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 temperatures above 0 °C (273.15 K, 32 °F) at sea level, but it often 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.

Water covers 71% of the Earth's surface, and is vital for all known forms of life. 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 as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Only 2.5% of the Earth's water 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.

Water on Earth moves continually through the hydrological cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. 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 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. There is a clear correlation between access to safe water and GDP per capita. However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability. A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. 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 fresh water used by humans goes to agriculture.

Read more about Water:  Chemical and Physical Properties, Taste and Odor, On Earth, Effects On Life, Effects On Human Civilization, Water Law, Water Politics and Water Crisis

Famous quotes containing the word water:

    I believe that water is the only drink for a wise man: wine is not so noble a liquor; and think of dashing the hopes of a morning with a cup of warm coffee, or of an evening with a dish of tea! Ah, how low I fall when I am tempted by them! Even music may be intoxicating. Such apparently slight causes destroyed Greece and Rome, and will destroy England and America.
    Henry David Thoreau (1817–1862)

    I think of the nestling fallen into the deep grass,
    The turtle gasping in the dusty rubble of the highway,
    The paralytic stunned in the tub, and the water rising,—
    All things innocent, hapless, forsaken.
    Theodore Roethke (1908–1963)

    And one rose in a tent of sea and gave
    A darkening shudder; water fell away;
    The whale stood shining, and then sank in spray.
    Yvor Winters (1900–1968)