Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms. It is vital for all known forms of life, even though it provides no calories or organic nutrients.

Its chemical formula H2O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds.

"Water" is the name of the liquid state of H2O at standard conditions for temperature and pressure.

A number of natural states of water exist. It forms precipitation in the form of rain and aerosols in the form of fog. Clouds consist of suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor.

Water covers approximately 70.9% of the Earth's surface, mostly in seas and oceans.

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