It is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state (water vapour or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.
Water covers 70. 9% of the Earth's surface, and is vital for all known forms of life. 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 ponds 0. 6%. A very small amount of the Earth's water is contained within biological bodies and manufactured products.
Water on Earth moves continually through a cycle of evaporation or transpiration (evapo-transpiration), precipitation, and runoff, usually reaching the sea. Over land, evaporation and transpiration contribute to the precipitation over land.