Water is a clear, pellucid, transparent fluid which forms the streams, lakes, oceans and rain, is the major constituent of the fluids of living things
chemical compound, and the most widely used of all solvents. A
water molecule contains one
oxygen and two
hydrogenatoms that are 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 covers 71% of the Earth's surface. It is vital for all known forms of
life. 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. 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.
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. Water used in the production of a good or service is known as
virtual water.
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. There is a clear correlation between access to safe water and
gross domestic product per capita. However, some observers have estimated that by 2025 more than half of the
world populationwill be facing water-based vulnerability. A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. 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.
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