Water is a common chemical substance that is essential to 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 a gaseous state, water vapor.
About 1,460 teratonnes (Tt) of water cover 71% of Earth's surface, 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.
Some of the Earth's water is a part of man-made and natural objects near the earth's surface such as water towers, and animal and plant bodies, manufactured products, and food stores.
Saltwater oceans hold 97.0% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as riverslakes 0.6%. Water in these forms moves perpetually through the water cycle of evaporation and transpiration, precipitation, and runoff usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt per year.
Over land, evaporation and transpiration contribute another 71 Tt per year to the precipitation of 107 Tt per year over land. Some water is trapped for periods in ice caps, glaciers, aquifers, or in lakes, for varying periods, sometimes providing fresh water for life on land. Clean, fresh water is essential to human and other life. In many parts of the world, it is in short supply.
Many very important chemical substances, such as salts, sugars, acids, alkalis, some gasesoxygen), and many organic molecules dissolve in water.
"To the people of poor nations, we
pledge to work alongside you to make
your farms flourish and LetCleanWatersFlow, to nourish starved
bodies and feed hungry minds. And to those nations like ours that enjoy
relative plenty, we say we can no longer afford indifference to
suffering outside our borders; nor can we consume the world's resources
without regard to effect. For the world has changed, and we must change
with it". Source: President Obama, Inaugural Speech, January 20, 2009, Washington, DC, USA.