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Water is an inorganic, transparent, , , and nearly colorless chemical substance, which is the main constituent of 's and the of all known living (in which it acts as a ). It is vital for all known forms of life, even though it provides no or . Its is H2O, meaning that each of its contains one and two , connected by .Two hydrogen atoms are attached to one oxygen atom at an angle of 104.45°.

"Water" is the name of the liquid state of H2O at standard ambient temperature and pressure. It forms in the form of and in the form of . are formed from suspended droplets of water and , its solid state. When finely divided, crystalline ice may precipitate in the form of . The gaseous state of water is or . Water moves continually through the of , (evapotranspiration), , precipitation, and , usually reaching the sea.

Water covers 71% of the 's , mostly in and . Small portions of water occur as (1.7%), in the and the of and (1.7%), and in the as , (formed of ice and liquid water suspended in air), and precipitation (0.001%). Water Vapor in the Climate System , Special Report, AGU, December 1995 (linked 4/2007). Vital Water .

Water plays an important role in the . Approximately 70% of the freshwater used by goes to . in salt and fresh water bodies is a major source of food for many parts of the . Much of the long-distance trade of (such as oil, natural gas, and manufactured products) is transported by through , , , and . Large quantities of water, ice, and are used for and , in and . Water is an excellent for a wide variety of substances both mineral and organic; as such it is widely used in industrial processes, and in cooking and . Water, ice and snow are also central to many and other forms of , such as , , , , , diving, and .

Etymology
The word water comes from wæter, from Proto-Germanic * watar (source also of watar, wetir, water, Old High German wazzar, Wasser, vatn, 𐍅𐌰𐍄𐍉 (), from Proto-Indo-European * wod-or, suffixed form of root * wed- ("water"; "wet"). Also , through the Indo-European root, with ύδωρ (), вода́ (), uisce, and Albanian ujë.

History

Chemical and physical properties
Water () is a polar inorganic compound that is at a and liquid, nearly colorless with a hint of blue. This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the " of life":
(2013). 9780321775658, Pearson.
indeed, water as found in nature almost always includes various dissolved substances, and special steps are required to obtain chemically . Water is the only common substance to exist as a , liquid, and in normal terrestrial conditions.
(2013). 9780321775658, Pearson.

States
Along with oxidane, water is one of the two official names for the chemical compound ;
(1998). 9780865426856, Blackwell Science. .
it is also the phase of . The other two common states of matter of water are the phase, , and the gaseous phase, or . The addition or removal of heat can cause : (water to ice), (ice to water), (water to vapor), (vapor to water), sublimation (ice to vapor) and deposition (vapor to ice).

Density
Water differs from most liquids in that it becomes less as it freezes. In 1 atm pressure, it reaches its maximum density of at . The density of ice is , an expansion of 9%.
(2021). 9780534395971, Thomson Brooks/Cole.
(2021). 9789814338967
This expansion can exert enormous pressure, bursting pipes and cracking rocks (see ).

In a lake or ocean, water at 4 °C (39.2 °F) sinks to the bottom, and ice forms on the surface, floating on the liquid water. This ice insulates the water below, preventing it from freezing solid. Without this protection, most aquatic organisms would perish during the winter.

Phase transitions
At a pressure of one atmosphere (atm), ice melts or water freezes at 0 °C (32 °F) and water boils or vapor condenses at 100 °C (212 °F). However, even below the boiling point, water can change to vapor at its surface by (vaporization throughout the liquid is known as ). Sublimation and deposition also occur on surfaces. For example, is deposited on cold surfaces while form by deposition on an aerosol particle or ice nucleus. In the process of , a food is frozen and then stored at low pressure so the ice on its surface sublimates.

The melting and boiling points depend on pressure. A good approximation for the rate of change of the melting temperature with pressure is given by the Clausius–Clapeyron relation:

$\frac\left\{d T\right\}\left\{d P\right\} = \frac\left\{T \left\left(v_\text\left\{L\right\}-v_\text\left\{S\right\}\right\right) \right\}\left\{L_\text\left\{f\right\}\right\},$
where $v_\text\left\{L\right\}$ and $v_\text\left\{S\right\}$ are the of the liquid and solid phases, and $L_\text\left\{f\right\}$ is the molar of melting. In most substances, the volume increases when melting occurs, so the melting temperature increases with pressure. However, because ice is less dense than water, the melting temperature decreases.
(2021). 9783662532072, Springer. .
In glaciers, pressure melting can occur under sufficiently thick volumes of ice, resulting in .

The Clausius-Clapeyron relation also applies to the boiling point, but with the liquid/gas transition the vapor phase has a much lower density than the liquid phase, so the boiling point increases with pressure.

(2021). 9780495126713, Cengage Learning. .
Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed in , a geyser in Yellowstone National Park. In hydrothermal vents, the temperature can exceed .

At , the boiling point of water is . As atmospheric pressure decreases with altitude, the boiling point decreases by 1 °C every 274 meters. High-altitude cooking takes longer than sea-level cooking. For example, at , cooking time must be increased by a fourth to achieve the desired result. (Conversely, a can be used to decrease cooking times by raising the boiling temperature.) In a vacuum, water will boil at room temperature.

Triple and critical points
On a pressure/temperature (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the , where all three phases can coexist. The triple point is at a temperature of and a pressure of ; it is the lowest pressure at which liquid water can exist. Until 2019, the triple point was used to define the Kelvin temperature scale.

The water/vapor phase curve terminates at and . This is known as the critical point. At higher temperatures and pressures the liquid and vapor phases form a continuous phase called a supercritical fluid. It can be gradually compressed or expanded between gas-like and liquid-like densities, its properties (which are quite different from those of ambient water) are sensitive to density. For example, for suitable pressures and temperatures it can with nonpolar compounds, including most . This makes it useful in a variety of applications including high-temperature and as an ecologically benign solvent or in chemical reactions involving organic compounds. In Earth's mantle, it acts as a solvent during mineral formation, dissolution and deposition.

Phases of ice and water
The normal form of ice on the surface of Earth is , a phase that forms crystals with hexagonal symmetry. Another with cubic crystalline symmetry, , can occur in the upper atmosphere. As the pressure increases, ice forms other crystal structures. As of 2019, 17 have been experimentally confirmed and several more are predicted theoretically. When sandwiched between layers of , ice forms a square lattice.

The details of the chemical nature of liquid water are not well understood; some theories suggest that its unusual behaviour is due to the existence of 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,Edmund T. Rolls (2005), "Emotion Explained". Oxford University Press, Medical. , 9780198570035. and frogs are known to be able to smell it.R. Llinas, W. Precht (2012), "Frog Neurobiology: A Handbook". Springer Science & Business Media. , 9783642663161 However, water from ordinary sources (including bottled mineral water) usually has many dissolved substances, that may give it varying tastes and odors. and other animals have developed senses that enable them to evaluate the of water by avoiding water that is too salty or .

Color and appearance
Pure water is visibly blue due to absorption of light in the region ca. 600 nm - 800 nm. The color can be easily observed in a glass of tap-water placed against a pure white background, in daylight. The principal absorption bands responsible for the color are of the O-H stretching vibrations. The apparent intensity of the color increases with the depth of the water column, following Beer's law. This also applies, for example, with a swimming pool when the light source is sunlight reflected from the pool's white tiles.

In nature, the color may also be modified from blue to green due to the presence of suspended solids or algae.

In industry, near-infrared spectroscopy is used with aqueous solutions as the greater intensity of the lower overtones of water means that glass with short path-length may be employed. To observe the fundamental stretching absorption spectrum of water or of an aqueous solution in the region around 3500 cm−1 (2.85 μm)

(1997). 9780471163947, Wiley.
a path length of about 25 μm is needed. Also, the cuvette must be both transparent around 3500 cm−1 and insoluble in water; is one material that is in common use for the cuvette windows with aqueous solutions.

The Raman-active fundamental vibrations may be observed with, for example, a 1  cm sample cell.

, , and other organisms can live in water up to hundreds of meters deep, because can reach them. Practically no sunlight reaches the parts of the oceans below of depth.

The of liquid water (1.333 at ) is much higher than that of air (1.0), similar to those of and , but lower than those of (1.473), (1.501), (1.627), and common types of glass (1.4 to 1.6). The refraction index of ice (1.31) is lower than that of liquid water.

Polar molecule
In a water molecule, the hydrogen atoms form a 104.5° angle with the oxygen atom. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. At the other two corners are of valence electrons that do not participate in the bonding. In a perfect tetrahedron, the atoms would form a 109.5° angle, but the repulsion between the lone pairs is greater than the repulsion between the hydrogen atoms. The O–H bond length is about 0.096nm.

Other substances have a tetrahedral molecular structure, for example, () and (). However, oxygen is more electronegative (holds on to its electrons more tightly) than most other elements, so the oxygen atom retains a negative charge while the hydrogen atoms are positively charged. Along with the bent structure, this gives the molecule an electrical dipole moment and it is classified as a .

Water is a good polar , that dissolves many salts and organic molecules such as sugars and simple alcohols such as . Water also dissolves many gases, such as oxygen and —the latter giving the fizz of beverages, and beers. In addition, many substances in living organisms, such as , and , are dissolved in water. The interactions between water and the subunits of these biomacromolecules shape , , and other phenomena crucial to life (hydrophobic effect).

Many organic substances (such as and ) are , that is, insoluble in water. Many inorganic substances are insoluble too, including most metal , , and .

Hydrogen bonding
Because of its polarity, a molecule of water in the liquid or solid state can form up to four with neighboring molecules. Hydrogen bonds are about ten times as strong as the Van der Waals force that attracts molecules to each other in most liquids. This is the reason why the melting and boiling points of water are much higher than those of other analogous compounds like hydrogen sulfide. They also explain its exceptionally high specific heat capacity (about 4.2 /g/K), heat of fusion (about 333 J/g), heat of vaporization (), and thermal conductivity (between 0.561 and 0.679 W/m/K). These properties make water more effective at moderating Earth's , by storing heat and transporting it between the oceans and the atmosphere. The hydrogen bonds of water are around 23 kJ/mol (compared to a covalent O-H bond at 492 kJ/mol). Of this, it is estimated that 90% is attributable to electrostatics, while the remaining 10% is partially covalent.

These bonds are the cause of water's high

(2021). 9780132508827, Pearson Prentice Hall. .
and capillary forces. The refers to the tendency of water to move up a narrow tube against the force of . This property is relied upon by all , such as trees. Capillary Action – Liquid, Water, Force, and Surface – JRank Articles . Science.jrank.org. Retrieved on 28 September 2015.

Self-ionisation
Water is a weak solution of hydronium hydroxide - there is an equilibrium ⇔ + , in combination with solvation of the resulting ions.

Electrical conductivity and electrolysis
Pure water has a low electrical conductivity, which increases with the dissolution of a small amount of ionic material such as .

Liquid water can be split into the hydrogen and oxygen by passing an electric current through it—a process called electrolysis. The decomposition requires more energy input than the heat released by the inverse process (285.8 kJ/mol, or 15.9 MJ/kg).

Mechanical properties
Liquid water can be assumed to be incompressible for most purposes: its compressibility ranges from 4.4 to in ordinary conditions. Even in oceans at 4 km depth, where the pressure is 400 atm, water suffers only a 1.8% decrease in volume.

The of water is about 10−3 Pa· or 0.01 poise at , and the speed of sound in liquid water ranges between depending on temperature. Sound travels long distances in water with little attenuation, especially at low frequencies (roughly 0.03 /km for 1 k), a property that is exploited by and humans for communication and environment sensing ().UK National Physical Laboratory, Calculation of absorption of sound in seawater . Online site, last accessed on 28 September 2016.

Reactivity
Metallic elements which are more electropositive than hydrogen, particularly the and alkaline earth metals such as , , , and displace hydrogen from water, forming and releasing hydrogen. At high temperatures, carbon reacts with steam to form and hydrogen.

On Earth
Hydrology is the study of the movement, distribution, and quality of water throughout the Earth. The study of the distribution of water is . The study of the distribution and movement of groundwater is , of glaciers is , of inland waters is and distribution of oceans is . Ecological processes with hydrology are in the focus of .

The collective mass of water found on, under, and over the surface of a planet is called the . Earth's approximate water volume (the total water supply of the world) is 1.386 × 109 cubic kilometers (3.33 × 108 cubic miles).

Liquid water is found in bodies of water, such as an ocean, sea, lake, river, stream, , pond, or . The majority of water on Earth is . Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as groundwater in .

Water is important in many geological processes. Groundwater is present in most rocks, and the pressure of this groundwater affects patterns of faulting. Water in the mantle is responsible for the melt that produces at . On the surface of the Earth, water is important in both chemical and physical processes. Water, and to a lesser but still significant extent, ice, are also responsible for a large amount of sediment transport that occurs on the surface of the earth. Deposition of transported sediment forms many types of , which make up the of Earth history.

Water cycle
The (known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the , between the , water, , , and plants.

Water moves perpetually through each of these regions in the water cycle consisting of the following transfer processes:

• from oceans and other water bodies into the air and from land plants and animals into the air.
• precipitation, from water vapor condensing from the air and falling to the earth or ocean.
• runoff from the land usually reaching the sea.
Most water vapors found mostly in the ocean returns to it, but winds carry water vapor over land at the same rate as runoff into the sea, about 47 Tt per year whilst evaporation and transpiration happening in land masses also contribute another 72 Tt per year. Precipitation, at a rate of 119 Tt per year over land, has several forms: most commonly rain, snow, and , with some contribution from and . Dew is small drops of water that are condensed when a high density of water vapor meets a cool surface. Dew usually forms in the morning when the temperature is the lowest, just before sunrise and when the temperature of the earth's surface starts to increase.
(2021). 9789814338967, World Scientific Publishing.
Condensed water in the air may also to produce .

Water runoff often collects over flowing into rivers. A mathematical model used to simulate river or stream flow and calculate water quality parameters is a hydrological transport model. Some water is diverted to for agriculture. Rivers and seas offer opportunities for travel and commerce. Through , runoff shapes the environment creating river and which provide rich soil and level ground for the establishment of population centers. A flood occurs when an area of land, usually low-lying, is covered with water which occurs when a river overflows its banks or a storm surge happens. On the other hand, drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation either due to its topography or due to its location in terms of .

Fresh water storage
Water occurs as both "stocks" and "flows." Water can be stored as lakes, water vapor, groundwater or "aquifers," and ice and snow. Of the total volume of global freshwater, an estimated 69 percent is stored in glaciers and permanent snow cover; 30 percent is in groundwater; and the remaining 1 percent in lakes, rivers, the atmosphere, and biota.
(1993). 9780195076271, Oxford University Press. .
The length of time water remains in storage is highly variable: some aquifers consist of water stored over thousands of years but lake volumes may fluctuate on a seasonal basis, decreasing during dry periods and increasing during wet ones. A substantial fraction of the water supply for some regions consists of water extracted from water stored in stocks, and when withdrawals exceed recharge, stocks decrease. By some estimates, as much as 30 percent of total water used for irrigation comes from unsustainable withdrawals of groundwater, causing groundwater depletion.

Sea water and tides
contains about 3.5% on average, plus smaller amounts of other substances. The physical properties of seawater differ from in some important respects. It freezes at a lower temperature (about ) and its density increases with decreasing temperature to the freezing point, instead of reaching maximum density at a temperature above freezing. The salinity of water in major seas varies from about 0.7% in the to 4.0% in the . (The , known for its ultra-high salinity levels of between 30–40%, is really a .)

are the cyclic rising and falling of local sea levels caused by the of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and water bodies and produce oscillating currents known as tidal streams. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the and the local . The strip of seashore that is submerged at high tide and exposed at low tide, the , is an important ecological product of ocean tides.

Effects on life
From a standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing to react in ways that ultimately allow . All known forms of life depend on water. Water is vital both as a in which many of the body's solutes dissolve and as an essential part of many processes within the body. Metabolism is the sum total of and . In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g., starches, triglycerides, and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g., glucose, fatty acids, and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.

Water is fundamental to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO2 (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO2 in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H+, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as a hydroxide ion (OH) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. have pH values less than 7 while bases have values greater than 7.

Aquatic life forms
Earth surface waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as , spend portions of their lives in water and portions on land. Plants such as and grow in the water and are the basis for some underwater ecosystems. is generally the foundation of the ocean .

Aquatic vertebrates must obtain oxygen to survive, and they do so in various ways. Fish have instead of , although some species of fish, such as the , have both. , such as dolphins, whales, , and need to surface periodically to breathe air. Some amphibians are able to absorb oxygen through their skin. Invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see insect and mollusc siphons) and ( ). However, as invertebrate life evolved in an aquatic habitat most have little or no specialization for respiration in water.

Effects on human civilization
Civilization has historically flourished around rivers and major waterways; , the so-called cradle of civilization, was situated between the major rivers and ; the ancient society of the depended entirely upon the . was also founded on the banks of the Italian river . Large like , , , , New York City, , , , , and owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like , have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.

Health and pollution
Water fit for human consumption is called or potable water. Water that is not potable may be made potable by filtration or , or by a range of . More than 660 million people do not have access to safe drinking water.

Water that is not fit for drinking but is not harmful to humans when used for swimming or bathing is called by various names other than potable or drinking water, and is sometimes called , or "safe for bathing". Chlorine is a skin and mucous membrane irritant that is used to make water safe for bathing or drinking. Its use is highly technical and is usually monitored by government regulations (typically 1 part per million (ppm) for drinking water, and 1–2 ppm of chlorine not yet reacted with impurities for bathing water). Water for bathing may be maintained in satisfactory microbiological condition using chemical disinfectants such as or or by the use of light.

In the US, non-potable forms of generated by humans may be referred to as , which is treatable and thus easily able to be made potable again, and blackwater, which generally contains and other forms of waste which require in order to be made reusable. Greywater composes 50–80% of residential wastewater generated by a household's sanitation equipment (, showers, and kitchen runoff, but not toilets, which generate blackwater.) These terms may have different meanings in other countries and cultures.

This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about a billion people around the world routinely drink unhealthy water. In 2000, the established the Millennium Development Goals for water to halve by 2015 the proportion of people worldwide without access to safe water and . Progress toward that goal was uneven, and in 2015 the UN committed to the following targets set by the Sustainable Development Goals of achieving universal access to safe and affordable water and sanitation by 2030. Poor and bad sanitation are deadly; some five million deaths a year are caused by water-related diseases. The World Health Organization estimates that could prevent 1.4 million child deaths from each year.

Water is not an infinite resource (meaning the availability of water is limited), but rather re-circulated as potable water in precipitation.

(2013). 9781136457043, Routledge. .

In the developing world, 90% of all still goes untreated into local rivers and streams.

(2021). 9781843390084, IWA Publishing.
Some 50 countries, with roughly a third of the world's population, also suffer from medium or high water stress and 17 of these extract more water annually than is recharged through their natural water cycles.
(2021). 9781402001048, Springer.
The strain not only affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

Human uses

Agriculture
The most important use of water in agriculture is for , which is a key component to produce enough food. Irrigation takes up to 90% of water withdrawn in some developing countries and significant proportions in more economically developed countries (in the United States, 42% of freshwater withdrawn for use is for irrigation).

Fifty years ago, the common perception was that water was an infinite resource. At the time, there was fewer than half the current number of people on the planet. People were not as wealthy as today, consumed fewer calories, and ate less meat, so less water was needed to produce their food. They required a third of the volume of water we presently take from rivers. Today, the competition for the fixed amount of water resources is much more intense, giving rise to the concept of . This is because there are now nearly eight billion people on the planet, their consumption of water-thirsty meat and vegetables is rising, and there is increasing competition for water from industry, urbanization and biofuel crops. In the future, even more quantities of water shall be needed to produce food because the Earth's population is forecast to rise to 9 billion by 2050.United Nations Press Release POP/952 (13 March 2007). World population will increase by 2.5 billion by 2050

An assessment of water management in agriculture was conducted in 2007 by the International Water Management Institute in Sri Lanka to see if the world had sufficient water to provide food for its growing population., Molden, D. (Ed). Water for food, Water for life: A Comprehensive Assessment of Water Management in Agriculture. Earthscan/IWMI, 2007. It assessed the current availability of water for agriculture on a global scale and mapped out locations suffering from water scarcity. It found that a fifth of the world's people, more than 1.2 billion, live in areas of physical water scarcity, where there is not enough water to meet all demands. A further 1.6 billion people live in areas experiencing economic water scarcity, where the lack of investment in water or insufficient human capacity make it impossible for authorities to satisfy the demand for water. The report found that it would be possible to produce the food required in the future, but that continuation of today's food production and environmental trends would lead to crises in many parts of the world. To avoid a global water crisis, farmers will have to strive to increase productivity to meet growing demands for food, while industries and cities find ways to use water more efficiently.Chartres, C. and Varma, S. (2010) Out of water. From Abundance to Scarcity and How to Solve the World's Water Problems. FT Press (US).

Water scarcity is also caused by production of : 1 kg of cotton—equivalent of a pair of jeans—requires water to produce. While cotton accounts for 2.4% of world water use, the water is consumed in regions which are already at a risk of water shortage. Significant environmental damage has been caused, such as disappearance of the .

File:Water requirement per tonne of food product, OWID.svg|Water requirement per tonne of food product File:Subsurface drip emission on loamy soil.ogv|Water distribution in subsurface File:SiphonTubes.JPG| of field crops

As a scientific standard
On 7 April 1795, the gram was defined in France to be equal to "the absolute weight of a volume of pure water equal to a cube of one-hundredth of a meter, and at the temperature of melting ice". Décret relatif aux poids et aux mesures. 18 germinal an 3 (7 April 1795) . Decree relating to the weights and measurements (in French). quartier-rural.org For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely the mass of one liter of water. In spite of the fact that the decreed definition of the gram specified water at —a highly reproducible temperature—the scientists chose to redefine the standard and to perform their measurements at the temperature of highest water density, which was measured at the time as . here L'Histoire Du Mètre, La Détermination De L'Unité De Poids . histoire.du.metre.free.fr

The Kelvin temperature scale of the SI system was based on the of water, defined as exactly , but as of May 2019 is based on the Boltzmann constant instead. The scale is an absolute temperature scale with the same increment as the Celsius temperature scale, which was originally defined according to the (set to ) and (set to ) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also a small quantity of heavier isotopes oxygen-18, oxygen-17, and hydrogen-2 (). The percentage of the heavier isotopes is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less heavy isotopes than seawater. Therefore, standard water is defined in the Vienna Standard Mean Ocean Water specification.

For drinking
The contains from 55% to 78% water, depending on body size. Re: What percentage of the human body is composed of water? Jeffrey Utz, M.D., The MadSci Network To function properly, the body requires between of water per day to avoid ; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though the British Dietetic Association advises that 2.5 liters of total water daily is the minimum to maintain proper hydration, including 1.8 liters (6 to 7 glasses) obtained directly from beverages. Medical literature favors a lower consumption, typically 1 liter of water for an average male, excluding extra requirements due to fluid loss from exercise or warm weather.
(2021). 9780781719360, Lippincott Williams & Wilkins. .

Healthy kidneys can excrete 0.8 to 1 liter of water per hour, but stress such as exercise can reduce this amount. People can drink far more water than necessary while exercising, putting them at risk of water intoxication (hyperhydration), which can be fatal. The popular claim that "a person should consume eight glasses of water per day" seems to have no real basis in science. Studies have shown that extra water intake, especially up to at mealtime was associated with weight loss. Adequate fluid intake is helpful in preventing constipation. Water, Constipation, Dehydration, and Other Fluids . Sciencedaily.com. Retrieved on 28 September 2015.

An original recommendation for water intake in 1945 by the Food and Nutrition Board of the United States National Research Council read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." The latest dietary reference intake report by the United States National Research Council in general recommended, based on the median total water intake from US survey data (including food sources): for men and of water total for women, noting that water contained in food provided approximately 19% of total water intake in the survey.

(2021). 9780309091695 .

Specifically, pregnant and women need additional fluids to stay hydrated. The Institute of Medicine (US) recommends that, on average, men consume and women ; pregnant women should increase intake to and breastfeeding women should get 3 liters (12 cups), since an especially large amount of fluid is lost during nursing. Also noted is that normally, about 20% of water intake comes from food, while the rest comes from drinking water and beverages ( included). Water is excreted from the body in multiple forms; through and , through , and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well.

Humans require water with few impurities. Common impurities include metal salts and oxides, including copper, iron, calcium and lead, Conquering Chemistry 4th Ed. Published 2008 and/or harmful bacteria, such as . Some are acceptable and even desirable for taste enhancement and to provide needed .

(1993). 9780139811760, Prentice Hall. .

The single largest (by volume) freshwater resource suitable for drinking is in Siberia.

(2021). 9781845451776, Berghahn Books. .

Washing
The propensity of water to form and is useful in various processes. Washing is also an important component of several aspects of personal . Most of the personal water use is due to , doing the and , reaching hundreds of liters per day per person in developed countries.

Transportation
The use of water for transportation of materials through rivers and canals as well as the international shipping lanes is an important part of the world economy.

Chemical uses
Water is widely used in chemical reactions as a or and less commonly as a or . In inorganic reactions, water is a common solvent, dissolving many ionic compounds, as well as other polar compounds such as and compounds closely related to water. In organic reactions, it is not usually used as a reaction solvent, because it does not dissolve the reactants well and is (acidic and basic) and . Nevertheless, these properties are sometimes desirable. Also, acceleration of Diels-Alder reactions by water has been observed. Supercritical water has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently. Water vapor is used for some processes in the chemical industry. An example is the production of acrylic acid from acrolein, propylene and propane. The possible effect of water in these reactions includes the physical-, chemical interaction of water with the catalyst and the chemical reaction of water with the reaction intermediates.

Heat exchange
Water and steam are a common fluid used for , due to its availability and high heat capacity, both for cooling and heating. Cool water may even be naturally available from a lake or the sea. It's especially effective to transport heat through and of water because of its large latent heat of vaporization. A disadvantage is that metals commonly found in industries such as steel and copper are faster by untreated water and steam. In almost all thermal power stations, water is used as the working fluid (used in a closed-loop between boiler, steam turbine, and condenser), and the coolant (used to exchange the waste heat to a water body or carry it away by in a ). In the United States, cooling power plants is the largest use of water. Water Use in the United States, National Atlas.gov

In the industry, water can also be used as a neutron moderator. In most , water is both a coolant and a moderator. This provides something of a passive safety measure, as removing the water from the reactor also . However other methods are favored for stopping a reaction and it is preferred to keep the nuclear core covered with water so as to ensure adequate cooling.

Fire considerations
Water has a high heat of vaporization and is relatively inert, which makes it a good fire extinguishing fluid. The evaporation of water carries heat away from the fire. It is dangerous to use water on fires involving oils and organic solvents because many organic materials float on water and the water tends to spread the burning liquid.

Use of water in fire fighting should also take into account the hazards of a , which may occur when water is used on very hot fires in confined spaces, and of a hydrogen explosion, when substances which react with water, such as certain metals or hot carbon such as coal, , or coke graphite, decompose the water, producing .

The power of such explosions was seen in the Chernobyl disaster, although the water involved did not come from fire-fighting at that time the reactor's own water cooling system. A steam explosion occurred when the extreme overheating of the core caused water to flash into steam. A hydrogen explosion may have occurred as a result of a reaction between steam and hot .

Some metallic oxides, most notably those of and alkaline earth metals, produce so much heat on reaction with water that a fire hazard can develop. The alkaline earth oxide is a mass-produced substance that is often transported in paper bags. If these are soaked through, they may ignite as their contents react with water.

Recreation
Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, , , and diving. In addition, some sports, like and , are played on ice. Lakesides, beaches and are popular places for people to go to relax and enjoy recreation. Many find the sound and appearance of flowing water to be calming, and fountains and other water features are popular decorations. Some keep fish and other flora and fauna inside or ponds for show, fun, and companionship. Humans also use water for snow sports i.e. , , or , which require the water to be frozen.

Water industry
The provides drinking water and services (including ) to households and industry. facilities include , for rainwater harvesting, water supply networks, and water purification facilities, , , including old aqueducts. Atmospheric water generators are in development.

Drinking water is often collected at springs, extracted from artificial borings (wells) in the ground, or pumped from lakes and rivers. Building more wells in adequate places is thus a possible way to produce more water, assuming the aquifers can supply an adequate flow. Other water sources include rainwater collection. Water may require purification for human consumption. This may involve the removal of undissolved substances, dissolved substances and harmful . Popular methods are filtering with sand which only removes undissolved material, while chlorination and kill harmful microbes. does all three functions. More advanced techniques exist, such as . of abundant is a more expensive solution used in coastal .

The distribution of drinking water is done through municipal water systems, tanker delivery or as . Governments in many countries have programs to distribute water to the needy at no charge.

Reducing usage by using drinking (potable) water only for human consumption is another option. In some cities such as Hong Kong, seawater is extensively used for flushing toilets citywide in order to conserve freshwater resources.

may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the polluter. Like other types of pollution, this does not enter standard accounting of market costs, being conceived as for which the market cannot account. Thus other people pay the price of water pollution, while the private firms' profits are not redistributed to the local population, victims of this pollution. consumed by humans often end up in the waterways and can have detrimental effects on life if they and if they are not .

Municipal and industrial wastewater are typically treated at wastewater treatment plants. Mitigation of polluted is addressed through a variety of prevention and treatment techniques. ( See Surface runoff#Mitigation and treatment.)

Industrial applications
Many industrial processes rely on reactions using chemicals dissolved in water, suspension of solids in water or using water to dissolve and extract substances, or to wash products or process equipment. Processes such as , , , paper manufacturing, textile production, dyeing, printing, and cooling of power plants use large amounts of water, requiring a dedicated water source, and often cause significant water pollution.

Water is used in . is electricity obtained from . Hydroelectric power comes from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable energy source. The energy is supplied by the motion of water. Typically a dam is constructed on a river, creating an artificial lake behind it. Water flowing out of the lake is forced through turbines that turn generators.

Pressurized water is used in and water jet cutters. Also, high pressure water guns are used for precise cutting. It works very well, is relatively safe, and is not harmful to the environment. It is also used in the cooling of machinery to prevent overheating, or prevent saw blades from overheating.

Water is also used in many industrial processes and machines, such as the and , in addition to its use as a chemical . Discharge of untreated water from industrial uses is . Pollution includes discharged solutes (chemical pollution) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and utilizes a variety of purification techniques both in water supply and discharge.

Food processing
, , and are popular cooking methods that often require immersing food in water or its gaseous state, steam. Water is also used for . Water also plays many critical roles within the field of .

such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water are affected by solutes, as well as , which is in turn affected by altitude. Water boils at lower temperatures with the lower air pressure that occurs at higher elevations. One mole of sucrose (sugar) per kilogram of water raises the boiling point of water by , and one mole of salt per kg raises the boiling point by ; similarly, increasing the number of dissolved particles lowers water's freezing point.

(2021). 9780387699394, Springer. .

Solutes in water also affect water activity that affects many chemical reactions and the growth of microbes in food.

(1999). 9780834212343, Springer. .
Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water. Solutes in water lower water activity—this is important to know because most bacterial growth ceases at low levels of water activity. Not only does microbial growth affect the safety of food, but also the preservation and shelf life of food.

is also a critical factor in food processing and may be altered or treated by using a chemical ion exchange system. It can dramatically affect the quality of a product, as well as playing a role in sanitation. Water hardness is classified based on concentration of calcium carbonate the water contains. Water is classified as soft if it contains less than 100 mg/l (UK) or less than 60 mg/l (US).

According to a report published by the Water Footprint organization in 2010, a single kilogram of beef requires of water; however, the authors also make clear that this is a global average and circumstantial factors determine the amount of water used in beef production.

Medical use
Water for injection is on the World Health Organization's list of essential medicines.

Distribution in nature

In the universe
Much of the universe's water is produced as a byproduct of . The formation of stars is accompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water observed is quickly produced in this warm dense gas.Melnick, Gary, Harvard-Smithsonian Center for Astrophysics and Neufeld, David, Johns Hopkins University quoted in:

On 22 July 2011, a report described the discovery of a gigantic cloud of water vapor containing "140 trillion times more water than all of Earth's oceans combined" around a located 12 billion light years from Earth. According to the researchers, the "discovery shows that water has been prevalent in the universe for nearly its entire existence".

Water has been detected in interstellar clouds within our , the .

(2009). 9780061854484, Zondervan. .
Water probably exists in abundance in other galaxies, too, because its components, hydrogen, and oxygen, are among the most abundant elements in the universe. Based on models of the formation and evolution of the Solar System and that of other star systems, most other are likely to have similar ingredients.

Water vapor
Water is present as vapor in:

Liquid water
Liquid water is present on Earth, covering 71% of its surface. Liquid water is also occasionally present in small amounts on Mars. Scientists believe liquid water is present in the Saturnian moons of Enceladus, as a 10-kilometre thick ocean approximately 30–40 kilometres below Enceladus' south polar surface, and Titan, as a subsurface layer, possibly mixed with . Jupiter's moon Europa has surface characteristics which suggest a subsurface liquid water ocean. Liquid water may also exist on Jupiter's moon Ganymede as a layer sandwiched between high pressure ice and rock.Dunham, Will. (3 May 2014) Jupiter's moon Ganymede may have 'club sandwich' layers of ocean | Reuters . In.reuters.com. Retrieved on 28 September 2015.

Water ice
Water is present as ice on:

And is also likely present on:

Exotic forms
Water and other probably comprise much of the internal structures of and and the water in the deeper layers may be in the form of in which the molecules break down into a soup of hydrogen and oxygen ions, and deeper still as in which the oxygen crystallises but the hydrogen ions float about freely within the oxygen lattice. Weird water lurking inside giant planets , New Scientist, 1 September 2010, Magazine issue 2776.

Water and habitable zone
The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of as we know it. The Earth is located in the of the ; if it were slightly closer to or farther from the (about 5%, or about 8 million kilometers), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.

Earth's allows it to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a temperature buffer (greenhouse effect) which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would allow temperature extremes, thus preventing the accumulation of water except in polar ice caps (as on ).

The surface temperature of Earth has been relatively constant through despite varying levels of incoming solar radiation (), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric . This proposal is known as the .

The state of water on a planet depends on ambient pressure, which is determined by the planet's gravity. If a planet is sufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused by gravity, as it was observed on exoplanets Gliese 436 b and GJ 1214 b.

Law, politics, and crisis
is politics affected by water and . For this reason, water is a strategic resource in the globe and an important element in many political conflicts. It causes health impacts and damage to biodiversity.

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 . However, some observers have estimated that by 2025 more than half of the will 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%.

1.6 billion people have gained access to a safe water source since 1990. The Millennium Development Goals Report , United Nations, 2008 The proportion of people in developing countries with access to safe water is calculated to have improved from 30% in 1970

(2021). 9780521010689, Cambridge University Press. .
to 71% in 1990, 79% in 2000 and 84% in 2004.

A 2006 United Nations report stated that "there is enough water for everyone", but that access to it is hampered by mismanagement and corruption., (2006), Water, a shared responsibility. The United Nations World Water Development Report 2 In addition, global initiatives to improve the efficiency of aid delivery, such as the Paris Declaration on Aid Effectiveness, have not been taken up by water sector donors as effectively as they have in education and health, potentially leaving multiple donors working on overlapping projects and recipient governments without empowerment to act.Welle, Katharina; Evans, Barbara; Tucker, Josephine, and Nicol, Alan (2008) Is water lagging behind on Aid Effectiveness?

The authors of the 2007 Comprehensive Assessment of Water Management in Agriculture cited poor governance as one reason for some forms of water scarcity. Water governance is the set of formal and informal processes through which decisions related to water management are made. Good water governance is primarily about knowing what processes work best in a particular physical and socioeconomic context. Mistakes have sometimes been made by trying to apply 'blueprints' that work in the developed world to developing world locations and contexts. The Mekong river is one example; a review by the International Water Management Institute of policies in six countries that rely on the Mekong river for water found that thorough and transparent cost-benefit analyses and environmental impact assessments were rarely undertaken. They also discovered that Cambodia's draft water law was much more complex than it needed to be.

The UN World Water Development Report (WWDR, 2003) from the World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%. 40% of the world's inhabitants currently have insufficient fresh water for minimal . More than 2.2 million people died in 2000 from waterborne diseases (related to the consumption of contaminated water) or drought. In 2004, the UK charity reported that a child dies every 15 seconds from easily preventable water-related diseases; often this means lack of disposal.

Organizations concerned with water protection include the International Water Association (IWA), , Water 1st, and the American Water Resources Association. The International Water Management Institute undertakes projects with the aim of using effective water management to reduce poverty. Water related conventions are United Nations Convention to Combat Desertification (UNCCD), International Convention for the Prevention of Pollution from Ships, United Nations Convention on the Law of the Sea and Ramsar Convention. World Day for Water takes place on 22 March and World Oceans Day on 8 June.

In culture

Religion
Water is considered a purifier in most religions. Faiths that incorporate ritual washing (ablution) include , , , , the Rastafari movement, , , and . Immersion (or or ) of a person in water is a central of Christianity (where it is called ); it is also a part of the practice of other religions, including Islam ( ), Judaism ( ) and ( ). In addition, a ritual bath in pure water is performed for the dead in many religions including Islam and Judaism. In Islam, the five daily prayers can be done in most cases after completing washing certain parts of the body using clean water ( ), unless water is unavailable (see ). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of ).

In Christianity, is water that has been sanctified by a priest for the purpose of , the blessing of persons, places, and objects, or as a means of repelling evil. Chambers's encyclopædia, Lippincott & Co (1870). p. 394.Altman, Nathaniel (2002) Sacred water: the spiritual source of life. pp. 130–133. .

In , water ( ) is respected as the source of life.

Philosophy
The Ancient Greek philosopher held that water is one of the four classical elements along with fire, earth and air, and was regarded as the , or basic substance of the universe. , who was portrayed by Aristotle as an astronomer and an engineer, theorized that the earth, which is denser than water, emerged from the water. Thales, a , believed further that all things are made from water. Plato believed the shape of water is an which accounts for why it is able to flow easily compared to the cube-shaped earth.Lindberg, D. (2008). The beginnings of western science: The European scientific tradition in a philosophical, religious, and institutional context, prehistory to A.D. 1450. (2nd ed.). Chicago: University of Chicago Press.

In the theory of the , water was associated with , as being cold and moist. The classical element of water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

Water is also taken as a role model in some parts of traditional and popular . 's 1891 translation of the Dao De Jing states, "The highest excellence is like (that of) water. The excellence of water appears in its benefiting all things, and in its occupying, without striving (to the contrary), the low place which all men dislike. Hence (its way) is near to (that of) the " and "There is nothing in the world more soft and weak than water, and yet for attacking things that are firm and strong there is nothing that can take precedence of it—for there is nothing (so effectual) for which it can be changed." Guanzi in the "Shui di" 水地 chapter further elaborates on the symbolism of water, proclaiming that "man is water" and attributing natural qualities of the people of different Chinese regions to the character of local water resources. Guanzi : Shui Di – Chinese Text Project . Ctext.org. Retrieved on 28 September 2015.

Art and activism
Painter and activist Fredericka Foster curated The Value of Water, at the Cathedral of St. John the Divine in New York City, which anchored a year long initiative by the Cathedral on our dependence on water. The largest exhibition to ever appear at the Cathedral, it featured over forty artists, including , , , William Kentridge, , , , William Kentridge, Alice Dalton Brown, Teresita Fernandez and . The Think About Water water advocacy website was created by Foster to highlight artists who use water as their subject or medium.

Dihydrogen monoxide parody
Water's technically correct but rarely used , "dihydrogen monoxide", has been used in a series of and that mock scientific illiteracy. This began in 1983, when an April Fools' Day article appeared in a newspaper in Durand, Michigan. The false story consisted of safety concerns about the substance.

• Outline of water
• Water (data page) is a collection of the chemical and physical properties of water.
• (fear of water)
• Oral rehydration therapy
• Water pinch analysis

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