'''पाणी''' (H<sub>2</sub>O) हे अणूंपासून बनलेला पदार्थ आहे. सामान्य तापमानाला पाणी द्रव स्वरुपात असते. पाण्याला वास व चव नसते. पृथ्वीवर पाणी विपुल प्रमाणात आढळते. 

thumb|पाण्याचा थेंब

पाणी हे रंगहीन असून वनस्पतींच्या सर्व जैविक प्रक्रियेत आवश्यक असते.  पाण्याच्या घन स्वरुपाला बर्फ व वायुरुपाला वाफ असे संबोधतात. अनेक पदार्थ पाण्यात विरघळतात व म्हणून पाण्याला वैश्विक द्रावक (Universal solvent) असे संबोधतात.

वर्ग:रसायनशास्त्र
वर्ग:पाणी
वर्ग:निसर्ग

als:Wasser
am:ውሃ

arc:ܡܝܐ

ay:Uma

ba:Һыу
bar:Wossa
bat-smg:Ondou
be-x-old:Вада

bm:Ji

cdo:Cūi

ch:Hånom
chr:ᎠᎹ
cr:ᓃᐲᔾ

cv:Шыв

eml:Aqua

eo:Akvo
es:Agua

fur:Aghe
ga:Uisce
gan:水
gd:Uisge

gn:Y
gv:Ushtey
hak:Súi

hsb:Woda

hy:Ջուր
ia:Aqua

ie:Aqua

jbo:djacu

right|350px|thumb|Water in three states: liquid, solid ([[ice), and (invisible) vapor in air. condensed from water vapor.]]

'''Water''' is a common state, but the substance also has a petatonnes&nbsp;(Pt) of water covers 71% of the air as precipitation.<ref>
[http://www.agu.org/sci_soc/mockler.html Water Vapor in the Climate System], Special Report, [AGU], December 1995 (linked 4/2007). [http://www.unep.org/dewa/assessments/ecosystems/water/vitalwater/ Vital Water] 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%. Some 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 runoff, usually reaching the developing countries - there is a water crisis, and it is estimated that by 2025 more than half of the world population will be facing water-based vulnerability.<ref name=Kulshreshtha1998></ref> Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of freshwater is consumed by agriculture.<ref name=Baroni2007></ref>

== Types of water ==
thumb|125px|Liquid water in motion

Water can appear in three phases. 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 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 to be able to evaluate the potability of water: animals generally dislike the taste of microbes. The taste advertised in microbes.

Different names are given to water's various forms:
right|thumb|125px|''[[Snowflakes'' by Wilson Bentley, 1902]]
*'''according to phase'''
**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 phase
|-
|&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 water
** fresh 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
** nuclear reactors.
** tritiated water

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

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

== Chemical and physical properties ==
{|align="right" border="1" cellspacing="0" cellpadding="3" style="margin: 0 0 0 0.5em; background: #FFFFFF; border-collapse: collapse; border-color: #C0C090;"
!  | <big>Water</big>
|-
| align="center" colspan="2" bgcolor="#ffffff" | 135px|The dimensions and geometric structure of a water molecule110px|This space-filled model shows the molecular structure of water.
Water is a necessary compound on the earth's surface.
|-
!  | Information and properties
|-
| Common name
| water
|-
| IUPAC name
| [http://www.acdlabs.com/iupac/nomenclature/93/r93_182.htm oxidane]
|-
| Alternative names
| aqua, more)
|-
| Molecular formula
| H<sub>2</sub>O
|-
| CAS number
| 7732-18-5
|-
| InChI
| InChI=1/H2O/h1H2
|-
| Molar mass
| 18.0153 g/mol
|-
| 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 °C (273.15 K) (32 °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>
|-
!  | Supplementary data page
|-
| align="center" cellspacing="3" style="border: 1px solid #C0C090; background-color: #F8EABA; margin-bottom: 3px;" colspan="2" |<small>Disclaimer and references</small>
|-
|}

left|thumb|model of [[hydrogen bonds between molecules of water]]
left|thumb|Impact from a water drop causes an upward "rebound" jet surrounded by circular [[capillary waves.]]
<!-- Image with unknown copyright status removed: thumb|left|Droplet frozen by flash. www.liquidartgallery.com -->
thumb|left|[[Dew drops adhering to a spider web]]
left|thumb|capillary action of water compared to [[Mercury (element)|mercury]]

Water is the H<sub>2</sub>O''': one 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 are, 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.<ref></ref>

* Water is transparent, and thus absorbed.

* Water is a liquid under standard conditions.

* Since oxygen has a higher 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 , compared to  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 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 salts, 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 <ref>Kotz, J. C., Treichel, P., & Weaver, G. C. (2005). Chemistry & Chemical Reactivity. Thomson Brooks/Cole.</ref>. 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 .

* 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. 

right|thumb|[[European Agreement concerning the International Carriage of Dangerous Goods by Road|ADR label for transporting goods dangerously reactive with water]]
* Some substances (sodium, lithium, calcium, potassium) emit a flammable gas (hydrogen) when wet, or react violently with water.

== 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.<ref>Gary Melnick, Harvard-Smithsonian Center for Astrophysics and David Neufeld, Johns Hopkins University quoted in:

(linked 4/2007)
</ref>

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, a large amounts of water in Mercury's exosphere<ref name="planetary society"></ref>
* 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
* ices only
* Enceladus (moon of Saturn) - 91% in the atmosphere
* exoplanets known as HD 189733 b<ref>[http://www.time.com/time/health/article/0,8599,1642811,00.html Water Found on Distant Planet] July 12, 2007 By Laura Blue, Time</ref> and HD 209458 b.<ref name="Space.com water"> [http://www.space.com/scienceastronomy/070410_water_exoplanet.html Water Found in Extrasolar Planet's Atmosphere] - Space.com</ref>

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<ref>http://www.spiegel.de/wissenschaft/weltall/0,1518,564911,00.html</ref>.

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
* Europa
* 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===
thumb|450px|right|The [[Solar System along center row range of possible habitable zones of varying size stars.]]
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 8 million kilometres or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.<ref>J. C. I. Dooge. "Integrated Management of Water Resources". in E. Ehlers, T. Krafft. (eds.) ''Understanding the Earth System: compartments, processes, and interactions. Springer, '''2001''', p. 116. More references are at the end of the article "Habitable Zone" at [http://www.daviddarling.info/encyclopedia/H/habzone.html The Encyclopedia of Astrobiology, Astronomy and Spaceflight].
</ref>

Earth's mass allows atmosphere. Water vapor and carbon dioxide in the atmosphere provide a 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.<!-- This refrence or common external link does not work now [http://www.cnn.com/2007/TECH/space/05/16/odd.exoplanet.reut/index.html]-->

There are various theories about origin of water on Earth.

==Water on Earth==

right|thumb|Water covers 71% of the Earth's surface; the [[oceans contain 97.2% of the Earth's water. The Antarctic ice sheet, which contains 90% of all fresh water on Earth, is visible at the bottom. Condensed atmospheric water can be seen as clouds, contributing to the Earth's albedo.]]
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 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 phases. It also exists as groundwater in aquifers.

The most important geological processes caused by water are: chemical weathering, water erosion, water ice erosion and sedimentation by glacier.

=== Water cycle ===

right|thumb|[[water cycle]]

The 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 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 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.

===Fresh water storage===

Some runoff water is trapped for periods, for example in lakes.
At high altitude, during winter, and in the far north and south, snow collects in ice caps, snow pack and glaciers.
Water also infiltrates the ground and goes into aquifers. This springs, or more spectacularly in wells.
This water storage is important, since clean, fresh water is essential to human and other land-based life. In many parts of the world, it is in short supply. 

===Tides===
{{imageframe
|width=240
|content=120px120px
|caption=High tide (left) and low tide (right).
}}

'''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 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 ==
thumb|An [[oasis is an isolated water source with vegetation in desert]]
thumb|right|Some of the [[biodiversity of a coral reef]]
thumb|right|Water reflecting light in [[Crissy Field]]

From a biological standpoint, water has many distinct properties that are critical for the proliferation of 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===

thumb|left|Some marine [[diatoms - a key phytoplankton group]]
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 seals need to surface periodically to breathe air. Smaller life forms are able to absorb oxygen through their skin.

==Effects on human civilization==
thumb|right| Water [[Fountain]]
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===
thumb|[[Environmental Scientist sampling water.]]
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).

This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about 1&nbsp;billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and 2003 G8 Evian summit.<ref> [http://www.g8.fr/evian/english/navigation/2003_g8_summit/summit_documents/water_-_a_g8_action_plan.html G8 "Action plan" decided upon at the 2003 Evian summit] </ref> Even if this difficult goal is met, it will still leave more than an estimated half a billion people without access to safe drinking water and over 1&nbsp;billion without access to adequate sanitation. Poor water quality and bad sanitation are deadly; some 5&nbsp;million deaths a year are caused by polluted drinking water. Water, however, is not a finite resource, but rather re-circulated as potable water in precipitation in quantities many degrees of magnitude higher than human consumption. Therefore, it is the relatively small quantity of water in reserve in the earth (about 1% of our drinking water supply, which is replenished in aquifers around every 1 to 10 years), that is a non-renewable resource, and it is, rather, the distribution of potable and irrigation water which is scarce, rather than the actual amount of it that exists on the earth. Water-poor countries use importation of goods as the primary method of importing water (to leave enough for local human consumption), since the manufacturing process uses around 10 to 100 times products' masses in water.

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

===Human uses===
====Agriculture====
thumb|right|[[irrigation of field crops]]
The most important use of water in agriculture is for an irrigation and irrigation is key component to produce enough food. Irrigation takes up to 90% of water withdrawn in some developing countries.<ref>[http://www.wbcsd.org/includes/getTarget.asp?type=d&id=MTYyNTA WBCSD Water Faacts & Trends]</ref>

====As a scientific standard====
On liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0&nbsp;°C&mdash;a highly stable ''temperature'' point&mdash;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.<ref>''[http://histoire.du.metre.free.fr/fr/index.htm here L'Histoire Du Mètre, La Détermination De L'Unité De Poids]''</ref>

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.

====For drinking====
thumb|right|A young girl drinking [[bottled water.]]

The human body is anywhere from 55% to 78% water depending on body size.<ref> [http://www.madsci.org/posts/archives/2000-05/958588306.An.r.html Re: What percentage of the human body is composed of water?] Jeffrey Utz, M.D., The MadSci Network </ref> To function properly, the body requires between one and seven liters 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 most advocates agree that 6–7 glasses of water (approximately 2 litres) daily is the minimum to maintain proper hydration.<ref></ref> 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.<ref name=Rhoades_2003></ref> For those who have healthy kidneys, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication (hyperhydration), which can be fatal. The "fact" that a person should consume eight glasses of water per day cannot be traced back to a scientific source.<ref>[http://ajpregu.physiology.org/cgi/content/full/283/5/R993 "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 × 8"?] by Heinz Valdin, Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire</ref> There are other myths such as the effect of water on weight loss and constipation that have been dispelled.<ref> [http://www.factsmart.org/h2o/h2o.htm Drinking Water - How Much?], Factsmart.org web site and references within</ref>

An original recommendation for water intake in 1945 by the pregnant and 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.

thumb|right|[[Hazard symbol for ''No drinking water'']]
Humans require water that does not contain too many impurities. Common impurities include metal salts and/or harmful bacteria, such as ''Vibrio''. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes. <ref>{{cite book
  | last = Maton
  | first = Anthea
  | authorlink = 
  | coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright
  | title = Human Biology and Health
  | publisher = Prentice Hall
  | date = 1993
  | location = Englewood Cliffs, New Jersey, USA
  | pages = 
  | url = 
  | doi = 
  | id = 
  | isbn = 0-13-981176-1}}</ref>