WATER
Water is a transparent and nearly colorless chemical substance that is the main constituent of Earth'sstreams , lakes, and oceans , and thefluids of most living organisms. Itschemical formula is H 2O , meaning that its molecule contains one oxygen and two hydrogen atoms , that are connected by covalent bonds . Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure; but it often refers also to its solid state (ice) or its gaseous state (steam or
water vapor ). It also occurs in nature as snow , glaciers, ice packs and
icebergs, clouds, fog, dew , aquifers , and atmospheric humidity .
Water covers 71% of the Earth's surface. [1] It is vital for all known forms of life . On Earth, 96.5% of the planet's crust 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 ice and liquid water suspended in air), and precipitation. [2]
[3] Only 2.5% of this water is
freshwater , and 98.8% of that water is in ice (excepting ice in clouds) 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. [2] A greater quantity of water is found in the earth's interior. [4]
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. Large amounts of water are also chemically combined or adsorbed in hydrated minerals .
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 . [5] There is a clear correlation between access to safe water and gross domestic product per capita . [6] However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.[7] A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. [8]
Water plays an important role in the
world economy . Approximately 70% of the freshwater used by humans goes to
agriculture . [9] Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities (such as oil and natural gas ) and manufactured products is transported by boats through seas, rivers, lakes, and canals. Large quantities of water, ice, and steam are used for cooling and
heating , in industry and homes. Water is an excellent solvent for a wide variety of chemical substances; as such it is widely used in industrial processes, and in cooking and washing. Water is also central to many sports and other forms of entertainment, such as swimming , pleasure boating, boat racing , surfing , sport fishing , and
diving.
Water is a liquid at the temperatures and pressures that are most adequate for life. Specifically, at a standard pressure of 1 atm (1.01325 bar , 101.325 k Pa , 14.69595 psi ), water is a liquid between the temperatures of 273.15 K (0 °C , 32 °F ) and 373.15 K (100 °C, 212 °F). Increasing the pressure slightly lowers the melting point , which is about −5 °C at 600 atm, −22 °C at 2100 atm. This effect is relevant, for example, to ice skating , to the buried lakes of Antartica , and to the movement of glaciers. (At pressures higher than 2100 atm the melting point rapidly increases again, and ice takes several exotic forms that do not exist at lower pressures.)
Increasing the pressure has a more dramatic effect on the boiling point, that is about 374 °C at 220 atm. This effect is important in, among other things, deep-sea hydrothermal vents and geysers , pressure cooking , and
steam engine design. At the top of
Mount Everest, where the atmospheric pressure is about 0.34 atm, water boils at 68 °C (154 °F).
At very low pressures (below about 0.006 atm), water cannot exist in the liquid state and passes directly from solid to gas by sublimation —a phenomenon exploited in the freeze drying of food. At very high pressures (above 221 atm), the liquid and gas states are no longer distinguishable, a state called supercritical steam.
Water also differs from most liquids in that it becomes less dense as it freezes. The maximum density of water in its liquid form (at 1atm) is 1,000 kg/m 3 (62.43 lb/cu ft); that occurs at 3.98 °C (39.16 °F). [15] The density of ice is 917 kg/m 3 (57.25 lb/cu ft). [16]
[17] Thus, water expands 9% in volume as it freezes, which accounts for the fact that ice floats on liquid water.
The details of the exact chemical nature of liquid water are not well understood; some theories suggest that water's unusual behaviour is as a result of it having 2 liquid states.
Taste and odor
Pure water is usually described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, [20] and frogs are known to be able to smell it. [21] However, water from ordinary sources (including bottled mineral water) usually has many dissolved substances, that may give it varying tastes and odors.
Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water that is too salty or putrid .[22]Color and appearance
The apparent color of natural bodies of water (and swimming pools) is often determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.
Light in the visible electromagnetic spectrum can traverse a couple meters of pure water (or ice) without significant absorption , so that it lookstransparent and colorless. [23] Thus aquatic plants, algae , and otherphotosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water vapour is essentially invisible a gas.Taste and odor Pure water is usually described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, [20] and frogs are known to be able to smell it. [21] However, water from ordinary sources (including bottled mineral water) usually has many dissolved substances, that may give it varying tastes and odors.Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water that is too salty orputrid .[22]Color and appearanceThe apparent color of natural bodies of water (and swimming pools) is often determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.Light in the visible electromagnetic spectrum can traverse a couple meters of pure water (or ice) without significant absorption , so that it lookstransparent and colorless. [23] Thus aquatic plants, algae , and otherphotosynthetic organisms can live in water up to hundreds of meters deep because sunlight can reach them. Water vapour is essentially invisible as gas.
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