TheInfoList

Water (chemical formula H2O) is an
inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
, transparent, tasteless, odorless, and
nearly colorless Jerome Dillon is a professional musician, best known for his tenure as drummer with industrial rock group Nine Inch Nails from 1999–2005. After his departure, his own project, Nearly, released its debut album ''reminder'' in December 2005, alo ...
chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
, which is the main constituent of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

's
hydrosphere The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
and the
fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
s of all known living organisms (in which it acts as a
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

). It is vital for all known forms of
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...

, even though it provides no
calories The calorie is a unit of energy defined as the amount of heat needed to raise the temperature of a quantity of water by one degree. For historical reasons, two main definitions of calorie are in wide use. The small calorie or gram calorie (usual ...
or organic
nutrient A nutrient is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and t ...
s. Its
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
, H2O, indicates that each of its
molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

s contains one
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

and two
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s, connected by
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
s. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°. "Water" is the name of the liquid state of H2O at
standard conditions for temperature and pressure Standard temperature and pressure (STP) are standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * Standard (metrology), ...
. A number of natural states of water exist. It forms
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...

in the form of rain and
aerosol An aerosol (abbreviation of "aero-solution") is a suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in math ...

s in the form of fog.
Cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

s consist of suspended droplets of water and
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

, its solid state. When finely divided,
crystalline A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and contain the least amount of kinet ...

ice may precipitate in the form of
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

. The gaseous state of water is
steam Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , ev ...

or
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
. Water covers approximately 70.9% of the Earth's surface, mostly in seas and oceans. Small portions of water occur as
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

(1.7%), in the
glaciers A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

and the
ice caps Image:Vatnajökull.jpeg, upright=1.35, Vatnajökull, Iceland In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Descript ...
of
Antarctica Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an autonomous territory An autonomous administrative division (also referred to as an autonomous area, entity, unit, region, subdivision, or territory) is a subnational administra ...

(1.7%), and in the air as
vapor In physics, a vapor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, America ...
, clouds (consisting of ice and liquid water suspended in air), and
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
(0.001%). Water moves continually through the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, describes the continuous movement of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, ...

of
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

,
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

(
evapotranspiration Evapotranspiration (ET) is the sum of water evaporation and transpiration from a surface area to the Atmosphere of Earth, atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and ...

),
condensation Condensation is the change of the state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

,
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
, and
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed (printing), bleed, printing that lies beyond the edges to wh ...
, usually reaching the sea. Water plays an important role in the
world economy The world economy or the global economy is the economy of all humans of the world, referring to the global economic system which includes all economic activities which are conducted both within and between nations, including production (economics ...
. Approximately 70% of the freshwater used by
humans Humans (''Homo sapiens'') are the most populous and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species ...
goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. 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 Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language ...
, from
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new ...
*''watar'' (source also of
Old Saxon Old Saxon, also known as Old Low German, was a Germanic language The Germanic languages are a branch of the Indo-European The Indo-European languages are a language family native to western and southern Eurasia. It comprises most of ...
,
Old Frisian Old Frisian was a West Germanic language spoken between the 8th and 16th centuries in the area between the Rhine ), Surselva Surselva Region is one of the eleven administrative districts Administrative division, administrative unitArticl ...
,
Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium (also referred as ''flemish'') Dutch may also refer to:" Castle * Dutch Castle Places * ...
,
Old High German Old High German (OHG, german: Althochdeutsch, German abbr. ) is the earliest stage of the German language German ( Standard High German: , ) is a West Germanic language mainly spoken in Central Europe Central Europe is an area of Euro ...
,
German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

, ,
Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ethnonym of a group of East Germanic tribes **Gothic language, an extinct East Germanic language spoken by the Goths **Crimean Gothic, the Gothic language spoken by ...
(), from
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
*''wod-or'', suffixed form of root *''wed-'' ("water"; "wet"). Also
cognate In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most langu ...
, through the Indo-European root, with
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
(),
Russian Russian refers to anything related to Russia, including: *Russians (русские, ''russkiye''), an ethnic group of the East Slavic peoples, primarily living in Russia and neighboring countries *Rossiyane (россияне), Russian language term ...
(),
Irish Irish most commonly refers to: * Someone or something of, from, or related to: ** Ireland, an island situated off the north-western coast of continental Europe ** Northern Ireland, a constituent unit of the United Kingdom of Great Britain and North ...
, and
Albanian Albanian may refer to: *Pertaining to Albania in Southeast Europe; in particular: **Albanians, an ethnic group native to the Balkans **Albanian language **Albanian culture **Demographics of Albania, includes other ethnic groups within the country ...

.

# Chemical and physical properties

Water () is a
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the clim ...
inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
that is at
room temperature Colloquially, room temperature is the range of air temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat ...
a
taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...

less and
odor An odor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. Cur ...
less liquid, nearly colorless with a hint of blue. This simplest
hydrogen chalcogenideHydrogen chalcogenides (also chalcogen hydrides or hydrogen chalcides) are binary compounds of hydrogen with chalcogen The chalcogens () are the chemical elements in group (periodic table), group 16 of the periodic table. This group is also known ...
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 "
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

of life": indeed, water as found in nature almost always includes various dissolved substances, and special steps are required to obtain chemically
pure water 200px, Royal Academy of Pharmacy (Spain) ">Royal Academy of Pharmacy">Royal Academy of Pharmacy (Spain) Purified water is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, ...

. Water is the only common substance to exist as a
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

, liquid, and
gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
in normal terrestrial conditions.

## States

Along with ''oxidane'', ''water'' is one of the two official names for the chemical compound ; it is also the
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

phase of . The other two common
states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...
of water are the
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

phase,
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

, and the gaseous phase,
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
or
steam Steam is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , ev ...

. The addition or removal of heat can cause
phase transition In , , and many other related fields, phase transitions (or phase changes) are the of transition between a state of a medium, identified by some parameters, and another one, with different values of the parameters. Commonly the term is used to ...
s:
freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid o ...

(water to ice),
melting Melting, or Enthalpy of fusion, fusion, is a physical process that results in the phase transition of a chemical substance, substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the applic ...

(ice to water),
vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon. ...
(water to vapor),
condensation Condensation is the change of the state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

(vapor to water), sublimation (ice to vapor) and
deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * List of deposed politicians, Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual ...
(vapor to ice).

### Density

Water differs from most liquids in that it becomes less
dense The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or ...

as it freezes. In 1 atm pressure, it reaches its maximum density of at . The density of ice is , an expansion of 9%. This expansion can exert enormous pressure, bursting pipes and cracking rocks (see
Frost weathering Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...
). 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 An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
(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
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

(vaporization throughout the liquid is known as
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...
). Sublimation and deposition also occur on surfaces. For example,
frost Frost is a thin layer of ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is ...

is deposited on cold surfaces while
snowflake A snowflake is a single ice crystal Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...

s form by deposition on an aerosol particle or ice nucleus. In the process of
freeze-drying Freeze drying, also known as lyophilisation or cryodesiccation, is a low temperature Food drying, dehydration process that involves freezing the product, lowering pressure, then removing the ice by Sublimation (phase transition), sublimation. Thi ...
, 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 The Clausius–Clapeyron relation, named after Rudolf Clausius Rudolf Julius Emanuel Clausius (; 2 January 1822 – 24 August 1888) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method ...
: :$\frac = \frac,$ where $v_\text$ and $v_\text$ are the
molar volume In chemistry and related fields, the molar volume, symbol ''V''m, or \tilde V of a substance is the occupied volume divided by the amount of substance at a given temperature and pressure. It is equal to the molar mass (''M'') divided by the mass de ...
s of the liquid and solid phases, and $L_\text$ is the molar
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system A thermodynamic system is a body of matter In classical physics and general chemistry, matter is any su ...
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. In glaciers, pressure melting can occur under sufficiently thick volumes of ice, resulting in
subglacial lake A subglacial lake is a lake that is found under a glacier A glacier ( or ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, abla ...
s. 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. Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed in
Old Faithful Old Faithful is a cone geyser A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeolog ...

, a geyser in
Yellowstone National Park Yellowstone National Park is an American national park#REDIRECT National park A national park is a park in use for Conservation (ethic), conservation purposes, created and protected by national governments. Often it is a reserve of natur ...

. In
hydrothermal vent A hydrothermal vent is a fissure A fissure is a long, narrow crack opening along the surface of the Earth. It is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in the Earth's crust, on ice sheets and glaciers, and ...
s, the temperature can exceed . At
sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in th ...

, 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
pressure cooker Pressure cooking is the process of cooking food under high pressure steam, employing water or a water-based cooking liquid, in a sealed vessel known as a ''pressure cooker''. High pressure limits boiling, and permits cooking temperatures well abo ...

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
phase diagram A phase diagram in physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical ins ...

(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 In , the triple point of a substance is the and at which the three (, , and ) of that substance coexist in .. It is that temperature and pressure at which the curve, curve and the curve meet. For example, the triple point of occurs at a tem ...
, 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 mix freely with nonpolar compounds, including most
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s. This makes it useful in a variety of applications including high-temperature
electrochemistry Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...

and as an ecologically benign solvent or
catalyst that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

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
Ice Ih Ice Ih (hexagonal ice crystal) (pronounced: ice one h, also known as ice-phase-one) is the hexagonal crystal form of ordinary , or frozen . Virtually all ice in the is ice Ih, with the exception only of a small amount of that is occasionally ...
, a phase that forms crystals with Hexagonal crystal family, hexagonal symmetry. Another with Cubic crystal system, cubic crystalline symmetry, Ice Ic, 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. 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. When sandwiched between layers of graphene, 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 Humans (''Homo sapiens'') are the most populous and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species ...
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. Humans and other animals have developed senses that enable them to evaluate the drinking water, potability of water by avoiding water that is too salty or putrid.

## Color and appearance

Pure water is Visual perception, visibly blue due to electromagnetic absorption by water, 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 Overtone band, overtones of the O–H stretching Molecular vibration, 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 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 3500 cm−1 (2.85 μm) 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 spectroscopy, Raman-active fundamental vibrations may be observed with, for example, a 1 cm sample cell. Aquatic plants, algae, and other Photosynthesis, 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 of depth. The refractive index of liquid water (1.333 at ) 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. The O–H bond length is about 0.096 nm. Other substances have a tetrahedral molecular structure, for example, methane () and hydrogen sulfide (). However, oxygen is more electronegativity, 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 polar molecule. Water is a good polar
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

, that dissolves many salt (chemistry), salts and hydrophilic organic molecules such as sugars and simple alcohols such as ethanol. Water also dissolves many gases, such as oxygen and carbon dioxide—the latter giving the fizz of carbonation, carbonated beverages, sparkling wines and beers. In addition, many substances in living organisms, such as proteins, DNA and polysaccharides, are dissolved in water. The interactions between water and the subunits of these biomacromolecules shape protein folding, Base pairing, DNA base pairing, and other phenomena crucial to life (hydrophobic effect). Many organic substances (such as lipids, fats and oils and alkanes) are 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 hydrogen bonds 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 Hydrogen chalcogenide, other analogous compounds like hydrogen sulfide. They also explain its exceptionally high specific heat capacity (about 4.2 Joule, J/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 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. These bonds are the cause of water's high surface tension 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.

## Self-ionisation

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

## Electrical conductivity and electrolysis

Pure water has a low electrical conductivity, which increases with the dissolution (chemistry), dissolution of a small amount of ionic material such as sodium chloride, common salt. Liquid water can be split into the Chemical element, elements hydrogen and oxygen by passing an electric current through it—a process called Electrolysis of water, electrolysis. The decomposition requires more energy input than the standard enthalpy of formation, heat released by the inverse process (285.8 kJ/mole (unit), 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 viscosity of water is about 10−3 Pa·second, s or 0.01 Poise (unit), 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 decibel, dB/km for 1 khertz, Hz), a property that is exploited by cetaceans and humans for communication and environment sensing (sonar).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 Electronegativity, electropositive than hydrogen, particularly the alkali metals and alkaline earth metals such as lithium, sodium, calcium, potassium and Caesium, 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 Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

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 The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
. 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 body of water, bodies of water, such as an ocean, sea, lake, river, stream, canal, pond, or puddle. The majority of water on Earth is sea water. 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 rock (geology), rocks, and the pressure of this groundwater affects patterns of fault (geology), faulting. Water in the Mantle (geology), mantle is responsible for the melt that produces volcanoes at subduction zones. On the surface of the Earth, water is important in both chemical and physical weathering 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 (geology), Deposition of transported sediment forms many types of sedimentary rocks, which make up the geologic record of History of the Earth, Earth history.

## Water cycle

The
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, describes the continuous movement of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, ...

(known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the
hydrosphere The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
, between the Earth atmosphere, atmosphere, soil water, surface water,
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

, and plants. Water moves perpetually through each of these regions in the ''water cycle'' consisting of the following transfer processes: *
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

from oceans and other water bodies into the air and
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

from land plants and animals into the air. *
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
, from water vapor condensing from the air and falling to the earth or ocean. * runoff (water), 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 Metric tonne unit, 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 hail, with some contribution from fog and dew. 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. Condensed water in the air may also refract sunlight to produce rainbows. Water runoff often collects over Drainage basin, watersheds 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 irrigation for agriculture. Rivers and seas offer opportunities for travel and commerce. Through erosion, runoff shapes the environment creating river valleys and river delta, 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 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. 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 overdrafting, groundwater depletion.

## Sea water and tides

Seawater, Sea water 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 ) 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 30–40%, is really a salt lake.) Tides are the cyclic rising and falling of local sea levels caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuary, estuarine 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 Coriolis effect, 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

From a biology, biological standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s to react in ways that ultimately allow Self-replication, replication. All known forms of life depend on water. Water is vital both as a
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

in which many of the body's solutes dissolve and as an essential part of many metabolism, 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. 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. Acids have pH values less than 7 while Base (chemistry), 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 instead of lungs, although some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and pinniped, seals 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 Siphon (insect), insect and Siphon (mollusc), mollusc siphons) and 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

Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates; the ancient society of the Egyptians depended entirely upon the Nile. The early 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.

## Human uses

### 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. 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. Access to fresh water is often taken for granted, especially in developed countries that have build 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. 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. 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. 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. 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 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. 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 drip irrigation File:SiphonTubes.JPG, Irrigation 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". 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 . The Kelvin temperature scale of the International System of Units, SI system was based on the
triple point In , the triple point of a substance is the and at which the three (, , and ) of that substance coexist in .. It is that temperature and pressure at which the curve, curve and the curve meet. For example, the triple point of occurs at a tem ...
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 boiling point (set to ) and melting point (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 (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

The human body contains from 55% to 78% water, depending on body size. To function properly, the body requires between 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. 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. 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. 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. Specifically, pregnant and breastfeeding 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 (Caffeine, caffeinated included). Water is excreted from the body in multiple forms; through urine and feces, through 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, and/or harmful bacteria, such as ''Vibrio''. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes. The single largest (by volume) freshwater resource suitable for drinking is Lake Baikal in Siberia.

### Washing

The propensity of water to form Solvation, solutions and emulsions is useful in various washing processes. Washing is also an important component of several aspects of personal body hygiene. Most of the personal water use is due to showering, doing the laundry and dishwashing, 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
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

or reactant and less commonly as a solute or catalyst. In inorganic reactions, water is a common solvent, dissolving many ionic compounds, as well as other polar compounds such as ammonia and Hydrogen chalcogenide, 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 amphoteric (acidic ''and'' basic) and nucleophilic. 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 heat exchanger, heat exchange, due to its availability and high Heat capacity of water, 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
vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon. ...
and
condensation Condensation is the change of the state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

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 oxidation, 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 Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

in a cooling tower). In the United States, cooling power plants is the largest use of water.Water Use in the United States
National Atlas.gov
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 void coefficient, 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

Water has a high heat of vaporization and is relatively inert, which makes it a good Fire fighting#Use of water, 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 (fuel), 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 alkali metals and alkaline earth metals, produce so much heat on reaction with water that a fire hazard can develop. The alkaline earth oxide Calcium 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.

### Recreation

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, waterskiing, boating, surfing and Underwater diving, diving. In addition, some sports, like ice hockey and ice skating, are played on ice. Lakesides, beaches and water parks 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 aquariums or ponds for show, fun, and companionship. Humans also use water for snow sports i.e. skiing, sledding, snowmobiling or snowboarding, which require the water to be frozen.

### Water industry

The water industry provides drinking water and wastewater services (including sewage treatment) to households and industry. Water supply facilities include water wells, cisterns for rainwater harvesting, water supply networks, and water purification facilities, water tanks, water towers, water pipes including old Aqueduct (watercourse), aqueducts. Atmospheric water generators are in development. Drinking water is often collected at spring (hydrosphere), springs, extracted from artificial Boring (earth), 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 microbes. Popular methods are filter (water), filtering with sand which only removes undissolved material, while Water chlorination, chlorination and
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...
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. 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 Water conservation, conserve freshwater resources. Water pollution, Polluting water 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 externality, externalities 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. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on marine biology, aquatic life if they bioaccumulation, bioaccumulate and if they are not biodegradable. Municipal and industrial wastewater are typically treated at 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 slurry, slurries or using water to dissolve and extract substances, or to wash products or process equipment. Processes such as mining, chemical pulping, pulp bleaching, 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 Hydrodemolition, 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 A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

. Discharge of untreated water from industrial uses is water pollution, pollution. 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

Boiling, steaming, and simmering are popular cooking methods that often require immersing food in water or its gaseous state, steam. Water is also used for dishwashing. Water also plays many critical roles within the field of food science. Solutes 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 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 (unit), 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. Solutes in water also affect water activity that affects many chemical reactions and the growth of microbes in food. 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. 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) 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 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. 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". Water has been detected in interstellar clouds within our galaxy, the Milky Way. 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: * Solar atmosphere, Atmosphere of the Sun: in detectable trace amounts * Atmosphere of Mercury: 3.4%, and large amounts of water in Mercury (planet), Mercury's exosphere * Atmosphere of Venus: 0.002% * Earth's atmosphere: ≈0.40% over full atmosphere, typically 1–4% at surface; as well as Atmosphere of the Moon, that of the Moon in trace amounts * Atmosphere of Mars: 0.03% * Atmosphere of Ceres * Atmosphere of Jupiter: 0.0004% – in volatiles, ices only; and that of its moon Europa (moon), Europa * Atmosphere of Saturn – in volatiles, ices only; Enceladus (moon), Enceladus: 91% and Dione (moon), Dione (exosphere) * Atmosphere of Uranus – in trace amounts below 50 bar * Atmosphere of Neptune – found in the deeper layers * Extrasolar planet atmospheres: including those of HD 189733 b and HD 209458 b,Water Found in Extrasolar Planet's Atmosphere
– Space.com
Tau Boötis b, HAT-P-11b, XO-1b, WASP-12b, WASP-17b, and WASP-19b. * Stellar atmospheres: not limited to cooler stars and even detected in giant hot stars such as Betelgeuse, Mu Cephei, Antares and Arcturus.Andersson, Jonas (June 2012)
Water in stellar atmospheres "Is a novel picture required to explain the atmospheric behavior of water in red giant stars?"
Lund Observatory, Lund University, Sweden
* Circumstellar disks: including those of more than half of T Tauri stars such as AA Tauri as well as TW Hydrae, IRC +10216 and APM 08279+5255, VY Canis Majoris and S Persei.

### Liquid water

Liquid water is present on Earth, covering 71% of its surface. Liquid water is also occasionally present in small amounts Water on Mars, on Mars. Scientists believe liquid water is present in the Saturnian moons of Enceladus (moon), Enceladus, as a 10-kilometre thick ocean approximately 30–40 kilometres below Enceladus' south polar surface, and Titan (moon), Titan, as a subsurface layer, possibly mixed with ammonia. Jupiter's moon Europa (moon), Europa has surface characteristics which suggest a subsurface liquid water ocean. Liquid water may also exist on Jupiter's moon Ganymede (moon), Ganymede as a layer sandwiched between high pressure ice and rock.

### Water ice

Water is present as ice on: * Water on Mars, Mars: under the regolith and at the poles. * Earth–Moon system: mainly as ice sheets on Earth and in Lunar craters and volcanic rocks NASA reported the detection of water molecules by NASA's Moon Mineralogy Mapper aboard the Indian Space Research Organization's Chandrayaan-1 spacecraft in September 2009. * Ceres (dwarf planet), Ceres * Jupiter's moons: Europa (moon), Europa's surface and also that of Ganymede (moon), Ganymede and Callisto (moon), Callisto * Saturn: in the Rings of Saturn, planet's ring system and on the surface and mantle of Titan (moon), Titan and Enceladus (moon), Enceladus (supporting online material, table S1) * Pluto–Charon (moon), Charon system * Comets and other related Kuiper belt and Oort cloud objects And is also likely present on: * Mercury (planet), Mercury's poles * Tethys (moon), Tethys

### Exotic forms

Water and other volatiles probably comprise much of the internal structures of Uranus and Neptune and the water in the deeper layers may be in the form of 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 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 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 Organism, life on Earth as we know it. The Earth is located in the 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. Earth's gravity allows it to hold an Celestial body atmosphere, 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). The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (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. 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.

# In culture

## Religion

Water is considered a purifier in most religions. Faiths that incorporate ritual washing (Ritual purification, ablution) include Christianity, Hinduism, Islam, Judaism, the Rastafari movement, Shinto, Taoism, and Wicca. Immersion (or aspersion or affusion) of a person in water is a central sacrament of Christianity (where it is called baptism); it is also a part of the practice of other religions, including Islam (''Ghusl''), Judaism (''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, holy water is water that has been sanctified by a priest for the purpose of baptism, the Blessing (Roman Catholic Church), blessing of persons, places, and objects, or as a means of repelling evil. In Zoroastrianism, water (''aban, āb'') is respected as the source of life.

## Philosophy

The Ancient Greek philosopher Empedocles saw Water (classical element) , water as one of the four classical elements (along with fire, earth, and Air (classical element), air), and regarded it as an ylem, or basic substance of the universe. Thales, whom Aristotle portrayed as an astronomer and an engineer, theorized that the earth, which is denser than water, emerged from the water. Thales, a monist, believed further that all things are made from water. Plato believed that the shape of water is an icosahedron - thus explaining why it flows easily compared to the cube-shaped earth. The theory of the humorism , four bodily humors associated water with phlegm, as being cold and moist. The Water (classical element), classical element of water was also one of the Five elements (Chinese philosophy), five elements in traditional Chinese philosophy (along with earth (classical element), earth, fire (classical element), fire, wood (classical element), wood, and metal (classical element), metal). Some traditional and popular Asian philosophy , Asian philosophical systems take water as a role-model. James Legge'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 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." ''Guanzi (text), 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.

## Folklore

"Living water" features in Germanic and Slavic Folklore, folktales as a means of bringing the dead back to life. Note the Grimms' Fairy Tales , Grimm fairy-tale (The Water of Life (German fairy tale) , "The Water of Life") and the Russian dichotomy of and dead water ). The Fountain of Youth represents a related concept of Magic (supernatural) , magical waters allegedly preventing aging.

## 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 Jenny Holzer, Robert Longo, Mark Rothko, William Kentridge, April Gornik, Kiki Smith, Pat Steir, William Kentridge, Alice Dalton Brown, Teresita Fernandez and Bill Viola. Foster create
an ecological collective of artists who use water as their subject or medium. Members includ
Basia Irland
Aviva Rahmani, Betsy Damon, Diane Burko, Leila Daw, Stacy Levy, Charlotte Coté, Meridel Rubenstein, Stacy Levy, Anna Macleod, and Aviva Rahmani. To mark the 10th anniversary of access to water and sanitation being declared a human right by the UN, the charity WaterAid commissioned ten visual artists to show the impact of clean water on people’s lives.

## Dihydrogen monoxide parody

Water's technically correct but rarely used chemical name, dihydrogen monoxide, has been used in a series of hoaxes and pranks 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. * Aquaphobia (fear of water) * Mpemba effect * Oral rehydration therapy * Thirst * Water pinch analysis

# References

* * Debenedetti, PG., and HE Stanley, "Supercooled and Glassy Water", ''Physics Today'' 56 (6), pp. 40–46 (2003)
* * Gleick, PH., (editor), ''The World's Water: The Biennial Report on Freshwater Resources''. Island Press, Washington, D.C. (published every two years, beginning in 1998.
The World's Water, Island Press
*
Journal of Contemporary Water Research & Education
* Postel, S., ''Last Oasis: Facing Water Scarcity''. W.W. Norton and Company, New York. 1992 * Reisner, M., ''Cadillac Desert: The American West and Its Disappearing Water''. Penguin Books, New York. 1986.
United Nations World Water Development Report
Produced every three years. * St. Fleur, Nicholas

"The water you drink is older than the planet you're standing on." ''The New York Times'' (15 April 2016)

OECD Water statistics

The World's Water Data Page

FAO Comprehensive Water Database, AQUASTAT

Water science school
(USGS)
Portal to The World Bank's strategy, work and associated publications on water resources

America Water Resources Association

Water on the web

Water structure and science

Why water is one of the weirdest things in the universe
BBC Ideas, Video, 3:16 minutes, 2019
The chemistry of water
(NSF special report)

* [https://www.pbs.org/wgbh/molecule-that-made-us H2O:The Molecule That Made Us], a 2020 PBS documentary {{Authority control Articles containing video clips Hydrogen compounds Inorganic solvents Liquids Materials that expand upon freezing Nuclear reactor coolants Oxides Oxygen compounds Water,