The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (February 2012)

Indoor Tap – commonly found in the bathroom or kitchen. This tap is a single-handle, double-spout tap (one spout for hot water, one spout for cold water).

Tap water (running water, city water, municipal water, etc.) is potable water supplied to a tap (valve) inside the household or workplace. It is a principal component of "indoor plumbing", which became available in urban areas of the developed world during the last quarter of the 19th century, and common during the mid-20th century. The application of technologies involved in providing clean (potable) water to homes, businesses and public buildings is a major subfield of sanitary engineering. Calling potable water "tap water" distinguishes it from the other main types of potable water, such as water from rainwater-collecting cisterns, from village pumps (town pumps), or from streams, rivers, or lakes (whose potability varies).

Background[link]

The availability of tap water has major public health benefits, since it typically vastly reduces the risk to the public of contracting water-borne diseases. Providing tap water to large urban or suburban populations requires a complex and carefully designed system of collection, storage, treatment and distribution, and is commonly the responsibility of a government agency, often the same agency responsible for the removal and treatment of wastewater.

Specific chemical compounds are often added to tap water during the treatment process to adjust the pH or remove contaminants, as well as chlorine to kill biological toxins. Local geological conditions affecting groundwater are determining factors for the presence of various metal ions, often rendering the water "soft" or "hard".

Tap water remains susceptible to biological or chemical contamination. In the event of contamination deemed dangerous to public health, government officials typically issue an advisory regarding water consumption. In the case of biological contamination, residents are usually advised to boil their water before consumption or to use bottled water as an alternative. In the case of chemical contamination, residents may be advised to refrain from consuming tap water entirely until the matter is resolved.

In many areas a compound of fluoride is added to tap water in an effort to improve dental health among the public. In some communities "fluoridation" remains a controversial issue. (See water fluoridation controversy.)

Potable water supply[link]

This supply may come from several possible sources.

• Municipal water supply
• Water wells
• Delivered by truck
• Processed water from creeks, streams, rivers, lakes, rainwater, etc.

Domestic water systems have been evolving since people first located their homes near a running water supply, e.g., a stream or river. The water flow also allowed sending waste water away from the domiciles.

Modern indoor plumbing delivers clean, safe, potable water to each service point in the distribution system. It is imperative that the clean water not be contaminated by the waste water (disposal) side of the process system. Historically, this contamination of drinking water has been the largest killer of humans.^[1]

Hot water supply[link]

Domestic hot water is provided by means of water heater appliances, or through district heating. The hot water from these units is then piped to the various fixtures and appliances that require hot water, such as lavatories, sinks, bathtubs, showers, washing machines, and dishwashers.

Fixtures and appliances[link]

Everything in a building that uses water falls under one of two categories; Fixture or Appliance. As the consumption points above perform their function, most produce waste/sewage components that will require removal by the waste/sewage side of the system. The minimum is an air gap. See cross connection control & backflow prevention for an overview of backflow prevention methods and devices currently in use, both through the use of mechanical and physical principles.

Fixtures are devices that use water without an additional source of power.

Pipe materials[link]

It has been suggested that this article or section be merged into Plumbing. (Discuss) Proposed since March 2012.

History[link]

The earliest known evidence of drain tile being used for plumbing was found in Mesopotamia and is estimated to have been made around 3000 BC. The tiles were made from clay mixed with short lengths of straw. Both brass and copper pipes have been found in Egypt believed to have been made close to 2500 BC. The Romans made extensive use of lead pipe by joining sheets of lead into piping to carry their water supply and waste.

During the Dark Ages following the fall of the Roman Empire, plumbing development virtually ceased for centuries except for isolated cases of plumbing installed in palaces and castles. In the 13th century, blacksmiths formed sheets of iron and lap welded the seam to create iron pipe. Though it is unclear as to when galvanized iron pipe was first used, a French chemist named Melouin is credited with developing the process in 1742. The earliest known use for cast iron pipe is for the water supply to a fountain in Langensalza,Germany, built around 1560. In 1819 the first cast iron pipe constructed in the US, was manufactured in Weymouth, New Jersey. Before that time, cast iron pipe and fittings had to be imported from Europe. It was not until the 1960s that the hubless cast iron pipe was brought to the U.S. from Europe by way of Canada.

During the early 1900s, heavy-walled copper joined with threaded fittings was in use, but limited to public buildings because of its high cost. However, during the 1930s light-gauge Copper tube and fittings were developed which made copper economically feasible and increased its popularity. Copper plumbing is bacteriostatic, which means that bacteria can't easily grow in copper pipes.

Polyvinyl chloride (PVC) was produced experimentally in the 19th century but did not become practical to manufacture until 1926, when Waldo Semon of BF Goodrich Co. developed a method to plasticize PVC, making it easier to process. PVC pipe began to be manufactured in the 1940s and was in wide use for Drain-Waste-Vent piping during the reconstruction of Germany and Japan following WWII. In the 1950s, plastics manufacturers in Western Europe and Japan began producing acrylonitrile butadiene styrene (ABS) pipe. The method for producing cross-linked polyethylene (PEX) was also developed in the 1950s. Plastic supply pipes have become increasingly common, with a variety of materials and fittings employed, however plastic water pipes do not keep water as clean as copper and brass piping does.

Plumbing codes define which materials may be used, and all materials used in the US must be proven by ASTM, UL, and/or NFPA testing.

Steel[link]

Galvanized steel potable water supply and distribution pipes are commonly found with nominal diameters from 3/8" to 2". It is rarely used today for new construction residential plumbing. Steel pipe has National Pipe Thread (NPT) standard tapered male threads, which connect with female tapered threads on elbows, tees, couplers, valves, and other fittings. Galvanized steel (often known simply as "galv" or "iron" in the plumbing trade) is relatively expensive, and difficult to work with due to weight and requirement of a pipe threader. It remains in common use for repair of existing "galv" systems and to satisfy building code non-combustibility requirements typically found in hotels, apartment buildings and other commercial applications. It is also extremely durable and resistant to mechanical abuse. Black lacquered steel pipe is the most widely used pipe material for fire sprinklers and natural gas.

Most single family homes' systems typically won't require supply piping larger than 3/4". In addition to expense, another downside is that steel pipe suffers from a tendency to become obstructed due to internal rusting and mineral deposits forming on the inside of the pipe over time, once the internal galvanizing zinc coating has degraded. In potable water distribution service, galvanized steel pipe has a service life of about 30 to 50 years, although it is not uncommon for it to be less in geographic areas with corrosive water contaminants.

Copper[link]

See also Copper tubing

Sizes[link]

Common wall-thicknesses of copper tubing in the USA are "Type K", "Type L" and "Type M":^[2]

• Type K has the thickest wall section of the three types of pressure rated tubing and is commonly used for deep underground burial such as under sidewalks and streets, with a suitable corrosion protection coating or continuous polyethylene sleeve as required by code.

• Type L has a thinner pipe wall section, and is used in residential and commercial water supply and pressure applications.

• Type M has the thinnest wall section, and is generally suitable for condensate and other drains, but sometimes illegal for pressure applications, depending on local codes.

Types K and L are generally available in both hard drawn "sticks" and in rolls of soft annealed tubing, whereas type M is usually only available in hard drawn "sticks".

In the plumbing trade the size of copper tubing is measured by its nominal diameter (average inside diameter). Some American trades, heating and cooling technicians for instance, use the outside diameter (OD) to designate copper tube sizes. The HVAC tradesman also use this different measurement to try and not confuse water pipe with copper pipe used for the HVAC trade, as pipe used in the air-conditioning trade uses copper pipe that is made at the factory without processing oils that would be incompatible with the oils used to lubricate the compressors in the AC system. The OD of copper tube is ¹⁄₈th inch larger than its nominal size. Therefore, 1 inch nominal copper tube and 1 ¹⁄₈th inch ACR tube are exactly the same tube with different size designations. The wall thickness of the tube, as mentioned above, never affects the sizing of the tube. Type K ¹⁄₂ inch nominal tube, is the same size as Type L ¹⁄₂ inch nominal tube (⁵⁄₈ inch ACR).

Common wall-thicknesses in Europe are "Type X", "Type Y" and "Type Z", defined by the EN 1057 standard.

• Type X is the most common, and is used in above groundservices including drinking water supply, hot and cold water systems, sanitation, central heating and other general purpose applications.

• Type Y is a thicker walled pipe, used for underground works and heavy duty requirements including hot and cold water supply, gas reticulation, sanitary plumbing, heating and general engineering.

• Type Z is a thinner walled pipe, also used for above groundservices including drinking water supply, hot and cold water systems, sanitation, central heating and other general purpose applications.

In the plumbing trade the size of copper tubing is measured by its outside diameter in millimetres. Common sizes are 15 mm and 22 mm.^[3]

Thin-walled types used to be relatively inexpensive, but since 2002 copper prices have risen considerably due to rising global demand and a stagnant supply.

Lead leaching[link]

Generally, copper tubes are soldered directly into copper or brass fittings, although compression, crimp, or flare fittings are also used. Formerly, concerns with copper supply tubes included the lead used in the solder at joints (50% tin and 50% lead). Some studies have shown significant "leaching" of the lead into the potable water stream, particularly after long periods of low usage, followed by peak demand periods. In hard water applications, shortly after installation, the interior of the pipes will be coated with the deposited minerals that had been dissolved in the water, and therefore the vast majority of exposed lead is prevented from entering the potable water. Building codes now require lead-free solder. Building Codes throughout the U.S. require the use of virtually "lead-free" (<0.2% lead) solder or filler metals in plumbing fittings and appliances as well.

Corrosion[link]

Copper water tubes are susceptible to: cold water pitting caused by contamination of the pipe interior, typically with soldering flux; erosion corrosion caused by high speed or turbulent flow; and stray current corrosion, caused by poor electrical wiring technique, such as improper grounding and bonding.

Pin holes[link]

Pin-hole leaks can occur if copper piping is improperly grounded or bonded; nonmetal piping, such as Pex or PVC, does not suffer from this problem. The phenomenon is known technically as stray current corrosion or electrolytic pitting. Pin-holing due to poor grounding or poor bonding occurs typically in homes where the original plumbing has been modified; homeowners may find that a new plastic water filtration device or plastic repair union has interrupted the water pipe's electrical continuity to ground, when they start seeing pinhole water leaks after a recent install. Damage occurs rapidly, usually becoming obvious about six months after the ground interruption. Correctly-installed plumbing appliances will have a copper bonding jumper cable connecting the interrupted pipe sections. Pinhole leaks from stray current corrosion can result in thousands of dollars in plumbing bills, and sometimes require the replacement of the entire affected water line. The cause is fundamentally an electrical defect, not a plumbing defect; once the plumbing damage is repaired, an electrician should promptly be consulted to evaluate the grounding and bonding of the entire plumbing and electrical systems.

The difference between a ground and a bond is subtle. See Ground, for a complete description.

Stray current corrosion occurs because: 1) the piping system has been connected accidentally or intentionally to a DC voltage source; 2) the piping does not have metal-to-metal electrical continuity throughout its length; or 3) if the voltage source is AC, one or more naturally-occurring minerals coating the pipe interior may act as a rectifier, converting AC current to DC . The DC voltage forces the water within the piping to act as an electrical conductor (an electrolyte). Electric current leaves the copper pipe, moves though the water across the nonconductive section (a plastic filter housing, for example), and reenters the pipe on the opposite side. Pitting occurs at the electrically negative side (the cathode), which may happen to be either upstream or downstream with respect to the water flow direction. Pitting occurs because the electrical voltage ionizes the pipe's interior copper metal, which reacts chemically with dissolved minerals in the water creating copper salts; these copper salts are soluble in water and wash away. Microscopic pits eventually grow and consolidate to form pin holes. When one is discovered, there are almost certainly more that have not yet leaked. A complete discussion of stray current corrosion can be found in chapter 11, section 11.4.3, of Handbook of Corrosion Engineering, by Pierre Roberge.^[4]

Detecting and eliminating poor bonding is relatively straightforward. Detection is accomplished using a simple DC voltmeter, with test probe leads placed in various locations in the plumbing. Typically, a probe on a hot pipe and a probe on a cold pipe will tell you if there is improper grounding. Anything beyond a few millivolts is signifiant, and potentials of 200 mV are common. A missing bond will show up best in the area of the gap, as the measured electrical potential dissipates over distance. The missing bond is usually located near the cold water inlet to the building, as filtration and treatment equipment are usually added there, but pinhole leaks can occur anywhere downstream or upstream from the interruption of electrical continuity.

Correcting the problem is a simple matter of either purchasing a copper bonding jumper kit, composed of copper cable at least #6 AWG in diameter and two bronze ground clamps for affixing it the plumbing. See NFPA 70, the U.S. National Electrical Code Handbook (NEC), section on bonding and ground for details on selecting the correct bonding conductor wire size.

A similar bonding jumper wire can also be seen crossing gas meters, but for a different reason.^{[further explanation needed]}

However, if homeowners are experiencing shocks or large sparks from plumbing fixtures or pipes, it is more serious than a missing bond. Larger voltages may be caused by a live electrical wire bridging to the plumbing, and improper or missing plumbing system grounding. Such a situation poses an electrical shock hazard and potential fire danger; an electrician should be consulted immediately.

Plastics[link]

Plastic piping placed for a sink

Plastic pipe is in wide use for domestic water supply and drain-waste-vent (DWV) pipe. For example, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene (PP), polybutylene (PB), and polyethylene (PE) may be allowed by code for specific uses. Some examples of plastics in water supply systems are:

• PVC/CPVC - rigid plastic pipes similar to PVC drain pipes but with thicker walls to deal with municipal water pressure, introduced around 1970. PVC should be used only for cold water, or for venting. CPVC can be used for hot and cold potable water supply. Connections are made with primers and solvent cements as required by code.

• PP - The material is used primarily in housewares, food packaging, and clinical equipment,^[5] but since the early 1970s has seen increasing use worldwide for both domestic hot and cold water. PP pipes are heat fused, being unsuitable for the use of glues, solvents, or mechanical fittings. PP pipe is often used in green building projects.^[6][7]

• PBT - flexible (usually gray or black) plastic pipe which is attached to barbed fittings and secured in place with a copper crimp ring. The primary manufacturer of PBT tubing and fittings was driven into bankruptcy by a class-action lawsuit over failures of this system. However, PB and PBT tubing has since returned to the market and codes, typically first for "exposed locations" such as risers.

• PEX - cross-linked polyethylene system with mechanically-joined fittings employing barbs, and crimped steel or copper rings.

• Polytanks - plastic polyethylene cisterns, underground water tanks, above ground water tanks, are usually made of linear polyethylene suitable as a potable water storage tank, provided in white, black or green, approved by NSF and made of FDA approved materials.

• Aqua - known as PEX-Al-PEX, for its PEX/aluminum sandwich, consisting of aluminum pipe sandwiched between layers of PEX, and connected with modified brass compression fittings. In 2005, a large number of these fittings were recalled.^{[further explanation needed]}

Fittings and valves[link]

Potable water supply systems require not only pipe, but also many fittings and valves which add considerably to their functionality as well as cost. The Piping and plumbing fittings and Valves articles discuss them further.

Regulation and compliance[link]

Before a water supply system is constructed or modified, the designer and contractor need to consult the local plumbing code and obtain a building permits prior to construction.^[8][9] Even replacing an existing water heater may require a permit and inspection of the work. NSF 61 is the U.S. national standard for potable water piping guidelines. National and local fire codes should be integrated in the design phase of the water system too to prevent "failure comply with regulations" notices. Some areas of the United States require on-site water reserves of potable and fire water by law.

Waste water[link]

The waste water from the various appliances, fixtures, and taps is transferred to the waste and sewage removal system via the sewage drain system. This system consists of larger diameter piping, water traps, and is well vented to prevent toxic gases from entering the living space. The plumbing drains and vents article discusses the topic further, and introduces sewage treatment.

Tap water versus bottled water[link]

The examples and perspective in this section may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (April 2010)

In modern Western society, levels of contaminants found in tap water vary for every household and plumbing system. A general conception regarding water is that bottled water is designed to be 'cleaner' than conventional tap water. However, in 1999, The Natural Resources Defense Council (NRDC) released controversial findings from a four year study on bottled water. The results of this study claimed that one-third of the waters tested contained levels of contamination—including synthetic organic chemicals, bacteria, and arsenic—in at least one sample that exceeded allowable limits under either state or bottled water industry standards or guidelines.^[10] However, the bottled water industry was quick to dispute the claim saying bottled water is one the most highly regulated food products under the FDA regulatory authority and that the FDA system worked extremely well when coupled with the International Bottled Water Association's Model Code and unannounced inspections.^[11]

Some municipalities in the United States are making an effort to use tap water over bottled water on government properties and events. However, others voted the idea down, including voters in the state of Washington who repealed a bottled water tax via citizen initiative.^[12]

James Workman, author of the book Heart of Dryness: How the Last Bushmen Can Help Us Endure the Coming Age of Permanent Drought and co-founder of SmartMarkets says that he doesn't believe that "tap water is bad and bottled water is good". Rather he cites differences in quality regulations and standards. "Bottled water is often tap water put through another filter and not held to the same quality regulations as public utility water is."^[13]

Chlorine is a disinfectant which is added to tap water in the United States. Chlorine can leave organic material like trihalomethanes and haloacetic acids in the water. The level of chlorine found is small, 1L of chlorinated water gives 0.2 mg of chlorine, which is too small to cause any health problems.^[14]

While most U.S. cities have what is considered safe tap water, contaminants ranging from bacteria to heavy metals are present in some tap water and violations of tap water standards have been well-publicized, such as the severe 1993 Cryptosporidium outbreak in Milwaukee, Wisconsin, which led to several deaths and around 400,000 illnesses (see: Milwaukee Cryptosporidium outbreak). The University of Cincinnati recently completed a Tap Water Quality Analysis, funded by PUR, for major US cities.^[15]

Cloudiness due to dissolved gases[link]

Tap water can sometimes appear cloudy, and this is often mistaken for a mineral impurity in the water. Cloudy water, also known as white water, is actually caused by air bubbles coming out of solution in the water. Because cold water holds more air than warm water, small bubbles will appear in water with a high dissolved oxygen content that is heated or depressurized, because this reduces how much dissolved gas the water can hold. This condition is completely harmless, and the cloudiness of the water disappears quickly as the gas is released from the water.^[16]

See also[link]

Water portal

References[link]

External links[link]

• The Water Information Center - An online resource for public water system basics and water management issues from the National Academy of Sciences.
• US Environmental Protection Agency Drinking water page
• U.S. Centers for Disease Control and Prevention (CDC) Healthy Water - Public Water Systems - One-stop resource for information on public water systems supplying tap water including information on drinking water, fluoridation, water testing, water-related diseases and contaminants, etc., plus links to EPA, WHO, and other resources.
• Bottled Water: Better than the Tap?^{[dead link]} - A Food and Drug Administration site, explains different sources & treatment of water in depth and compares them.
• the International Code Council
• the American Society for Testing and Materials
• the National Ground Water Association
• The Copper Development Association
• 2008 Municipal Water Pricing Report(Canada)
• Notes on Pipe -- Copper Pipe weights and max PSI

Look up tap in Wiktionary, the free dictionary.

Tap or tapped may refer to:

Mechanical and electrical[link]

• Tap (valve), a device for controlling the release of a liquid or gas
• Tap (transformer), part of an electrical device
• A cutting tool, part of a tap and die set

Entertainment[link]

• Tap dance
• Tap!, a magazine for owners of Apple's iOS devices
• Tap (film)
• TAP (novelette), by Greg Egan
• Tapping, a playing technique for stringed instruments
• The Amazing Pudding, a magazine about Pink Floyd
• Topfield Application Program, a software application extending functionality of Topfield TV products
• Tap, a Magic: The Gathering keyword
• Lagging, a method of cheating in online games
• Tapped (film), 2009 documentary about bottled water's negative impact on human health and the environment

Computing and telecommunications[link]

• Tap, to interface with a touchscreen on a mobile device, tablet or monitor
• Tap, the act of transferring data from one computer device to another by tapping the devices together, i.e., NFC
• Network tap, a device that monitors data on a computer network
• Telephone tap, a device that monitors phone conversations
• Tap (signal processing)
• test access port (TAP) in JTAG
• TAP (network driver)
• Telelocator Alphanumeric Protocol, a protocol for sending messages to a cellular or pager service
• Test Anything Protocol, a protocol for communication of software unit testing results
• Time-sharing Assembly Program, an application for the SDS 940 computer
• Test Access Port, part of the Joint Test Action Group boundary scan architecture
• TAP, a phreaking magazine published by the Youth International Party
• Transferred Account Procedure, a GSMA Data Record Format

Transportation[link]

• TAP Portugal, an airline
• Transit Access Pass, a Los Angeles fare card
• Tai Po Market Station, a rail station, MTR code TAP
• Tapachula International Airport, IATA code TAP

Pipelines[link]

• Trans Adriatic Pipeline‎, a proposed pipeline from Greece to Italy via Albania
• Trans-Afghanistan Pipeline, a proposed pipeline from Turkmenistan to India
• Trans-Alaska Pipeline System
• Trans-Arabian Pipeline, a 1947-1990 pipeline from Saudi Arabia to Lebanon

Other uses[link]

• Tap, Azerbaijan
• Táp, Hungary
• TAP Boyz (The Arabian Posse), a street gang
• Tap code, a cipher used by prisoners to communicate
• Tap consonant, a term in phonetics
• TAP Pharmaceutical Products
• Tajna Armia Polska, a WWII Resistance movement
• Tandem Affinity Purification, a technique for studying protein interactions
• Transporter associated with antigen processing, a protein
• Molson Coors Brewing Company, NYSE symbol TAP
• Tasmanians Against the Pulpmill, a group opposing the Bell Bay Pulp Mill
• Tunis Afrique Presse, a Tunisian news agency
• Tuition Assistance Program, New York State

See also[link]

• Taps (disambiguation)
• Tapping (disambiguation)

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

Water in three states: liquid, solid (ice), and (invisible) water vapor in the air. Clouds are accumulations of water droplets, condensed from vapor-saturated air.

Water is a chemical substance with the chemical formula H₂O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.^[1][2] Under nomenclature used to name chemical compounds, dihydrogen monoxide is the scientific name for water, though it is almost never used.^[3]

Water covers 70.9% of the Earth's surface,^[4] and is vital for all known forms of life.^[5] On Earth, 96.5% of the planet's water is found in oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.^[6][7] Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products.^[6]

Water on Earth moves continually through the hydrological cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. Evaporation and transpiration contribute to the precipitation over land.

Safe drinking water is essential to humans and other lifeforms. Access to safe drinking water has improved over the last decades in almost every part of the world, but approximately one billion people still lack access to safe water and over 2.5 billion lack access to adequate sanitation.^[8] There is a clear correlation between access to safe water and GDP per capita.^[9] However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability.^[10] A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.^[11] 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 the fresh water used by humans goes to agriculture.^[12]

Chemical and physical properties[link]

Model of hydrogen bonds (1) between molecules of water

Impact from a water drop causes an upward "rebound" jet surrounded by circular capillary waves.

Snowflakes by Wilson Bentley, 1902

Dew drops adhering to a spider web

Capillary action of water compared to mercury

Water is the chemical substance with chemical formula H₂O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom.

Water appears in nature in all three common states of matter and may take many different forms on Earth: water vapor and clouds in the sky; seawater in the oceans; icebergs in the polar oceans; glaciers and rivers in the mountains; and the liquid in aquifers in the ground.

At high temperatures and pressures, such as in the interior of very large planets, it is argued that water exists as ionic water in which the molecules break down into a soup of hydrogen and oxygen ions, and at even higher pressures as superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.^[13]

The major chemical and physical properties of water are:

• Water is a liquid at standard temperature and pressure. It is tasteless and odorless. The intrinsic colour of water and ice is a very slight blue hue, although both appear colorless in small quantities. Water vapour is essentially invisible as a gas.^[14]
• Water is transparent in the visible electromagnetic spectrum. Thus aquatic plants can live in water because sunlight can reach them. Infrared light is strongly absorbed by the hydrogen-oxygen or OH bonds.
• Since the water molecule is not linear and the oxygen atom has a higher electronegativity than hydrogen atoms, it carries a slight negative charge, whereas the hydrogen atoms are slightly positive. As a result, water is a polar molecule with an electrical dipole moment. Water also can form an unusually large number of intermolecular hydrogen bonds (four) for a molecule of its size. These factors lead to strong attractive forces between molecules of water, giving rise to water's high surface tension^[15] 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.^[16]
• Water is a good solvent and is often referred to as the universal solvent. Substances that 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 are immiscible 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 with the dissolution of a small amount of ionic material such as sodium chloride.
• The boiling point of water (and all other liquids) is dependent on the barometric pressure. For example, on the top of Mt. Everest water boils at 68 °C (154 °F), compared to 100 °C (212 °F) at sea level. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and remain liquid.
• At 4181.3 J/(kg·K), water has a high specific heat capacity, as well as a high heat of vaporization (40.65 kJ·mol⁻¹), 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 occurs at 3.98 °C (39.16 °F).^[17] It has the anomalous property of becoming less dense, not more, when it is cooled down to its solid form, ice. It expands to occupy 9% greater volume in this solid state, which accounts for the fact of ice floating on liquid water, as in icebergs.
• Its density is 1,000 kg/m³ (62.428 lb/cu ft or 8.3454 lb/US gal) liquid (at 4 °C; ice has a density of 917 kg/m³).^[18]
  

  

  ADR label for transporting goods dangerously reactive with water
• Water is miscible with many liquids, such as 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 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 that can be collected when the hydrogen and oxygen recombine.^[19]
• 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 may be violently explosive.

Taste and odor[link]

Water can dissolve many different substances, giving it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water that is too salty or putrid. The taste of spring water and mineral water, often advertised in marketing of consumer products, derives from the minerals dissolved in it. However, pure H₂O is tasteless and odorless. The advertised purity of spring and mineral water refers to absence of toxins, pollutants and microbes, not the absence of naturally occurring minerals.

Distribution in nature[link]

In the universe[link]

Much of the universe's water is 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.^[20]

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."^[21][22]

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. Interstellar clouds eventually condense into solar nebulae and solar systems such as ours.

Water vapor is present in

• Atmosphere of Mercury: 3.4%, and large amounts of water in Mercury's exosphere^[23]
• Atmosphere of Venus: 0.002%
• Earth's atmosphere: ~0.40% over full atmosphere, typically 1–4% at surface
• Atmosphere of Mars: 0.03%
• Atmosphere of Jupiter: 0.0004%
• Atmosphere of Saturn – in ices only
• Enceladus (moon of Saturn): 91%
• exoplanets known as HD 189733 b^[24] and HD 209458 b.^[25]

Liquid water is present on

• Earth: 71% of surface
• Europa: 100 km deep subsurface ocean

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

Water ice is present on

• Earth – mainly as ice sheets
• polar ice caps on Mars
• Moon
• Titan
• Europa
• Saturn's rings^[26]
• Enceladus
• Pluto and Charon^[26]
• Comets and comet source populations (Kuiper belt and Oort cloud objects).

Water ice may be present on Ceres and Tethys. 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 down as superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.^[13]

Some of the Moon's minerals contain water molecules. For instance, in 2008 a laboratory device which ejects and identifies particles found small amounts of the compound in the inside of volcanic rock brought from Moon to Earth by the Apollo 15 crew in 1971.^[27] 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.^[28]

Water and habitable zone[link]

The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are 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 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.^[29][30]

Earth's gravity allows it to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a temperature buffer (greenhouse effect) which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would allow temperature extremes, thus preventing the accumulation of water except in polar ice caps (as on 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^[31] and GJ 1214 b.^[32]

There are various theories about origin of water on Earth.

On Earth[link]

A graphical distribution of the locations of water on Earth.

Water covers 71% of the Earth's surface; the oceans contain 96.5% of the Earth's water. The Antarctic ice sheet, which contains 61% 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 the hydrosphere. Earth's approximate water volume (the total water supply of the world) is 1,338,000,000 km³ (321,000,000 mi³).^[6]

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.

Water is important in many geological processes. Groundwater is present in most rocks, and the pressure of this groundwater affects patterns of faulting. Water in the mantle is responsible for the melt that produces 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 of transported sediment forms many types of sedimentary rocks, which make up the geologic record of Earth history.

Water cycle[link]

Water cycle

The water cycle (known scientifically as the hydrologic cycle) refers to the continuous exchange of water within the hydrosphere, between 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 47 Tt per year. Over land, evaporation and transpiration 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.^[33] 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 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[link]

The Bay of Fundy at high tide (left) and low tide (right)

Some runoff water is trapped for periods of time, 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 groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially 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.

Sea water[link]

Sea water contains about 3.5% salt on average, plus smaller amounts of other substances. The physical properties of sea water differ from fresh water in some important respects. It freezes at a lower temperature (about −1.9 °C) 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.

Tides[link]

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 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[link]

Overview of photosynthesis and respiration. Water (at right), together with carbon dioxide (CO₂), form oxygen and organic compounds (at left), which can be respired to water and (CO₂).

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). 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 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 bases have values greater than 7.

Some of the biodiversity of a coral reef

Aquatic life forms[link]

Some marine diatoms – a key phytoplankton group

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 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 insect and mollusc siphons) and gills (Carcinus). However as invertebrate life evolved in an aquatic habitat most have little or no specialisation for respiration in water.

Effects on human civilization[link]

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, Buenos Aires, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore, have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.

Health and pollution[link]

Environmental Science Program, Iowa State University student sampling water.

Water fit for human consumption is called drinking water or potable water. Water that is not potable may be made potable by filtration or distillation, or by a range of other methods.

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). Water for bathing may be maintained in satisfactory microbiological condition using chemical disinfectants such as chlorine or ozone or by the use of ultraviolet light.

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

This natural resource is becoming scarcer in certain places, and its availability is a major social and economic concern. Currently, about a billion people around the world routinely drink unhealthy water. Most countries accepted the goal of halving by 2015 the number of people worldwide who do not have access to safe water and sanitation during the 2003 G8 Evian summit.^[34] 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 a billion without access to adequate sanitation. Poor water quality and bad sanitation are deadly; some five million deaths a year are caused by polluted drinking water. The World Health Organization estimates that safe water could prevent 1.4 million child deaths from diarrhea each year.^[35] 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.^[36] 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.^[37] The strain not only affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

Human uses[link]

Agriculture[link]

Irrigation of field crops

The most important use of water in agriculture is for irrigation, which is a key component to produce enough food. Irrigation takes up to 90% of water withdrawn in some developing countries^[38] and significant proportions in more economically developed countries (United States, 30% of freshwater usage is for irrigation).^[39] It takes around 3,000 litres of water, converted from liquid to vapour, to produce enough food to satisfy one person's daily dietary need. This is a considerable amount, when compared to that required for drinking, which is between two and five litres. To produce food for the 6.5 billion or so people who inhabit the planet today requires the water that would fill a canal ten metres deep, 100 metres wide and 7.1 million kilometres long – that's enough to circle the globe 180 times.

Fifty years ago, the common perception was that water was an infinite resource. At this time, there were fewer than half the current number of people on the planet. People were not as wealthy as today, consumed fewer calories and ate less meat, so less water was needed to produce their food. They required a third of the volume of water we presently take from rivers. Today, the competition for the fixed amount of water resources is much more intense, giving rise to the concept of peak water.^[40] This is because there are now nearly seven billion people on the planet, their consumption of water-thirsty meat and vegetables is rising, and there is increasing competition for water from industry, urbanisation and biofuel crops. In future, even more water will be needed to produce food because the Earth's population is forecast to rise to 9 billion by 2050.^[41] An additional 2.5 or 3 billion people, choosing to eat fewer cereals and more meat and vegetables could add an additional five million kilometres to the virtual canal mentioned above.

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.^[42] 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 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 industry and cities find ways to use water more efficiently.^[43]

As a scientific standard[link]

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."^[44] For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely the mass of one liter of water. In spite of the fact that the decreed definition of the gram specified water at 0 °C — a highly reproducible temperature — the scientists chose to redefine the standard and to perform their measurements at the temperature of highest water density, which was measured at the time as 4 °C (39 °F).^[45]

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 an absolute temperature scale with the same increment as the Celsius temperature scale, which was originally defined according the boiling point (set to 100 °C) and melting point (set to 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, standard water is defined in the Vienna Standard Mean Ocean Water specification.

For drinking[link]

A young girl drinking bottled water

Water quality: fraction of population using improved water sources by country

The human body contains from 55% to 78% water, depending on body size.^[46] 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 approximately 2 liters (6 to 7 glasses) of water daily is the minimum to maintain proper hydration.^[47] 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.^[48] 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.^[49][50] The popular claim that "a person should consume eight glasses of water per day" seems to have no real basis in science.^[51] Similar misconceptions concerning the effect of water on weight loss and constipation have also been dispelled.^[52]

Hazard symbol for non-potable water

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."^[53] The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 3.7 liters for men and 2.7 liters of water total for women.^[54] Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated. The Institute of Medicine (U.S.) recommends that, on average, men consume 3.0 liters and women 2.2 liters; pregnant women should increase intake to 2.4 liters (10 cups) and breastfeeding women should get 3 liters (12 cups), since an especially large amount of fluid is lost during nursing.^[55] Also noted is that normally, about 20% of water intake comes from food, while the rest comes from drinking water and beverages (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,^[56] and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for taste enhancement and to provide needed electrolytes.^[57]

The single largest (by volume) freshwater resource suitable for drinking is Lake Baikal in Siberia.^[58]

Washing[link]

The propensity of water to form solutions and emulsions is useful in various washing processes. Many industrial processes rely on reactions using chemicals dissolved in water, suspension of solids in water slurries or using water to dissolve and extract substances. Washing is also an important component of several aspects of personal body hygiene.

Transportation[link]

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[link]

Water is widely used in chemical reactions as a solvent or reactant and less commonly as a solute or catalyst. In inorganic reactions, water is a common solvent, dissolving many ionic compounds. 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.

Heat exchange[link]

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 stations, water is the coolant, which vaporizes and drives steam turbines to drive generators. In the U.S., cooling power plants is the largest use of water.^[39]

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

Fire extinction[link]

Water is used for fighting wildfires.

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

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

The power of such explosions was seen in the Chernobyl disaster, although the water involved did not come from fire-fighting at that time but 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 reaction between steam and hot zirconium.

Recreation[link]

Grand Anse Beach, St. George's, Grenada, West Indies.

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, waterskiing, boating, surfing and 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 life in aquariums or ponds for show, fun, and companionship. Humans also use water for snow sports i.e. skiing, sledding, snowmobiling or snowboarding, which requires the water to be frozen.

Water industry[link]

A manual water pump in China

Water purification facility

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 network, water purification facilities, water tanks, water towers, water pipes including old aqueducts. Atmospheric water generators are in development.

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

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

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

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 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 victim of this pollution. Pharmaceuticals consumed by humans often end up in the waterways and can have detrimental effects on aquatic life if they bioaccumulate and if they are not biodegradable.

Wastewater facilities are storm sewers and wastewater treatment plants. Another way to remove pollution from surface runoff water is bioswale.

Industrial applications[link]

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.

Three Gorges Dam is the largest hydro-electric power station.

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

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

Food processing[link]

Water can be used to cook foods such as noodles.

Water plays many critical roles within the field of food science. It is important for a food scientist to understand the roles that water plays within food processing to ensure the success of their products.

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 which occurs at higher elevations. One mole of sucrose (sugar) per kilogram of water raises the boiling point of water by 0.51 °C, and one mole of salt per kg raises the boiling point by 1.02 °C; similarly, increasing the number of dissolved particles lowers water's freezing point.^[59] Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food.^[60] Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.^[59] Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity.^[60] 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. It can dramatically affect the quality of a product as well as playing a role in sanitation. Water hardness is classified based on the amounts of removable calcium carbonate salt it contains per gallon. Water hardness is measured in grains; 0.064 g calcium carbonate is equivalent to one grain of hardness.^[59] Water is classified as soft if it contains 1 to 4 grains, medium if it contains 5 to 10 grains and hard if it contains 11 to 20 grains.^{[vague] [59]} The hardness of water may be altered or treated by using a chemical ion exchange system. The hardness of water also affects its pH balance which plays a critical role in food processing. For example, hard water prevents successful production of clear beverages. Water hardness also affects sanitation; with increasing hardness, there is a loss of effectiveness for its use as a sanitizer.^[59]

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 law, water politics and water crisis[link]

An estimate of the share of people in developing countries with access to potable water 1970–2000

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

1.6 billion people have gained access to a safe water source since 1990.^[61] The proportion of people in developing countries with access to safe water is calculated to have improved from 30% in 1970^[62] to 71% in 1990, 79% in 2000 and 84% in 2004. This trend is projected to continue.^[8] To halve, by 2015, the proportion of people without sustainable access to safe drinking water is one of the Millennium Development Goals. This goal is projected to be reached.

A 2006 United Nations report stated that "there is enough water for everyone", but that access to it is hampered by mismanagement and corruption.^[63] In addition, global initiatives to improve the efficiency of aid delivery, such as the Paris Declaration on Aid Effectiveness, have not been taken up by water sector donors as effectively as they have in education and health, potentially leaving multiple donors working on overlapping projects and recipient governments without empowerment to act.^[64]

The authors of the 2007 Comprehensive Assessment of Water Management in Agriculture cited poor governance as one reason for some forms of water scarcity. Water governance is the set of formal and informal processes through which decisions related to water management are made. Good water governance is primarily about knowing what processes work best in a particular physical and socioeconomic context. Mistakes have sometimes been made by trying to apply 'blueprints' that work in the developed world to developing world locations and contexts. The Mekong river is one example; a review by the International Water Management Institute of policies in six countries that rely on the Mekong river for water found that thorough and transparent cost-benefit analyses and environmental impact assessments were rarely undertaken. They also discovered that Cambodia's draft water law was much more complex than it needed to be.^[65]

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

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

Water used in the production of a good or service is virtual water.

In culture[link]

Religion[link]

Water is considered a purifier in most religions. Major faiths that incorporate ritual washing (ablution) include Christianity, Hinduism, Islam, Judaism, 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 completing 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). Water is mentioned numerous times in the Bible, for example: "The earth was formed out of water and by water" (NIV). In the Qur'an it is stated that "Living things are made of water" and it is often used to describe paradise.

Philosophy[link]

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 bodily humors, water was associated with phlegm. The classical element of Water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

Water is also taken as a role model in some parts of traditional and popular Asian philosophy. 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."^[66]

Literature[link]

Water is used 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.

Sherlock Holmes held that "From a drop of water, a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other."^[67]

See also[link]

	Water portal
	Sustainable development portal

• The water (data page) is a collection of the chemical and physical properties of water.

Water is described in many terms and contexts:

Liquid water and ice structures

• according to state
  • solid – ice
  • liquid – water
  • gaseous – water vapor
  • plasma

• according to meteorology:
  • hydrometeor
    • precipitation

	precipitation according to movement		precipitation according to state
	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		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
    • mist
  • ascending particles (drifted by wind)
    • spindrift
    • stirred snow
• according to occurrence
  • groundwater
  • meltwater
  • meteoric water
  • connate water
  • fresh water
  • surface water
  • mineral water – contains many minerals
  • brackish water
  • dead water – strange phenomenon which can occur when a layer of fresh or brackish water rests on top of denser 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
    • reverse osmosis plant water

• according to other features
  • soft water – contains fewer 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 reactors.
  • tritiated water

• according to microbiology
  • drinking water
  • wastewater
  • stormwater or surface water

• according to religion
  • holy water

Other topics[link]

• Dihydrogen monoxide hoax
• Water Pasteurization Indicator
• Water intoxication
• Water pinch analysis
• Mpemba effect

References[link]