WATER
This article is about general aspects of water. For a
detailed discussion of its chemical properties, see Properties of water. For other uses, see Water (disambiguation).
Water in three states: liquid, solid (ice), and gas (invisible water vapor
in the air). Clouds
are accumulations of water droplets, condensed
from vapor-saturated air.
Water is a chemical compound with the chemical formula H
2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at standard ambient temperature and pressure, but it often co-exists on Earth with its solid state, ice, and gaseous state, steam (water vapor). Water also exists in a liquid crystal state near hydrophilic surfaces.[1][2][3]
2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at standard ambient temperature and pressure, but it often co-exists on Earth with its solid state, ice, and gaseous state, steam (water vapor). Water also exists in a liquid crystal state near hydrophilic surfaces.[1][2][3]
Water covers 71% of the Earth's surface,[4]
and is vital for all known forms of life.[5]
On Earth, 96.5% of the planet's water is found in seas and oceans, 1.7% in
groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a
small fraction in other large water bodies, and 0.001% in the air
as vapor,
clouds
(formed of solid and liquid water particles suspended in air), and precipitation.[6][7]
Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice
and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the
atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is
contained within biological bodies and manufactured products.[6]
Water on Earth moves continually through the water cycle
of evaporation
and transpiration (evapotranspiration),
condensation,
precipitation, and runoff,
usually reaching the sea. Evaporation and transpiration contribute to the
precipitation over land.
Safe drinking water
is essential to humans and other lifeforms even though it provides no calories
or organic nutrients. Access to safe drinking water has improved over the last
decades in almost every part of the world, but approximately one billion people
still lack access to safe water and over 2.5 billion lack access to adequate
sanitation.[8]
There is a clear correlation between access to safe water and GDP per capita.[9]
However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.[10]
A report, issued in November 2009, suggests that by 2030, in some developing
regions of the world, water demand will exceed supply by 50%.[11]
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 the fresh water
used by humans goes to agriculture.[12]
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