# Water

Water
Names
IUPAC name
Water
Systematic IUPAC name
Oxidane
Identifiers
ECHA InfoCard
100.028.902
Properties
H2O
Molar mass 18.015 g·mol−1
Appearance Nearly colourless liquid or nearly colourless solid (blue if looked through thick layers of water) or colourless gas
Density Liquid:
0.9998396 g/mL at 0 °C
0.999972 at ~4 °C (Max.)
0.9970474 g/mL at 25 °C
Solid:
0.9167 g/mL at 0 °C
Melting point 0.00 °C (32.00 °F; 273.15 K)
Boiling point 99.98 °C (211.96 °F; 373.13 K)
Supplementary data page
Water (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Earth's atmosphere condense from gaseous water vapor
.

Water is an inorganic compound with the chemical formula H2O. It is a transparent, tasteless, odorless, and nearly colorless chemical substance, and it is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent[1]). It is vital for all known forms of life, despite not providing food, energy or organic micronutrients. Its chemical formula, H2O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°.[2] "Water" is also the name of the liquid state of H2O at standard temperature and pressure.

Because Earth's environment is relatively close to water's triple point, water exists on earth as a solid, liquid, and gas.[3] It forms precipitation in the form of rain and aerosols in the form of fog. Clouds consist of suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor.

Water covers about 71% of the Earth's surface, with seas and oceans making up most of the water volume on earth (about 96.5%).

ice caps of Antarctica and Greenland (1.7%), and in the air as vapor, clouds (consisting of ice and liquid water suspended in air), and precipitation (0.001%).[5][6] Water moves continually through the water cycle of evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff
, usually reaching the sea.

Water plays an important role in the

humans goes to agriculture.[7] Fishing in salt and fresh water bodies has been, and continues to be, a major source of food for many parts of the world, providing 6.5% of global protein.[8] Much of the long-distance trade of commodities (such as oil, 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 substances both mineral and organic; as such it is widely used in industrial processes, and in cooking and washing. Water, ice and snow are also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, diving, ice skating
and skiing.

## Etymology

The word water comes from Old English wæter, from Proto-Germanic *watar (source also of Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, vatn, Gothic 𐍅𐌰𐍄𐍉 (wato), from Proto-Indo-European *wod-or, suffixed form of root *wed- ('water'; 'wet').[9] Also cognate, through the Indo-European root, with Greek ύδωρ (ýdor; from Ancient Greek ὕδωρ (hýdōr), whence English 'hydro-'), Russian вода́ (vodá), Irish uisce, and Albanian ujë.

## Properties

Water (

pure water. Water is the only common substance to exist as a solid, liquid, and gas in normal terrestrial conditions.[13]

### States

Along with oxidane, water is one of the two official names for the chemical compound H
2
O
;

states of matter of water are the solid phase, ice, and the gaseous phase, water vapor or steam. The addition or removal of heat can cause phase transitions: freezing (water to ice), melting (ice to water), vaporization (water to vapor), condensation (vapor to water), sublimation (ice to vapor) and deposition (vapor to ice).[16]

#### Density

Water differs from most liquids in that it becomes less dense as it freezes.[a] In 1 atm pressure, it reaches its maximum density of 999.972 kg/m3 (62.4262 lb/cu ft) at 3.98 °C (39.16 °F), or almost 1,000 kg/m3 (62.43 lb/cu ft) at almost 4 °C (39 °F).[18][19] The density of ice is 917 kg/m3 (57.25 lb/cu ft), an expansion of 9%.[20][21] This expansion can exert enormous pressure, bursting pipes and cracking rocks.[22]

In a lake or ocean, water at 4 °C (39 °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 residing in lakes would perish during the winter.[23]

#### Magnetism

Water is a diamagnetic material.[24] Though interaction is weak, with superconducting magnets it can attain a notable interaction.[24]

#### Phase transitions

At a pressure of one

freeze-drying, a food is frozen and then stored at low pressure so the ice on its surface sublimates.[26]

${\displaystyle {\frac {dT}{dP}}={\frac {T\left(v_{\text{L}}-v_{\text{S}}\right)}{L_{\text{f}}}},}$

where ${\displaystyle v_{\text{L}}}$ and ${\displaystyle v_{\text{S}}}$ are the molar volumes of the liquid and solid phases, and ${\displaystyle L_{\text{f}}}$ is the molar latent heat 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.[17] In glaciers, pressure melting can occur under sufficiently thick volumes of ice, resulting in subglacial lakes.[27][28]

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.[29] Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed 205 °C (401 °F) in Old Faithful, a geyser in Yellowstone National Park.[30] In hydrothermal vents, the temperature can exceed 400 °C (752 °F).[31]

At

pressure cooker can be used to decrease cooking times by raising the boiling temperature.[33]
) In a vacuum, water will boil at room temperature.[34]

#### Triple and critical points

On a pressure/temperature phase diagram (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the triple point, where all three phases can coexist. The triple point is at a temperature of 273.16 K (0.01 °C; 32.02 °F) and a pressure of 611.657 pascals (0.00604 atm; 0.0887 psi);[35] it is the lowest pressure at which liquid water can exist. Until 2019, the triple point was used to define the Kelvin temperature scale.[36][37]

The water/vapor phase curve terminates at 647.096 K (373.946 °C; 705.103 °F) and 22.064 megapascals (3,200.1 psi; 217.75 atm).[38] 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 mix freely with nonpolar compounds, including most organic compounds. This makes it useful in a variety of applications including high-temperature electrochemistry and as an ecologically benign solvent or catalyst in chemical reactions involving organic compounds. In Earth's mantle, it acts as a solvent during mineral formation, dissolution and deposition.[39][40]

#### Phases of ice and water

The normal form of ice on the surface of Earth is Ice Ih, a phase that forms crystals with hexagonal symmetry. Another with cubic crystalline symmetry, Ice Ic, can occur in the upper atmosphere.[41] As the pressure increases, ice forms other crystal structures. As of 2019, 17 have been experimentally confirmed and several more are predicted theoretically (see Ice).[42] The 18th form of ice, ice XVIII, a face-centred-cubic, superionic ice phase, was discovered when a droplet of water was subject to a shock wave that raised the water's pressure to millions of atmospheres and its temperature to thousands of degrees, resulting in a structure of rigid oxygen atoms in which hydrogen atoms flowed freely.[43][44] When sandwiched between layers of graphene, ice forms a square lattice.[45]

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.[19][46][47][48]

### Taste and odor

Pure water is usually described as tasteless and odorless, although

putrid.[52]

### Color and appearance

Pure water is

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 cuvettes 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 3,500 cm−1 (2.85 μm)[54] a path length of about 25 μm is needed. Also, the cuvette must be both transparent around 3500 cm−1 and insoluble in water; calcium fluoride 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.

Aquatic plants, algae, and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Practically no sunlight reaches the parts of the oceans below 1,000 meters (3,300 ft) of depth.

The refractive index of liquid water (1.333 at 20 °C (68 °F)) is much higher than that of air (1.0), similar to those of alkanes and ethanol, but lower than those of glycerol (1.473), benzene (1.501), carbon disulfide (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

lone pairs 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.[55][56] The O–H bond length is about 0.096 nm.[57]

Other substances have a tetrahedral molecular structure, for example,

Water is a good polar

DNA base pairing, and other phenomena crucial to life (hydrophobic effect
).

Many organic substances (such as

hydrophobic, that is, insoluble in water. Many inorganic substances are insoluble too, including most metal oxides, sulfides, and silicates
.

### Hydrogen bonding

Because of its polarity, a molecule of water in the liquid or solid state can form up to four

heat of vaporization (2257 J/g), and thermal conductivity (between 0.561 and 0.679 W/m/K). These properties make water more effective at moderating Earth's climate, 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.[59]

These bonds are the cause of water's high surface tension[60] and capillary forces. The capillary action refers to the tendency of water to move up a narrow tube against the force of gravity. This property is relied upon by all vascular plants, such as trees.[citation needed]

Specific heat capacity of water[61]

### Self-ionization

Water is a weak solution of hydronium hydroxide – there is an equilibrium 2H
2
O
H
3
O+
+ OH
, in combination with solvation of the resulting hydronium ions.

### Electrical conductivity and electrolysis

Pure water has a low

dissolution of a small amount of ionic material such as common salt
.

Liquid water can be split into the elements 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).[62]

### Mechanical properties

Liquid water can be assumed to be incompressible for most purposes: its compressibility ranges from 4.4 to 5.1×10−10 Pa−1 in ordinary conditions.[63] Even in oceans at 4 km depth, where the pressure is 400 atm, water suffers only a 1.8% decrease in volume.[64]

The

cetaceans and humans for communication and environment sensing (sonar).[65]

### Reactivity

Metallic elements which are more

alkaline earth metals such as lithium, sodium, calcium, potassium and cesium displace hydrogen from water, forming hydroxides and releasing hydrogen. At high temperatures, carbon reacts with steam to form carbon monoxide
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 hydrography. The study of the distribution and movement of groundwater is hydrogeology, of glaciers is glaciology, of inland waters is limnology and distribution of oceans is oceanography. Ecological processes with hydrology are in the focus of ecohydrology.

The collective mass of water found on, under, and over the surface of a planet is called the hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1.386 billion cubic kilometres (333 million cubic miles).[5]

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

Water is important in many geological processes. Groundwater is present in most

Earth history
.

### Water cycle

The water cycle (known scientifically as the hydrologic cycle) is the continuous exchange of water within the

, groundwater, and plants.

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

• evaporation from oceans and other water bodies into the air and transpiration 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

refract sunlight to produce rainbows
.

Water runoff often collects over watersheds flowing into rivers. Through erosion, runoff shapes the environment creating river valleys and deltas 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 latitude.

### Water resources

Water resources are natural resources of water that are potentially useful for humans,[68] for example as a source of drinking water supply or irrigation water. 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.[69] 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.[70]

### Seawater and tides

Seawater contains about 3.5% sodium chloride on average, plus smaller amounts of other substances. The physical properties of seawater differ from fresh water in some important respects. It freezes at a lower temperature (about −1.9 °C (28.6 °F)) 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 Baltic Sea to 4.0% in the Red Sea. (The Dead Sea, known for its ultra-high salinity levels of between 30 and 40%, is really a salt lake.)

effects of Earth rotation and the local bathymetry. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone
, is an important ecological product of ocean tides.

## Effects on life

Overview of photosynthesis (green) and respiration
(red)

From a biological standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. 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.[71] Hydrogen is combined with CO
2
(absorbed from air or water) to form glucose and release oxygen.[citation needed] All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO
2
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

Acids 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 amphibians, spend portions of their lives in water and portions on land. Plants such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton is generally the foundation of the ocean food chain.

Aquatic vertebrates must obtain oxygen to survive, and they do so in various ways. Fish have

gills (Carcinus
). However, as invertebrate life evolved in an aquatic habitat most have little or no specialization for respiration in water.

## Effects on human civilization

Water fountain

Civilization has historically flourished around rivers and major waterways;

Indus Valley civilization (c. 3300 BCE to 1300 BCE) developed along the Indus River and tributaries that flowed out of the Himalayas. Rome was also founded on the banks of the Italian river Tiber. Large metropolises like Rotterdam, London, Montreal, Paris, New York City, Buenos Aires, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore
, 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

An environmental science program – a student from Iowa State University
sampling water

Water fit for human consumption is called drinking water or potable water. Water that is not potable may be made potable by filtration or distillation, or by a range of other methods. More than 660 million people do not have access to safe drinking water.[72][73]

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

safe water, 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 chlorine or ozone or by the use of ultraviolet
light.

Water reclamation is the process of converting wastewater (most commonly sewage, also called municipal wastewater) into water that can be reused for other purposes. There are 2.3 billion people who reside in nations with water scarcities, which means that each individual receives less than 1,700 cubic metres (60,000 cu ft) of water annually. 380 billion cubic metres (13×1012 cu ft) of municipal wastewater are produced globally each year.[74][75][76]

Freshwater is a renewable resource, recirculated by the natural

diarrhoea each year.[77]

In developing countries, 90% of all municipal wastewater still goes untreated into local rivers and streams.[78] Some 50 countries, with roughly a third of the world's population, also suffer from medium or high water scarcity and 17 of these extract more water annually than is recharged through their natural water cycles.[79] The strain not only affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

### Human uses

Total water withdrawals for agricultural, industrial and municipal purposes per capita, measured in cubic metres (m3) per year in 2010[80]

#### Agriculture

The most substantial human use of water is for agriculture, including irrigated agriculture, which accounts for as much as 80 to 90 percent of total human water consumption.[81] In the United States, 42% of freshwater withdrawn for use is for irrigation, but the vast majority of water "consumed" (used and not returned to the environment) goes to agriculture.[82]

Access to fresh water is often taken for granted, especially in developed countries that have built sophisticated water systems for collecting, purifying, and delivering water, and removing wastewater. But growing economic, demographic, and climatic pressures are increasing concerns about water issues, leading to increasing competition for fixed water resources, giving rise to the concept of peak water.[83] As populations and economies continue to grow, consumption of water-thirsty meat expands, and new demands rise for biofuels or new water-intensive industries, new water challenges are likely.[84]

An assessment of water management in agriculture was conducted in 2007 by the

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.[86]

Water scarcity is also caused by production of water intensive products. For example, cotton: 1 kg of cotton—equivalent of a pair of jeans—requires 10.9 cubic meters (380 cu ft) water to produce. While cotton accounts for 2.4% of world water use, the water is consumed in regions that are already at a risk of water shortage. Significant environmental damage has been caused: for example, the diversion of water by the former Soviet Union from the Amu Darya and Syr Darya rivers to produce cotton was largely responsible for the disappearance of the Aral Sea.[87]

#### 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".[88] 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 0 °C (32 °F)—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 4 °C (39 °F).[89]

The

absolute temperature scale with the same increment as the Celsius temperature scale, which was originally defined according to the boiling point (set to 100 °C (212 °F)) and melting point
(set to 0 °C (32 °F)) 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 (deuterium). 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

A young girl drinking bottled water
Roadside fresh water outlet from glacier, Nubra

The

user-generated source?] To function properly, the body requires between one and seven liters (0.22 and 1.54 imp gal; 0.26 and 1.85 U.S. gal)[citation needed] of water per day to avoid dehydration; 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.[91] 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.[92]

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.[93][94] The popular claim that "a person should consume eight glasses of water per day" seems to have no real basis in science.[95] Studies have shown that extra water intake, especially up to 500 milliliters (18 imp fl oz; 17 U.S. fl oz) at mealtime, was associated with weight loss.[96][97][98][99][100][101] Adequate fluid intake is helpful in preventing constipation.[102]

An original recommendation for water intake in 1945 by the Food and Nutrition Board of the

U.S. 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."[103] The latest dietary reference intake report by the U.S. National Research Council in general recommended, based on the median total water intake from US survey data (including food sources): 3.7 liters (0.81 imp gal; 0.98 U.S. gal) for men and 2.7 liters (0.59 imp gal; 0.71 U.S. gal) of water total for women, noting that water contained in food provided approximately 19% of total water intake in the survey.[104]

Specifically, pregnant and

sweating
, 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,

solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes.[107]

#### Washing

Washing is a method of cleaning, usually with water and soap or detergent. Washing and then rinsing both body and clothing is an essential part of good hygiene and health. [citation needed]

Often people use soaps and detergents to assist in the

emulsification of oils and dirt particles so they can be washed away. The soap can be applied directly, or with the aid of a washcloth
.

People wash themselves, or bathe periodically for religious ritual or therapeutic purposes[109] or as a recreational activity.

In

anal cleansing.[111]

More frequent is washing of just the hands, e.g. before and after preparing food and eating, after using the toilet, after handling something dirty, etc. Hand washing is important in reducing the spread of germs.[112][113][114] Also common is washing the face, which is done after waking up, or to keep oneself cool during the day. Brushing one's teeth is also essential for hygiene and is a part of washing.

'Washing' can also refer to the washing of clothing or other cloth items, like bedsheets, whether by hand or with a washing machine. It can also refer to washing one's car, by lathering the exterior with car soap, then rinsing it off with a hose, or washing cookware.

A private home washing machine
Excessive washing may damage the hair, causing dandruff, or cause rough skin/skin lesions.[115][116]

#### Transportation

waterways. Freight transport by sea has been widely used throughout recorded history. The advent of aviation has diminished the importance of sea travel for passengers, though it is still popular for short trips and pleasure cruises. Transport by water is cheaper than transport by air or ground,[117] but significantly slower for longer distances. Maritime transport accounts for roughly 80% of international trade, according to UNCTAD
in 2020.

Maritime transport can be realized over any distance by boat, ship, sailboat or

bulk transport
.

Containerization revolutionized maritime transport starting in the 1970s. "General cargo" includes goods packaged in boxes, cases, pallets, and barrels. When a cargo is carried in more than one mode, it is intermodal or co-modal.

#### Chemical uses

Water is widely used in chemical reactions as a

Supercritical water
has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently.

#### Heat exchange

Water and steam are a common fluid used for

oxidized 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 evaporation in a cooling tower). In the United States, cooling power plants is the largest use of water.[118]

In the nuclear power industry, water can also be used as a neutron moderator. In most nuclear reactors, water is both a coolant and a moderator. This provides something of a passive safety measure, as removing the water from the reactor also slows the nuclear reaction down. 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

fighting wildfires
.

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 steam explosion, 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, charcoal, or coke graphite, decompose the water, producing water gas.

The power of such explosions was seen in the Chernobyl disaster, although the water involved in this case did not come from fire-fighting but from 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 zirconium.

Some metallic oxides, most notably those of

alkaline earth metals, produce so much heat on reaction with water that a fire hazard can develop. The alkaline earth oxide quicklime 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.[119]

#### Recreation

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming,

snowmobiling or snowboarding, which the require the water to be at a low temperature either as ice or crystallized into snow
.

#### Water industry

The

are in development.

Drinking water is often collected at

filtering with sand which only removes undissolved material, while chlorination and boiling kill harmful microbes. Distillation does all three functions. More advanced techniques exist, such as reverse osmosis. Desalination of abundant seawater is a more expensive solution used in coastal arid climates
.

The distribution of drinking water is done through

municipal water systems, tanker delivery or as bottled water
. Governments in many countries have programs to distribute water to the needy at no charge.

.

Municipal and

wastewater treatment plants. Mitigation of polluted surface runoff 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

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

power generation. Hydroelectricity is electricity obtained from hydropower
. 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 water blasting 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 steam turbine and heat exchanger, in addition to its use as a chemical solvent. Discharge of untreated water from industrial uses is pollution. Pollution includes discharged solutes (chemical pollution) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and uses a variety of purification techniques both in water supply and discharge.

#### Food processing

noodles

Boiling, steaming, and simmering are popular cooking methods that often require immersing food in water or its gaseous state, steam.[120] Water is also used for dishwashing. Water also plays many critical roles within the field of food science.

air pressure, 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 0.51 °C (0.918 °F), and one mole of salt per kg raises the boiling point by 1.02 °C (1.836 °F); similarly, increasing the number of dissolved particles lowers water's freezing point.[121]

Solutes in water also affect water activity that affects many chemical reactions and the growth of microbes in food.[122] Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.[121] Solutes in water lower water activity—this is important to know because most bacterial growth ceases at low levels of water activity.[122] Not only does microbial growth affect the safety of food, but also the preservation and shelf life of food.

Water hardness 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)[123] or less than 60 mg/L (US).[124]

#### Medical use

Water for injection is on the

list of essential medicines.[126]

## Distribution in nature

### In the universe

Band 5 ALMA receiver is an instrument specifically designed to detect water in the universe.[127]

Much of the universe's water is produced as a byproduct of star formation. 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.[128]

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 quasar 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".[129][130]

Water has been detected in interstellar clouds within the Milky Way.[131] 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 planetary systems 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.

Enceladus, as a 10-kilometre thick ocean approximately 30–40 kilometres below Enceladus' south polar surface,[153][154] and Titan, as a subsurface layer, possibly mixed with ammonia.[155] Jupiter's moon Europa has surface characteristics which suggest a subsurface liquid water ocean.[156] Liquid water may also exist on Jupiter's moon Ganymede as a layer sandwiched between high pressure ice and rock.[157]

#### Water ice

Water is present as ice on:

And is also likely present on:

#### Exotic forms

Water and other

ionic water in which the molecules break down into a soup of hydrogen and oxygen ions, and deeper still as superionic water in which the oxygen crystallizes, but the hydrogen ions float about freely within the oxygen lattice.[176]

### Water and planetary habitability

The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of

habitable zone of the Solar System; if it were slightly closer to or farther from the Sun (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.[177][178]

Earth's

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 Mars).[citation needed
]

The surface temperature of Earth has been relatively constant through

insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.[citation needed
]

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

## Law, politics, and crisis

potable water
1970–2000

Water politics is politics affected by water and water resources. Water, particularly fresh water, is a strategic resource across the world 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 sanitation.[181] However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.[182] A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.[183]

1.6 billion people have gained access to a safe water source since 1990.[184] The proportion of people in developing countries with access to safe water is calculated to have improved from 30% in 1970[185] to 71% in 1990, 79% in 2000, and 84% in 2004.[181]

A 2006 United Nations report stated that "there is enough water for everyone", but that access to it is hampered by mismanagement and corruption.

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.[187]

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.[188]

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 hygiene. 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 WaterAid reported that a child dies every 15 seconds from easily preventable water-related diseases; often this means lack of sewage disposal.[citation needed
]

Organizations concerned with water protection include the

World Day for Water takes place on 22 March[189] and World Oceans Day on 8 June.[190]

## In culture

### Religion

People come to Inda Abba Hadera spring (Inda Sillasie, Ethiopia
) to wash in holy water

Water is considered a purifier in most religions. Faiths that incorporate ritual washing (

mikvah) and Sikhism (Amrit Sanskar). 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 washing certain parts of the body using clean water (wudu), unless water is unavailable (see Tayammum). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of misogi
).

In Christianity,

blessing of persons, places, and objects, or as a means of repelling evil.[192][193]

In Zoroastrianism, water (āb) is respected as the source of life.[194]

### Philosophy

Icosahedron as a part of Spinoza monument in Amsterdam
.

The Ancient Greek philosopher

monist, believed further that all things are made from water. Plato believed that the shape of water is an icosahedron – flowing easily compared to the cube-shaped earth.[195]

The theory of the

metal
).

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 Tao" 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."[196] 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.[197]

### Folklore

"Living water" features in Germanic and Slavic folktales as a means of bringing the dead back to life. Note the Grimm fairy-tale ("The Water of Life") and the Russian dichotomy of living [ru] and dead water [ru]. The Fountain of Youth represents a related concept of magical waters allegedly preventing aging.

### Art and activism

Painter and activist

Teresita Fernandez and Bill Viola.[202][203] Foster created Think About Water,[204][full citation needed] an ecological collective of artists who use water as their subject or medium. Members include Basia Irland,[205][full citation needed] Aviva Rahmani, Betsy Damon, Diane Burko, Leila Daw, Stacy Levy, Charlotte Coté,[206] Meridel Rubenstein, Stacy Levy, Anna Macleod, and Aviva Rahmani
.

### Dihydrogen monoxide parody

Water's technically correct but rarely used

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.[209]

## Notes

1. ^ Other substances with this property include bismuth, silicon, germanium and gallium.[17]

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