Editing Water

'''Water''' is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, ''water'' refers only to its liquid form or state, but the substance also has a solid state, ''ice'', and a gaseous state, ''water vapor'' or ''steam''. About 1.460 petatonnes&nbsp;(Pt) (10<sup>21</sup>kilograms) of water covers 71% of the Earth's surface, mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the atmosphere as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. A very small amount of the Earth's water is contained within water towers, biological bodies, manufactured products, and food stores. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land.

Water moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff , usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36&nbsp;Tt (10<sup>12</sup>kilograms) per year. Over land, evaporation and transpiration contribute another 71&nbsp;Tt per year to the precipitation of 107&nbsp;Tt per year over land.  Clean, fresh drinking water is essential to human and other life. 
==Types of water==


Water can appear in three states; it is one of the very few substances to be found naturally in all three states on earth. Water takes many different forms on Earth: water vapor and clouds in the sky; seawater and rarely icebergs in the ocean; glaciers and rivers in the mountains; and the liquid in aquifers in the ground. 

Water can dissolve many different substances, giving it different tastes and odors. In fact, humans and other animals have developed senses which are, to a degree, able to evaluate the drinking water|potability of water, avoiding water that is too salty or putrid. Humans also tend to prefer cold water to lukewarm; cold water is likely to contain fewer microorganism|microbes. The taste advertised in spring water or mineral water derives from the minerals dissolved in it, as pure H<sub>2</sub>O is tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microorganism|microbes.

Different names are given to water's various forms:

*'''according to state'''
**solid - ice
**liquid - water
**gaseous - water vapor

*'''according to meteorology''':
**hydrometeor
***Precipitation
:{| border=0|
'''
|-
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;||precipitation according to moves||&nbsp;&nbsp;||precipitation according to state
|-
|&nbsp;
|valign=top |
*vertical (falling) precipitation
**rain
**freezing rain
**drizzle
**freezing drizzle
**snow
**snow pellets
**snow grains
**ice pellets
**frozen rain
**hail
**ice crystals
*horizontal (seated) precipitation
** dew
** hoarfrost
** atmospheric icing
** glaze ice
||&nbsp;&nbsp;
|valign=top |
*liquid precipitation
**rain
**freezing rain
**drizzle
**freezing drizzle
**dew
*solid precipitation
**snow
**snow pellets
**snow grains
**ice pellets
**frozen rain
**hail
**ice crystals
** hoarfrost
** atmospheric icing
** glaze ice
*mixed precipitation
**in temperatures around 0 °C
|}
** levitating particles
*** clouds
*** fog
*** BR (according to METAR)
** ascending particles (drifted by wind)
*** spindrift
*** ''stirred snow''
*'''according to occurrence'''
** groundwater
** meltwater
** meteoric water
** connate fluids
** fresh water
** surface water
** mineral water – contains much minerals
** brackish water
** dead water – strange phenomenon which can occur when a layer of fresh or brackish water rests on top of more dense salt water, without the two layers mixing. It is dangerous for ship traveling.
** seawater
** brine
*'''according to uses'''
** tap water
** bottled water
** drinking water or potable water – useful for everyday drinking, without fouling, it contains balanced minerals that are not harmful to health (see below)
** purified water, laboratory-grade, analytical-grade or reagent-grade water – water which has been highly purified for specific uses in science or engineering. Often broadly classified as Type I, Type II, or Type III, this category of water includes, but is not limited to the following:
***distilled water
***double distilled water
***deionized water

*'''according to other features'''
** soft water – contains less minerals
** hard water – from underground, contains more minerals
** distilled water, double distilled water, deionized water - contains no minerals
** Water of crystallization — water incorporated into crystalline structures
** Hydrates — water bound into other chemical substances
** heavy water – made from heavy atoms of hydrogen - deuterium. It is in nature in normal water in very low concentration. It was used in construction of first Nuclear reactor technology|nuclear reactors.
** tritiated water

*'''according to microbiology'''
**drinking water
**wastewater
**stormwater or surface water

*'''according to religion'''
**holy water

==Information and properties==
*Common name: water
*IUPAC name : oxidane
*Alternative names:  aqua, dihydrogen monoxide, hydrogen hydroxide 
* Molecular formula:  H<sub>2</sub>O
*CAS number:  7732185
* International Chemical Identifier:  InChI=1/H2O/h1H2
* Molar mass:  18.0153 g/mol
* Density and Phase (matter)phase: 0.998 g/cm³ <small>(liquid at 20 °C, 1 atm)</small><br /> 0.917 g/cm³ <small>(solid at 0 °C, 1 atm)</small>
*Melting point:  0 Celsius°C (273.15 kelvinK) (32 Fahrenheit°F)
* Boiling point:  99.974 °C (373.124 K) (211.95 °F)
* Specific heat capacity:  4.184 J/(g·K) <small>(liquid at 20 °C)</small><br/> 74.539 J/ (mol·K) <small>(liquid at 25 °C)</small>


Water is the chemical substance with chemical formula '''H<sub>2</sub>O''': one molecule of water has two hydrogen atoms covalently chemical bonded to a single oxygen atom.

The major chemical and physical properties of water are:

* Water is a tasteless, odorless liquid at ambient temperature and pressure. The color of water and ice is, intrinsically, a very light blue hue, although water appears colorless in small quantities. Ice also appears colorless, and water vapor is essentially invisible as a gas.

* Water is transparent, and thus aquatic plants can live within the water because sunlight can reach them. Only strong UV light is slightly absorbed.

* Since oxygen has a higher electronegativity than hydrogen, water is a polar molecule. The oxygen has a slight negative charge while the hydrogens have a slight positive charge giving the article a strong effective dipole Electrical dipole moment.  The interactions between the different dipoles of each molecule cause a net attraction force associated with water's high amount of surface tension.

* Another very important force that causes the water molecules to stick to one another is the hydrogen bond.

* The boiling point of water (and all other liquids) is directly related to the barometric pressure. For example, on the top of Mt. Everest water boils at about 68°C, compared to 100°C at sea level. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.

* Water sticks to itself. Water has a high surface tension caused by the strong cohesion between water molecules because it is polar molecule|polar. The apparent elasticity caused by surface tension drives the capillary waves.

* Water also has high adhesion properties because of its polar nature.

* 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.

* Water is a very strong solvent, referred to as ''the universal solvent'', dissolving many types of substances. Substances that will mix well and dissolve in water, e.g. salts, sugars, acids, alkalis, and some gases: especially oxygen, carbon dioxide (carbonation), are known as "hydrophilic" (water-loving) substances, while those that do not mix well with water (e.g. fats and oils), are known as "hydrophobic" (water-fearing) substances.

* All the major components in cells (proteins, DNA and polysaccharides) are also dissolved in water.

* Pure water has a ''low'' electrical conductivity, but this increases significantly upon solvation of a small amount of ionic material such as sodium chloride.

* Water has the second highest specific heat capacity of any known chemical compound, after ammonia, as well as a high heat of vaporization (40.65 kJ mol<sup>−1</sup>), both of which are a result of the extensive hydrogen bonding between its molecules. These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.

* The maximum density of water is at 3.98°C Water becomes even less dense upon freezing, expanding 9%. This causes an unusual phenomenon: ice floats upon water, and so water organisms can live inside a partly frozen pond because the water on the bottom has a temperature of around convert|4|°C.

* Water is miscible with many liquids, for example ethanol, in all proportions, forming a single homogeneous liquid. On the other hand, water and most oils are ''immiscible'' usually forming layers according to increasing density from the top. As a gas, water vapor is completely miscible with air.

* Water forms an azeotrope with many other solvents.

*Water can be Electrolysis of water|split by electrolysis into hydrogen and oxygen. 

* As an oxide of hydrogen, water is formed when hydrogen or hydrogen-containing compounds burn or react with oxygen or oxygen-containing compounds. Water is not a fuel, it is an end-product of the combustion of hydrogen. The energy required to split water into hydrogen and oxygen by electrolysis or any other means is greater than the energy released when the hydrogen and oxygen recombine

* Elements which are more electropositive than hydrogen such as lithium, sodium, calcium, potassium and caesium displace hydrogen from water, forming hydroxides. Being a flammable gas, the hydrogen given off is dangerous and the reaction of water with the more electropositive of these elements is violently explosive.

==Distribution of water in nature==

===Water in the Universe===
Much of the universe's water may be produced as a byproduct of star formation. When stars are born, their birth 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. Water has been detected in interstellar clouds within our galaxy, the Milky Way. It is believed that water exists in abundance in other galaxies too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Interstellar clouds eventually condense into solar nebulae and solar systems, such as ours.

Water vapor is present on:
* Mercury - 3.4% in the atmosphere, and large amounts of water in Mercury's exosphere
* Venus - 0.002% in the atmosphere
* Earth - trace in the atmosphere (varies with climate)
* Mars - 0.03% in the atmosphere
* Jupiter - 0.0004% in the atmosphere
* Saturn - in volatiles|ices only
* Enceladus (moon of Saturn) - 91% in the atmosphere
* exoplanets known as HD 189733 
Liquid water is present on:
* Earth - 71% of surface
* Moon - small amounts of water have been found (in 2008) in the inside of volcanic pearls brought from Moon to Earth by the Apollo 15 crew in 1971

Strong evidence suggests that liquid water is present just under the surface of Saturn's moon Enceladus. Probably some liquid water is on Europa.

Water ice is present on:
* Earth - mainly on ice sheets
* polar ice caps on Mars
* Titan (moon)|Titan
* Europa (moon)|Europa
* Enceladus (moon)|Enceladus

Probability or possibility of distribution of water ice is at: lunar ice on the Moon, Ceres (dwarf planet), Tethys (moon). Ice is probably in internal structure of Uranus, Neptune, and Pluto and on comets.

===Water and habitable zone===

The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth is vital to the existence of 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 further from the Sun (about 5%, or about 8 million kilometres), the conditions which allow the three forms to be present simultaneously would be far less likely to exist

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

It has been proposed that life itself may maintain the conditions that have allowed its continued existence. 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 also depends on a 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. There are various theories about origin of water on Earth.

==Water 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 focus of ecohydrology.

The collective mass of water found on, under, and over the surface of a planet is called hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1&nbsp;360&nbsp;000&nbsp;000&nbsp;km³ (326&nbsp;000&nbsp;000&nbsp;mi³). Of this volume:

* 1&nbsp;320&nbsp;000&nbsp;000&nbsp;km³ (316&nbsp;900&nbsp;000&nbsp;mi³ or 97.2%) is in the sea water|oceans.
* 25&nbsp;000&nbsp;000&nbsp;km³ (6&nbsp;000&nbsp;000&nbsp;mi³ or 1.8%) is in glaciers, ice caps and ice sheets.
* 13&nbsp;000&nbsp;000&nbsp;km³ (3,000,000 mi³ or 0.9%) is groundwater.
* 250&nbsp;000&nbsp;km³ (60,000 mi³ or 0.02%) is fresh water in lakes, inland seas, and rivers.
* 13&nbsp;000&nbsp;km³ (3,100 mi³ or 0.001%) is atmospheric water vapor at any given time.

Groundwater and fresh water are useful or potentially useful to humans as water resources.

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 sea water. Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as groundwater in aquifers.

The most important geological processes caused by water are: Weathering, erosion, water sediment transport and sedimentation, mudflows, ice erosion and sedimentation by glacier.

===Water cycle===
Main|Water cycle

The water cycle (known scientifically as the '''hydrologic cycle''') refers to the continuous exchange of water within the hydrosphere, between the Earth atmosphere|atmosphere, soil water, surface water, groundwater, and plants.

Water moves perpetually through each of these regions in the ''water cycle'' consisting of following transfer processes:
*evaporation from oceans and other water bodies into the air and transpiration from land plants and animals into air.
*precipitation, from water vapor condensing from the air and falling to earth or ocean.
*runoff from the land usually reaching the sea.
Most water vapor over the oceans returns to the oceans, but winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tonne#Multiples|Tt per year. Over land, evaporation and transpiration contribute another 71 Tt per year. Precipitation, at a rate of 107 Tt per year over land, has several forms: most commonly rain, snow, and hail, with some contribution from fog and dew. Condensed water in the air may also refract sunlight to produce rainbows.

Water runoff often collects over watersheds flowing into rivers. A mathematical model used to simulate river or stream flow and calculate water quality parameters is hydrological transport model. Some of water is diverted to irrigation for agriculture. Rivers and seas offer opportunity 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. It is when a river overflows its banks or flood from the sea. A 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.

===Tides===

'''Tides''' are the cyclic rising and falling of Earth's ocean surface 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 biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. 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). Water is thus essential and central to these metabolic processes. Therefore, without water, these metabolic processes would cease to exist, leaving us to muse about what processes would be in its place, such as gas absorption, dust collection, etc.

Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO<sub>2</sub> (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 CO<sub>2</sub> in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H<sup>+</sup>, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH<sup>−</sup>) 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 bases have values greater than 7. Stomach acid (HCl) is useful to digestion. However, its corrosive effect on the esophagus during reflux can temporarily be neutralized by ingestion of a base such as aluminum hydroxide to produce the neutral molecules water and the salt aluminum chloride. Human biochemistry that involves enzymes usually performs optimally around a biologically neutral pH of 7.4.

For example a cell of ''Escherichia coli'' contains 70% of water, a human body 60–70%, plant body up to 90% and the body of an adult jellyfish is made up of 94–98% water.

===Aquatic life forms===

Earth's 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 that also live in the water. 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 animals 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 seals need to surface periodically to breathe air. Smaller life forms are able to absorb oxygen through their skin.

==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. Large metropolises like Rotterdam, London, Montreal, Paris, New York City, 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===

Water fit for human consumption is called drinking water or potable water. Water that is not potable can be made potable by filtration or distillation (heating it until it becomes water vapor, and then capturing the vapor without any of the impurities it leaves behind), or by other methods (chemical or heat treatment that kills bacteria). Sometimes the term safe water is applied to potable water of a lower quality threshold (i.e., it is used effectively for nutrition in humans that have weak access to water cleaning processes, and does more good than harm). Water that is not fit for drinking but is not harmful for 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).

===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 to the temperature of the 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 how massive one liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0&nbsp;°C—a highly stable ''temperature'' point—the scientists chose to redefine the standard and to perform their measurements at the most stable ''density'' point: the temperature at which water reaches maximum density, which was measured at the time as 4&nbsp;°C

The Kelvin temperature scale of the SI system is based on the triple point of water, defined as exactly 273.16 K or 0.01 °C. The scale is a more accurate development of the Celsius temperature scale, which is defined by the boiling point (100 °C) and melting point (0 °C) of water.

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

===As a heat transfer fluid===

Water and steam are used as heat transfer fluids in diverse heat exchange systems, due to its availability and high heat capacity, both as a coolant and for heating. Cool water may even be naturally available from a lake or the sea. Condensing steam is a particularly efficient heating fluid because of the large heat of vaporization. A disadvantage is that water and steam are somewhat corrosive. In almost all electric power plants, water is the coolant, which vaporizes and drives steam turbines to drive generators.

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

===Chemical uses===
Organic reactions are usually quenched with water or a water solution of a suitable acid, base or buffer. Water is generally effective in removing inorganic salts. In inorganic reactions, water is a common solvent. In organic reactions, it is usually not 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.

==Religion, philosophy, and literature==

Water is considered a purifier in most religions. Major faiths that incorporate ritual washing (ablution) include Christianity, Hinduism, Rastafarianism, Islam, Shinto, Taoism, and Judaism. 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 Judaism (''mikvah'') and Sikhism (''Amrit Sanskar''). In addition, a ritual bath in pure water is performed for the dead in many religions including Judaism and Islam. In Islam, the five daily prayers can be done in Tayammum|most cases after completing washing certain parts of the body using clean water (''wudu''). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of ''misogi''). Water is mentioned in the Bible 442 times in the New International Version and 363 times in the King James Version: 2 Peter 3:5(b) states, "The earth was formed out of water and by water" (NIV).

Some faiths use water especially prepared for religious purposes (holy water in some Christian denominations, ''Amrita'' in Sikhism and Hinduism). Many religions also consider particular sources or bodies of water to be sacred or at least auspicious; examples include Lourdes in Roman Catholicism, the Jordan River (at least symbolically) in some Christian churches, the Zamzam Well in Islam and the River Ganges (among many others) in Hinduism.

Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganga in Hinduism|Ganges is also personified as a goddess, while Saraswati have been referred to as goddess in Vedas. Also water is one of the "panch-tatva"s (basic 5 elements, others including fire, earth, space, air). Alternatively, gods can be patrons of particular springs, rivers, or lakes: for example in Greek mythology|Greek and Roman mythology|Roman mythology, Peneus was a river god, one of the three thousand Oceanids. In Islam, not only does water give life, but every life is itself made of water: "We made from water every living thing". (Sura of Al-Anbiya 21:30)

The Ancient Greek philosopher Empedocles held that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic substance of the universe. Water was considered cold and moist. In the theory of the four four bodily humors, water was associated with phlegm. Water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

Water also plays an important role in literature as a symbol of purification. Examples include the critical importance of a river in As I Lay Dying'' by William Faulkner and the drowning of Ophelia in ''Hamlet''.

[[Category: Science]]