The water

Water is a chemical substance that is composed of hydrogen and oxigen and is vital for all known forms of life.

In typical usage ,water refers only to its liquid form or state,but the substance also has a solid state,ice and gaseous state,water vapor or steam.Water covers 71% of earths surface.On earth it is found mostly in oceans and other large water bodies,with 1.6% of water below ground in aquifers and 0.001% in the air as vapor,clouds(formed of solid and liquid water particles suspended in air ), and precipitation.Oceans hold 97% of surface water,glaciers and polar ice caps 2.4%, and other land surface water such as rivers,lakes and pounds 0.6%.A very small amounts of the Earths water is contained within biological bodies and manufactured products.

Water on earth moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff,usually reaching the sea.Over land,evaporation and transpiration contribute to the precipitation over land.

Clean,fresh drinking water is essential to human and other life forms.Access to safe drinking water has improved steadily and substantially over the last decades in almost every part of the world.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 fresh water is consumed by agriculture.