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Water in three states: liquid, solid (ice), and gas (invisible water vapor in the air). Clouds are accumulations of water droplets, condensed from vapor-saturated air.

Water (chemical formula: H2O) is a transparent fluid which forms the world's streams, lakes, oceans and rain, and is the major constituent of the fluids of organisms. As a chemical compound, a water molecule contains one oxygen and two hydrogen atoms that are connected by covalent bonds. Water is a liquid at standard ambient temperature and pressure, but it often co-exists on Earth with its solid state, ice; and gaseous state, steam (water vapor). It also exists as snow, fog, dew and cloud.

Water covers 71% of the Earth's surface.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is vital for all known forms of life. On Earth, 96.5% of the planet's crust water is found in seas and 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 as vapor, clouds (formed of ice and liquid water suspended in air), and precipitation.<ref name=b1>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>Water Vapor in the Climate System, Special Report, [AGU], December 1995 (linked 4/2007). Vital Water UNEP. Archived 24 March 2015 at the Wayback Machine</ref> Only 2.5% of this water is freshwater, and 98.8% of that water is in ice (excepting ice in clouds) 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.<ref name=b1/> A greater quantity of water is found in the earth's interior.<ref name="crockett">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Water on Earth moves continually through the water cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. Evaporation and transpiration contribute to the precipitation over land. Water used in the production of a good or service is known as virtual water.

Safe drinking water is essential to humans and other lifeforms even though it provides no calories or organic nutrients. Access to 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 to adequate sanitation.<ref name=UN/> There is a clear correlation between access to safe water and gross domestic product per capita.<ref>"Public Services", Gapminder video</ref> However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of the freshwater used by humans goes to agriculture.<ref name=Baroni2007>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Water sections
Intro  Chemical and physical properties  Taste and odor  Distribution in nature  On Earth  Effects on life  Effects on human civilization  Law, politics, and crisis  In culture  See also  References  Further reading  External links  

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