025 more than half of theworld population will befacing water-based vulnerability.[9] A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.[10] Water plays an...