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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 coalent bonds. !ater is

a li"uid at standard ambient temperature and pressure, but it often co#exists on $arth with

its solid state, ice% and gaseous state, steam (water apor ). &t also exists as snow, fog, dew and cloud.

!ater coers of the $arth's surface. *+ &t is ital for all known forms of life. n $arth, -./ of the planet's crust water is found in seas and oceans, . in groundwater, . in glaciers and the ice caps of 

Antarctica and 0reenland, a small fraction in other large water bodies, and 1.11 in

the air  as apor , clouds (formed of ice and li"uid water suspended in air), and precipitation.*2+*3+ nly 2./

of this water is freshwater , and -4.4 of that water is in ice (excepting ice in clouds) and groundwater . 

5ess than 1.3 of all freshwater is in riers, lakes, and the atmosphere, and an een smaller amount of the

$arth's freshwater (1.113) is contained within biological bodies and manufactured products.*2+ A greater

"uantity of water is found in the earth's interior .*6+

!ater on $arth moes continually through the water

cycle of eaporation and transpiration (eapotranspiration), condensation, precipitation, and runoff , 

usually reaching the sea. $aporation and transpiration contribute to the precipitation oer land. !ater

used in the production of a good or serice is known as irtual water .

7afe drinking water  is essential to humans and other lifeforms een though it proides

no calories or  organic nutrients. Access to safe drinking water has improed oer the last decades in

almost eery part of the world, but approximately one billion people still lack access to safe water and

oer 2./ billion lack access to ade"uate sanitation.*/+ 8here is a clear correlation between access to safe

water andgross domestic product per capita.*+ 9oweer, some obserers hae estimated that by 212/

more than half of the world population will be facing water#based ulnerability.*+ A report, issued in

 oember 211-, suggests that by 2131, in some deeloping regions of the world, water demand will

exceed supply by /1.*4+ !ater plays an important role in the world economy, as it functions as

a solent for a wide ariety of chemical substances and facilitates industrial cooling and transportation.

Approximately 1 of the freshwater used by humans goes to agriculture.*-+

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