Biology> Chapter 1-Chemicals of Life

Water



This article is about the chemical substance. For a discussion of its properties, see water (molecule). For other uses, see Water (disambiguation).



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


Water is a common chemical substance that is essential to all known forms of life.[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor. About 1,460 teratonnes (Tt) of water covers 71% of the Earth's su***ce, mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.[2] Some of the Earth's water is contained within man-made and natural objects near the Earth's su***ce such as water towers, animal and plant bodies, manufactured products, and food stores.


Saltwater oceans hold 97% of su***ce water, glaciers and polar ice caps 2.4%, and other land su***ce water such as rivers and lakes 0.6%. Water moves continually through a cycle of evaporation or transpiration, precipitation, and runoff, usually reaching the sea. 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 to the precipitation of 107 Tt per year over land. Some water is trapped for varying periods in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land. Clean, fresh water is essential to human and other life. In many parts of the world, it is in short supply. Many organic molecules as well as salts, sugars, acids, alkalis, and some gases (especially oxygen), are soluble in water.

Beyond the Earth, a significant quantity of water is thought to exist underground on the planet Mars, on the moons Europa and Enceladus, and on the exoplanets known as HD 189733 b[3] and HD 209458 b.[4]




Water covers 71% of the Earth's su***ce; 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.



Chemical and physical properties

Water


Water is the base of all life, and
an abundant compound on the earth's su***ce.

Information and properties

Systematic name



water

Alternative names



aqua, dihydrogen monoxide,
hydrogen hydroxide, (more)

Molecular formula



H2O

InChI



InChI=1/H2O/h1H2

Molar mass



18.0153 g/mol

Density and phase



0.998 g/cm³ (liquid at 20 °C)
0.92 g/cm³ (solid)

Melting point



0 °C (273.15 K) (32 °F)

Boiling point



100 °C (373.15 K) (212 °F)

Specific heat capacity



4.184 J/(g·K) (liquid at 20 °C)

Supplementary data page

Disclaimer and references


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 is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue. Ice also appears colorless, and water vapor is essentially invisible as a gas.[5] Water is primarily a liquid under standard conditions, which is not predicted from its relationship to other analogous hydrides of the oxygen family in the periodic table, which are gases such as hydrogen sulfide. Also the elements surrounding oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur and chlorine, all combine with hydrogen to produce gases under standard conditions. The reason that oxygen hydride (water) forms a liquid is that it is more electronegative than all of these elements (other than fluorine). Oxygen attracts electrons much more strongly than hydrogen, resulting in a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom. The presence of a charge on each of these atoms gives each water molecule a net dipole moment. Electrical attraction between water molecules due to this dipole pulls individual molecules closer together, *** it more difficult to separate the molecules and therefore raising the boiling point. This attraction is known as hydrogen bonding. Water can be described as a polar liquid that dissociates disproportionately into the hydronium ion (H3O+(aq)) and an associated hydroxide ion (OH−(aq)). Water is in dynamic equilibrium between the liquid, gas and solid states at standard temperature and pressure, and is the only pure substance found naturally on Earth to be so.


Electronegative Polarity

Water has a partial negative charge (σ-) near the oxygen atom due to the unshared pairs of electrons, and partial positive charges (σ+) near the hydrogen atoms. In water, this happens because the oxygen atom is more electronegative than the hydrogen atoms — that is, it has a stronger "pulling power" on the molecule's electrons, drawing them closer (along with their negative charge) and *** the area around the oxygen atom more negative than the area around both of the hydrogen atoms.


Adhesion



Dew drops adhering to a spider web

Water sticks to itself (cohesion) because it is polar. Water also has high adhesion properties because of its polar nature. On extremely clean/smooth glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. In biological cells and organelles, water is in contact with membrane and protein su***ces that are hydrophilic; that is, su***ces that have a strong attraction to water. Irving Langmuir observed a strong repulsive force between hydrophilic su***ces. To dehydrate hydrophilic su***ces—to remove the strongly held layers of water of hydration—requires doing substantial work against these forces, called hydration forces. These forces are very large but decrease rapidly over a nanometer or less. Their importance in biology has been extensively studied by V. Adrian Parsegian of the National Institute of Health.[6] They are particularly important when cells are dehydrated by exposure to dry atmospheres or to extracellular freezing.


Su***ce tension

Main article: Su***ce tension



This daisy is under the water level, which has risen gently and smoothly. Su***ce tension prevents the water from submerging the flower.

Water has a high su***ce tension caused by the strong cohesion between water molecules. This can be seen when small quantities of water are put onto a non-soluble su***ce such as polythene; the water stays together as drops. Just as significantly, air trapped in su***ce disturbances forms bubbles, which sometimes last long enough to transfer gas molecules to the water. Another su***ce tension effect is capillary waves which are the su***ce ripples that form from around the impact of drops on water su***ces, and some times occur with strong subsu***ce currents flow to the water su***ce. The apparent elasticity caused by su***ce tension drives the waves.


Capillary action

Main article: Capillary action

Capillary action refers to the process of water moving up a narrow tube against the force of gravity. It occurs because water adheres to the sides of the tube, and then su***ce tension tends to straighten the su***ce *** the su***ce rise, and more water is pulled up through cohesion. The process is repeated as the water flows up the tube until there is enough water that gravity can counteract the adhesive force.


Solvation



High concentrations of dissolved lime make the water of Havasu Falls appear turquoise.

Water is a very strong solvent, referred to as the universal solvent, dissolving many types of substances. Substances that will mix well and dissolve in water (e.g. salts) are known as "hydrophilic" (water-loving) substances, while those that do not mix well with water (e.g. fats and oils), are known as "hydrophobic" (water-fearing) substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. If a substance has properties that do not allow it to overcome these strong intermolecular forces, the molecules are "pushed out" from the water, and do not dissolve. Contrary to the common misconception, water and hydrophobic substances does not "repel", and the hydration of a hydrophobic su***ce is energetically, but not entropically, favorable.


Electrical conductivity

Pure water has a low electrical conductivity, but this increases significantly upon solvation of a small amount of ionic material water such as hydrogen chloride. Thus the risks of electrocution are much greater in water with the usual impurities not found in pure water. Any electrical properties observable in water are from the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize where two water molecules become one hydroxide anion and one hydronium cation, but not enough to carry enough electric current to do any work or harm for most operations. In pure water, sensitive equipment can detect a very slight electrical conductivity of 0.055 µS/cm at 25 °C. Water can also be electrolyzed into oxygen and hydrogen gases but in the absence of dissolved ions this is a very slow process, as very little current is conducted. While electrons are the primary charge carriers in water (and metals), in ice (and some other electrolytes), protons are the primary carriers (see proton conductor).


Heavy Water and isotopologues of water

Hydrogen has three isotopes. The most common, *** up more than 95% of water, has 1 proton and 0 neutrons. A second isotope, deuterium (short form "D"), has 1 proton and 1 neutron. Deuterium, D2O, is also known as heavy water and is used in nuclear reactors as a neutron moderator. The third isotope, tritium, has 1 proton and 2 neutrons, and is radioactive, with a half-life of 12.32 years. T2O exists in nature only in tiny quantities, being produced primarily via cosmic ray-driven nuclear reactions in the atmosphere. D2O is stable, but differs from H2O in in that it is more dense - hence, "heavy water" - and in that several other physical properties are slightly different from those of common, Hydrogen-1 containing "light water". D2O occurs naturally in ordinary water in very low concentrations. Consumption of pure isolated D2O may affect biochemical processes - ingestion of large amounts impairs kidney and central nervous system function. However, very large amounts of heavy water must be consumed for any toxicity to be apparent, and smaller quantities can be consumed with no ill effects at all.


Water, ice, and vapor


Heat capacity and heat of vaporization

Main article: Enthalpy of vaporization

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−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.


Freezing point

A simple but environmentally important and unusual property of water is that its usual solid form, ice, floats on its liquid form. This solid state is not as dense as liquid water because of the geometry of the hydrogen bonds which are formed only at lower temperatures. For almost all other substances the solid form has a greater density than the liquid form. Fresh water at standard atmospheric pressure is most dense at 3.98 °C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the su***ce and progress downward, while the majority of the water underneath will hold a constant 4 °C. This effectively insulates a lake floor from the cold. The water will freeze at 0 °C (32 °F, 273 K), however, it can be supercooled in a fluid state down to its crystal homogeneous nucleation at almost 231 K (−42 °C)[7]. Ice also has a number of more exotic phases not commonly seen (go to the full article on Ice).


Triple point

The various triple points of water[8]

Phases in stable equilibrium



Pressure



Temperature

liquid water, ice I, and water vapour



611.73 Pa



273.16 K

liquid water, ice Ih, and ice III



209.9 MPa



251 K (-22 °C)

liquid water, ice Ih, and gaseous water



612 Pa



0.01 °C

liquid water, ice III, and ice V



350.1 MPa



-17.0 °C

liquid water, ice V, and ice VI



632.4 MPa



0.16 °C

ice Ih, Ice II, and ice III



213 MPa



-35 °C

ice II, ice III, and ice V



344 MPa



-24 °C

ice II, ice V, and ice VI



626 MPa



-70 °C


The triple point of water (the single combination of pressure and temperature at which pure liquid water, ice, and water vapor can coexist in a stable equilibrium) is used to define the kelvin, the SI unit of thermodynamic temperature. As a consequence, water's triple point temperature is a prescribed value rather than a measured quantity: 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (approximately 0.0060373 atm). This is approximately the combination that exists with 100% relative humidity at sea level and the freezing point of water.


Although it is commonly named as "the triple point of water", the stable combination of liquid water, ice I, and water vapour is but one of several triple points on the phase diagram of water. Gustav Heinrich Johann Apollon Tammann in Göttingen produced data on several other triple points in the early 20th century. Kamb and others documented further triple points in the 1960s.[9][8][10]


Miscibility and condensation

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.




Red line shows saturation

As a gas, water vapor is completely miscible with air. On the other hand the maximum water vapor pressure that is thermodynamically stable with the liquid (or solid) at a given temperature is relatively low compared with total atmospheric pressure. For example, if the vapor partial pressure[11] is 2% of atmospheric pressure and the air is cooled from 25 °C, starting at about 22 °C water will start to condense, defining the dew point, and creating fog or dew. The reverse process accounts for the fog burning off in the morning. If one raises the humidity at room temperature, say by running a hot shower or a bath, and the temperature stays about the same, the vapor soon reaches the pressure for phase change, and condenses out as steam. A gas in this context is referred to as saturated or 100% relative humidity, when the vapor pressure of water in the air is at the equilibrium with vapor pressure due to (liquid) water; water (or ice, if cool enough) will fail to lose mass through evaporation when exposed to saturated air. Because the amount of water vapor in air is small, relative humidity, the ratio of the partial pressure due to the water vapor to the saturated partial vapor pressure, is much more useful. Water vapor pressure above 100% relative humidity is called super-saturated and can occur if air is rapidly cooled, say by rising suddenly in an updraft.[12]

Water on Earth


Origin and planetary effects



The Solar System along center row range of possible habitable zones of varying size stars.

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.[13]


Solar distance and Earth gravity

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. 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 kilometers or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.[14]


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


It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The su***ce 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 su***ce 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.


Tides


High tide (left) and low tide (right).


Tides are the cyclic rising and falling of Earth's ocean su***ce 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.


Water cycle

The biosphere can be roughly divided into oceans, land, and atmosphere. 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. Some of this 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.

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 groundwater later flows back to the su***ce 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.




Snowflakes by Wilson Bentley, 1902


Forms of water

For more details on this topic, see Category: Forms of water.

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 odours. In fact, humans and other animals have developed senses to be able to evaluate the potability of water: animals generally dislike the taste of salty sea water and the putrid swamps and favor the purer water of a mountain spring or aquifer. The taste advertised in spring water or mineral water derives from the minerals dissolved in it, as pure H2O is tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microbes.


Effects on life



Some of the biodiversity of a coral reef

From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Water is thus essential and central to these metabolic processes. Therefore, without water, these metabolic processes would cease to exist, leaving us to muse about what processes would be in its place, such as gas absorption, dust collection, etc.


Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with 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. 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.


Aquatic life forms



Some marine diatoms - a key phytoplankton group

Earth's waters are filled with life. 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.

Different water creatures have found different solutions to obtaining oxygen in the water. Fish have gills instead of lungs, though some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to su***ce periodically to breathe air.

Effects on human civilization

A shower

Civilization has historically flourished around rivers and major waterways; Mesopotamia, the so-called cradle of civilization, was situated between the major rivers Tigris and Euphrates; the ancient society of the Egyptians depended entirely upon the Nile. Large metropolises like Rotterdam, London, Montreal, Paris, New York City, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like Singapore, have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.


Health and pollution

Water fit for human consumption is called drinking water or potable water. Water that is not potable can be made potable by 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 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.[15] 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 billion without access to adequate sanitation. Poor water quality and bad sanitation are deadly; some 5 million deaths a year are caused by polluted drinking water. Water, however, is not a finite resource (like petroleum is), 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. 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 [citation needed]. The strain not only affects su***ce freshwater bodies like rivers and lakes, but it also degrades groundwater resources.


Human uses

Agriculture

In many developing nations, irrigation accounts for over 90% of water withdrawn from available sources for use. In England where rain is abundant year round, water used for agriculture accounts for less than 1% of human usage. Yet even on the same continent, water used for irrigation in Spain, Portugal and Greece exceeds 70% of total usage. Irrigation has been a key component of the green revolution that has enabled many developing countries to produce enough food to feed everyone. More water will be needed to produce more food for 3 billion more people. But increasing competition for water and inefficient irrigation practices could constrain future food production. Globally, roughly 15-35% of irrigation withdrawals are estimated to be unsustainable. The map indicates where there is insufficient freshwater to fully satisfy irrigated crop demands.


As a scientific standard

On 7 April 1795, the gram was defined in France to be equal to "the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, and to the temperature of the melting ice."For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely how massive one liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0 °C—a highly stable temperature point—the scientists chose to redefine the standard and to perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C.


Neutral pH is defined as the natural pH of pure water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is a also small quantity of heavier 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



A manual water pump in China

Main article: Drinking water

About 70% of the fat free mass of the human body is made of water.[citation needed] 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 experts agree that 8–10 glasses of water (approximately 2 liters) daily is the minimum to maintain proper hydration.[19] 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, 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.[20] There are other myths such as the effect of water on weight loss and constipation that have been dispelled.[21]

Original recommendation for water intake in 1945 by the Food and Nutrition Board of the 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."[22] The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men.[23] Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated. According to the Institute of Medicine—who recommend that, on average, women consume 2.2 litres and men 3.0 litres—this is recommended to be 2.4 litres (approx. 9 cups) for pregnant women and 3 litres (approx. 12.5 cups) for breastfeeding women since an especially large amount of fluid is lost during nursing.[24] Also noted is that normally, about 20 percent 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 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.

The single largest freshwater resource suitable for drinking is Lake Baikal in Siberia, which has a very low salt and calcium content and is very clean.

As a solvent

Dissolving (or suspending) is used to wash everyday items such as the human body, clothes, floors, cars, food, and pets. Also, human wastes are carried by water in the sewage system. Its use as a cleaning solvent consumes most of water in industrialized countries.

An advantage of water is that biological processing of wastewater is easy and treated wastewater can be safely led into the sea or a river. However, if the contaminants are toxic and not processable biologically, wastewater has to be incinerated, which is expensive.

As a heat transfer fluid

Water and steam are commonly used as heat transfer fluids in diverse heat exchanger systems, because of the 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 power station, water is the coolant, which vaporizes and drives steam turbines to generate electricity.

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

Extinguishing fires

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. However, water cannot be used to fight fires of electric equipment, because impure water is electrically conductive, or oils and organic solvents, because they float on water and the explosive boiling of water tends to spread the burning liquid.

Decomposition of water that has been realized in the Chernobyl disaster. Initially, cooling of the incandescent reactor was attempted, but the result was an explosion, when the extreme heat decomposed water into hydrogen and oxygen, which subsequently exploded.

Chemical uses

Organic reactions are usually quenched with water or a water solution of a suitable acid, base or buffer. Water is generally effective in removing inorganic salts. In inorganic reactions, water is a common solvent. In organic reactions, it is usually not used as a reaction solvent, because it does not dissolve the reactants well and is amphoteric (acidic and basic) and nucleophilic. Nevertheless, these properties are sometimes desirable. Also, acceleration of Diels-Alder reactions by water has been observed. Supercritical water has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently.

Recreation

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, waterskiing, boating, fishing, and diving. In addition, some sports, like ice hockey and ice skating, are played on ice. Lakesides and beaches are popular places for people to go to relax and enjoy recreation. Many find the sound of flowing water to be calming, too. Some keep fish and other life in water tanks or ponds for show, fun, and companionship. Humans also use water for snow sports i.e. skiing or snowboarding, which requires the water to be frozen. People may also use water for play fighting such as with snowballs, water guns or water balloons. They may also make fountains and use water in their public or private decorations.

Industrial applications

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.


Food processing

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 is affected by solutes. One mole of sucrose (sugar) raises the boiling point of water by 0.52 °C, and one mole of salt raises the boiling point by 1.04 °C while lowering the freezing point of water in a similar way.[25] Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food.[26] Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.[25] Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity.[26] 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.[25] 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.[25] 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.[25]

Boiling, steaming, and simmering are popular cooking methods that often require immersing food in water or its gaseous state, steam.

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 sun. Heat from the sun evaporates water, which condenses as rain in higher altitudes, from

where it flows down.


Politics

See also: Water resources and Category:Water and politics



People waiting in line to gather water during the Siege of Sarajevo

Because of overpopulation, mass consumption, misuse, and water pollution, the availability of drinking water per capita is inadequate and shrinking as of the year 2006. For this reason, water is a strategic resource in the globe and an important element in many political conflicts. Some have predicted that clean water will become the "next oil", *** Canada, with this resource in abundance, possibly the richest country in the world.[citation needed] There is a long history of conflict over water, including efforts to gain access to water, the use of water in wars started for other reasons, and tensions over shortages and control.[27] UNESCO's World Water Development Report (WWDR, 2003) from its 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 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. The United Nations Development Programme sums up world water distribution in the 2006 development report: "While one part of the world sustains a designer bottled-water market that generates no tangible health benefits, another part suffers acute public health risks because people have to drink water from drains or from lakes and rivers."[28] Fresh water — now more precious than ever in our history for its extensive use in agriculture, high-tech manufacturing, and energy production — is increasingly receiving attention as a resource requiring better management and sustainable use.

OECD countries



Hopetoun Falls near Otway National Park, Victoria, Australia

With nearly 2,000 cubic metres (70,000 ft3) of water per person per year , the United States leads the world in water consumption per capita. In the Organization for Economic Co-operation and Development (OECD) countries, the U.S. is first for water consumption, then Canada with 1,600 cubic meters (56,000 ft3) of water per person per year, which is about twice the amount of water used by the average person from France, three times as much as the average German, and almost eight times as much as the average Dane. Since 1980, overall water use in Canada has increased by 25.7%. This is five times higher than the overall OECD increase of 4.5%. In contrast, nine OECD nations were able to decrease their overall water use since 1980 (Sweden, the Netherlands, the United States, the United Kingdom, the Czech Republic, Luxembourg, Poland, Finland and Denmark).[29][30]

United States

Ninety-five percent of the United States' fresh water is underground. One crucial source is a huge underground reservoir, the 1,300-kilometer (800 mi) Ogallala aquifer which stretches from Texas to South Dakota and waters one fifth of U.S. irrigated land. Formed over millions of years, the Ogallala aquifer has since been cut off from its original natural sources. It is being depleted at a rate of 12 billion cubic meters (420 billion ft3) per year, amounting to a total depletion to date of a volume equal to the annual flow of 18 Colorado Rivers. Some estimates say it will dry up in as little as 25 years. Many farmers in the Texas High Plains, which rely particularly on the underground source, are now turning away from irrigated agriculture as they become aware of the hazards of overpumping.[31]

Mexico



In Mexico City, an estimated 40% of the city's water is lost through leaky pipes built at the turn of the 20th century.[32]

Middle East

The Middle East region has only 1% of the world's available fresh water, which is shared among 5% of the world's population. Thus, in this region, water is an important strategic resource. By 2025, it is predicted that the countries of the Arabian peninsula will be using more than double the amount of water naturally available to them.[33] According to a report by the Arab League, two-thirds of Arab countries have less than 1,000 cubic meters (35,000 ft3) of water per person per year available, which is considered the limit.[34]

Jordan, for example, has little water, and dams in other countries have reduced its available water sources over the years. The 1994 Israel-Jordan Treaty of Peace stated that Israel would give 50 million cubic meters of water (1.7 billion ft3) per year to Jordan, which it refused to do in 1999 before backtracking. The 1994 treaty stated that the two countries would cooperate in order to allow Jordan better access to water resources, notably through dams on the Yarmouk River.[35] Confronted by this lack of water, Jordan is preparing new techniques to use non-conventional water resources, such as second-hand use of irrigation water and desalinization techniques, which are very costly and are not yet used. A desalinization project will soon be started in Hisban, south of Amman. The Disi groundwater project, in the south of Jordan, will cost at least $250 million to bring out water. Along with the Unity Dam on the Yarmouk River, it is one of Jordan's largest strategic projects. Born in 1987, the "Unity Dam" would involve both Jordan and Syria. This "Unity Dam" still has not been implemented because of Israel's opposition, Jordan and Syrian conflictive relations and refusal of world investors. However, Jordan's reconciliation with Syria following the death of King Hussein represents the removal of one of the project's greatest obstacles.[36]



The Jordan River

Both Israel and Jordan rely on the Jordan River, but Israel controls it, as well as 90% of the water resources in the region. Water is also an important issue in the conflict with the Palestinians—indeed, according to former Israeli prime minister Ariel Sharon quoted by Abel Darwish in the BBC, it was one of the causes of the 1967 Six-Day War. In practice the access to water has been a casus belli for Israel. The Israeli army prohibits Palestinians from pumping water, and settlers use much more advanced pumping equipment. Palestinians complain of a lack of access to water in the region.[37] Israelis in the West Bank use four times as much water as their Palestinian neighbors.[38] According to the World Bank, 90% of the West Bank's water is used by Israelis.[36] Article 40 of the appendix B of the September 28, 1995 Oslo accords stated that "Israel recognizes Palestinians' rights on water in the West Bank".

The Golan Heights provide 770 million cubic meters (27 billion ft3) of water per year to Israel, which represents a third of its annual consumption. The Golan's water goes to the Sea of Galilee—Israel's largest reserve—which is then redistributed throughout the country by the National Water Carrier. However, the level on the Sea of Galilee has dropped over the years, sparking fears that Israel's main water reservoir will become salinated. On its northern border, Israel threatened military action in 2002 when Lebanon opened a new pumping station taking water from a river feeding the Jordan. To help ease the crisis, Israel has agreed to buy water from Turkey and is investigating the construction of desalination plants.[39]

Iraq and Syria watched with apprehension the construction of the Atatürk Dam in Turkey and a projected system of 22 dams on the Tigris and Euphrates rivers.[40] According to the BBC, the list of 'water-scarce' countries in the region grew steadily from three in 1955 to eight in 1990 with another seven expected to be added within 20 years, including three Nile nations (the Nile is shared by nine countries).

Asia



Three Gorges Dam, receiving, upstream side, 26 July, 2004

In Asia, Cambodia and Vietnam are concerned by China's and Laos' attempts to control the flux of water. China is also preparing the Three Gorges Dam project on the Yangtze River, which would become the world's largest dam, causing many social and environmental problems. It also has a project to divert water from the Yangtze to the dwindling Yellow River, which feeds China's most important farming region.



Ganges river delta, Bangladesh and India

The Ganges is disputed between India and Bangladesh. The water reserves are being quickly depleted and polluted, while the glacier feeding the sacred Hindu river is retreating hundreds of feet each year because of global warming[citation needed] and deforestation in the Himalayas, which is causing subsoil streams flowing into the Ganges river to dry up. Downstream, India controls the flow to Bangladesh with the Farakka Barrage, 10 kilometers (6 mi) on the Indian side of the border. Until the late 1990s, India used the barrage to divert the river to Calcutta to keep the city's port from drying up during the dry season. This denied Bangladeshi farmers water and silt, and it left the Sundarban wetlands and mangrove forests at the river's delta seriously threatened. The two countries have now signed an agreement to share the water more equally. Water quality, however, remains a problem, with high levels of arsenic and untreated sewage in the river water.[41]

South America

The Guaraní Aquifer, located between the Mercosur countries of Argentina, Brazil, Bolivia and Paraguay, with a volume of about 40,000 km³, is an important source of fresh potable water for all four countries.

Privatization

Privatization of water companies has been contested on several occasions because of poor water quality, increasing prices, and ethical concerns. In Bolivia for example, the proposed privatization of water companies by the IMF was met by popular protests in Cochabamba in 2000, which ousted Bechtel, an American engineering firm based in San Francisco. SUEZ has started retreating from South America because of similar protests in Buenos Aires, Santa Fe, and Córdoba, Argentina. Consumers took to the streets to protest water rate hikes of as much as 500% mandated by SUEZ. In South and Central America, SUEZ has water concessions in Argentina, Bolivia, Brazil and Mexico. "Bolivian officials fault SUEZ for not connecting enough households to water lines as mandated by its contract and for charging as much as $455 a connection, or about three times the average monthly salary of an office clerk", according to the Mercury News.[42]

South Africa also made moves to privatize water, provoking an outbreak of cholera killing 200.[43]

In 1997, World Bank consultants assisted the Philippine government in the privatization of the city of Manila's Metropolitan Waterworks and Sewerage Systems (MWSS). By 2003, water price increases registered at 81% in the east zone of the Philippines and 36% in the west region. As services became more expensive and inefficient under privatization, there was reduced access to water for poor households. In October 2003, the Freedom from Debt Coalition reported that the diminished access to clean water resulted in an outbreak of cholera and other gastro-intestinal diseases.[44]

Regulation



A water-carrier in India, circa ~1882. In many places where running water is not available, water has to be transported by people.

Drinking water is often collected at springs, extracted from artificial borings in the ground, or wells. 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 are rainwater and river or lake water. This su***ce water, however, must be purified 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 ocean or seawater is a more expensive solution used in coastal arid climates.

The distribution of drinking water is done through municipal water systems or as bottled water. ***s in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource and to finance the boring of wells or the construction of dams and reservoirs.

Reducing waste by using drinking 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.

Religion, philosophy, and literature



A Hindu ablution as practiced in Tamil Nadu

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

Some faiths use water especially prepared for religious purposes (holy water in some Christian denominations, Amrit in Sikhism and Hinduism). Many religions also consider particular sources or bodies of water to be sacred or at least auspicious; examples include Lourdes in Roman Catholicism, the Zamzam Well in Islam and the River Ganges (among many others) in Hinduism. In Neo-Paganism water is often combined with salt in the first steps of a ritual, to act as a purifier of worshippers and the altar, symbolising both cleansing tears and the ocean.

Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganges is also personified as a goddess, while Saraswati have been referred to as goddess in Vedas. Also water is one of the "panch-tatva"s (basic 5 elements, others including fire, earth, space, air). Alternatively, gods can be patrons of particular springs, rivers, or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids. In Islam, not only does water give life, but every life is itself made of water: "We made from water every living thing".[45]

The 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. Water was also one of the five elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.

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

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Extra notes

1.水真的是universal solvent 吗？
The medical alcemist find out nothing works better than water.However, water is not a universal solvent; if it were,it would dissolve any container in which it was stored,including our cells .But water is a very versatile solvent,a quality we can trace to the polarity of the water molecule.

2. Water strider has hydrophobic legs which can help it float on water.

3. Adhesive force also allowed water in the top soil.

4.为什么水的温度不会超过100C
The temperature of water remains at 100 as it continues to be heated because the evaporatin molecules take the extra energy with them.