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 covers 71% of the 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 contained within man-made and natural objects near the Earth’s surface such as water towers, animal and plant bodies, manufactured products, and food stores.

Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers and lakes 0.6%. Water moves continually through a cycle of evaporation or 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 varying periods in ice caps, glaciers, aquifers, or in lakes, 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 organic molecules as well as salts, sugars, acids, alkalis, and some gases (especially oxygen), are soluble in water.

Beyond the Earth, a significant quantity of water is thought to exist underground on the planet Mars, on the moons Europa and Enceladus, and on the exoplanets known as HD 189733 b and HD 209458 b.

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