Water has the chemical formula H2O, meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. It can also be described ionically as HOH, with a hydrogen ion (H+) that is bonded to a hydroxide ion (OH-). It is in dynamic equilibrium between the liquid and vapor states at standard temperature and pressure. At room temperature, it is a nearly colorless, tasteless, and odorless liquid. It is often referred to in the sciences as the universal solvent and the only pure substance found naturally in all three states of matter.
Molecular properties
- Main article: Water (molecule)
Water has the chemical formula H2O, meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. It can also be described ionically as HOH, with a hydrogen ion (H+) that is bonded to a hydroxide ion (OH-). It is in dynamic equilibrium between the liquid and vapor states at standard temperature and pressure. At room temperature, it is a nearly colorless, tasteless, and odorless liquid. It is often referred to in the sciences as the universal solvent and the only pure substance found naturally in all three states of matter.
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Water in biology
From a biological standpoint, water has many distinct properties that are critical for the proliferation of life that set it apart from other substances. Water carries out this role by allowing organic compounds to react in ways that ultimately allows replication. It is a good solvent and has a high surface tension, and thus allows organic compounds and living things to be transported in it. Fresh water has its greatest density under normal atmospheric pressure at 4 °C, then becoming less dense as it freezes or heats up from this point. As a stable, polar molecule prevalent in the atmosphere, it plays an important role as a greenhouse gas absorbing infrared radiation, without which, Earth's average surface temperature would be −18 ° Celsius. Water also has an unusually high specific heat, which allows it to play many roles in regulating global and regional climate. Because it absorbs strongly in the infrared portion of the light spectrum, a small amount of visible red light is absorbed as well, resulting in water's slightly blue color when seen in mass quantities such as a lake or ocean.
Water is a very good solvent, dissolving many types of substances, such as various salts and sugar. It facilitates chemical interactions such as the process of metabolism.
Some substances, however, do not mix well with water, including lipids, some proteins and other hydrophobic substances. The chemical force explaining (among other things) why oil and water, famously, do not mix is Van der Waals force. Cell membranes take advantage of this property to carefully control interactions between their contents and external chemicals, which is facilitated somewhat by the surface tension of water.
Water has a high surface tension caused by the strong cohesion between water molecules. This can be seen when small quantities of water are put onto a nonsoluble surface such as polythene: the water stays together as drops. On extremely clean 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 surfaces that are hydrophilic, that is, surfaces that have a strong attraction to water. Irving Langmuir observed a strong repulsive force between hydrophilic surfaces. To dehydrate hydrophilic surfaces—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 A simple but environmentally important and unique property of water is that its common solid form, ice, floats on its liquid form. This solid phase is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases of water ice except ice-XI, the solid form is more dense than the liquid form. Fresh water at standard atmospheric pressure is most dense at 4 °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 surface 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. While this behavior may seem obvious, even intuitive, it should be noted that almost all other chemicals are denser as solids than they are as liquids, and freeze from the bottom up.
A common misconception about water is that it is a powerful conductor of electricity, with risks of electrocution explaining this popular belief. Any electrical properties observable in water are due to 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. Pure water can also be electrolized into oxygen and hydrogen gases but without any dissolved ions, this is a very slow process and thus very little current is conducted. Many bottled water companies exploit another common misconception, advertising both purity and taste, even though pure water is tasteless.
Forms of water
Water can take many forms. The solid state of water is commonly known as ice (while many other forms exist, see amorphous solid water); the gaseous state is known as water vapor (or steam), and the common liquid phase is generally taken as simply water. Above a certain critical temperature and pressure (647 K and 22.064 MPa), water molecules assume a supercritical condition, in which liquid-like clusters float within a vapor-like phase.
Heavy water is water in which the hydrogen atoms are replaced by its heavier isotope, deuterium. It is chemically almost identical to normal water. Heavy water is used in the nuclear industry to slow down neutrons.
A common substance
Water in the Universe
Water has been found in interstellar clouds within our galaxy, the Milky Way. It is believed that water exists in abundance in other galaxies too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe.
Interstellar clouds eventually condense into solar nebulae and solar systems, such as ours. The initial water can then be found in comets, planets, and their satellites. In our solar system, water, in ice form, has been found :
- on the Moon,
- on the planets Mercury, Mars, Neptune, and Pluto,
- on satellites of planets, such as Triton and Europa.
The liquid form of water is only known to occur on Earth, though strong evidence suggests that it is present just under the surface of Saturn's moon Enceladus.
Water on Earth
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.
Earth's approximate water volume (the total water supply of the world) is 1,360,000,000 km³ (326,000,000 mi³). Of this volume:
- 1,320,000,000 km³ (316,900,000 mi³ or 97.2%) is in the oceans
- 25,000,000 km³ (6,000,000 mi³ or 1.8%) is in glaciers and icecaps
- 13,000,000 km³ (3,000,000 mi³ or 0.9%) is groundwater.
- 250,000 km³ (60,000 mi³ or 0.02%) is fresh water in lakes, inland seas, and rivers.
- 13,000 km³ (3,100 mi³ or 0.001%) is atmospheric water vapor at any given time.
Liquid water is found in bodies of water, such as an ocean, sea, lake, river, stream, canal, or pond. The majority of water on Earth is sea water. Water is also present in the atmosphere in both liquid and vapor phases. It also exists as groundwater in aquifers. Although water normally boils at about 100 °C, in deep sea vents the pressurised superheated water reaches a natural temperature of 400 °C, whereas at the top of Mount Everest, the low pressure allows water to boil at a mere 70 °C.
Physics and chemistry of water
Density of water and ice
For most substances, the solid form of the substance is more dense than the liquid form; thus, a block of pure solid substance will sink in a tub of pure liquid substance. But, by contrast, a block of common ice will float in a tub of water because solid water is less dense than liquid water. This is an extremely important characteristic property of water. At room temperature, liquid water becomes denser with lowering temperature, just like other substances. But at 4 °C, just above freezing, water reaches its maximum density, and as water cools further toward its freezing point, the liquid water, under standard conditions, expands to become less dense. The physical reason for this is related to the crystal structure of ordinary ice, known as hexagonal ice Ih. Water, gallium, bismuth, acetic acid, antimony and silicon are some of the few materials which expand when they freeze; most other materials contract. It should be noted however, that not all forms of ice are less dense than liquid water. For example HDA and VHDA are both more dense than liquid phase pure water. Thus, the reason that the common form of ice is less dense than water is a bit non-intuitive, and relies heavily on the unusual properties inherent to the hydrogen bond.
Generally, water expands when it freezes because of its molecular structure, in tandem with the unusual elasticity of the hydrogen bond and the particular lowest energy hexagonal crystal conformation that it adopts under standard conditions. That is, when water cools, it tries to stack in a crystalline lattice configuration that stretches the rotational and vibrational components of the bond, so that the effect is that each molecule of water is pushed further from each of its neighboring molecules. This effectively reduces the density ρ of water when ice is formed under standard conditions.
The importance of this property cannot be overemphasized for its role on the ecosystem of Earth. For example, if water were more dense when frozen, lakes and oceans in a polar environment would eventually freeze solid (from top to bottom). This would happen because frozen ice would settle on the lake and riverbeds, and the necessary warming phenomenon (see below) could not occur in summer, as the warm surface layer would be less dense than the solid frozen layer below. It is a significant feature of nature that this does not occur naturally in the environment, but under synthetic laboratory conditions where HDA and VHDA form, specialized forms of ice are more dense, and do sink to the bottom in liquid water.
Nevertheless, the unusual expansion of freezing water (in ordinary natural settings in relevant biological systems), due to the hydrogen bond, from 4 °C above freezing to the freezing point offers an important advantage for freshwater life in winter. Water chilled at the surface becomes denser and sinks, forming convection currents that cool the whole water body, but when the temperature of the lake water reaches 4 °C, water on the surface, as it chills further, becomes less dense, and stays as a surface layer which eventually freezes and forms ice. Since downward convection of colder water is blocked by the density change, any large body of fresh water frozen in winter will have the coldest water near the surface, away from the riverbed or lakebed. This accounts for various little known phenomenon of ice characteristics as they relate to ice in lakes and "ice falling out of lakes" as described by early 20th century scientist Horatio D. Craft.
The following table gives the density of water in grams per cubic centimeter at various temperatures in degrees Celsius:
| Temp (°C) | Density (g/cm3) |
|---|---|
| 30 | 0.9957 |
| 20 | 0.9982 |
| 10 | 0.9997 |
| 0 | 0.9998 |
The values below 0 °C refer to supercooled water.
Density of saltwater and ice
The situation in salt water is somewhat different. Ice still floats to keep the oceans from freezing solid (see following paragraph). However, the salt content of oceans both lowers the colligative freezing point by about 2 °C and lowers the temperature of the density maximum of water to be about at the freezing point. Hence, in ocean water, because of the salt content, the downward convection of colder water is not blocked by an expansion of water as it becomes colder near the freezing point; thus the oceans' cold water near the freezing point continues to sink. For this reason, any creature attempting to survive at the bottom of such cold water as the Arctic Ocean generally lives in water that is 4 °C colder than the temperature at the bottom of frozen-over fresh water lakes and rivers in winter.
As the surface of salt water begins to freeze (at −1.9 °C for normal salinity seawater, 35‰) the ice that forms is essentially salt free with a density approximately that of freshwater ice. This ice floats on the surface and the salt that is "frozen out" adds to the salinity and density of the seawater just below it. This more dense saltwater sinks by convection and the replacing seawater is subject to the same process. This provides essentially freshwater ice at −1.9 °C on the surface. The increased density of the seawater beneath the forming ice sinks towards the bottom, thus the deep ocean waters should have a minimum temperature of −1.9 °C also. However the temperature of the deep oceans is about 4 °C.
Triple point
The temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium is called the triple point of water. This point is used to define the units of temperature (the kelvin and, indirectly, the degree Celsius and even the degree Fahrenheit). The triple point is at a temperature of 273.16 K (0.01 °C) by convention, and at a pressure of 611.73 Pa. This pressure is quite low, about 1/166 of the normal sea level barometric pressure of 101,325 Pa. The atmospheric surface pressure on planet Mars is remarkably close to the triple point pressure, and the zero-elevation or "sea level" of Mars is defined by the height at which the atmospheric pressure corresponds to the triple point of water.
Mpemba effect
The Mpemba effect is the surprising phenomenon whereby hot water can, under certain conditions, freeze faster than cold water, even though it must pass the lower temperature on the way to freezing. However, this can be explained with evaporation, convection, supercooling, and the insulating effect of frost.
Hot ice
Hot ice is the name given to another surprising phenomenon in which water at room temperature can be turned into ice at room temperature by supplying an electric field of the order of 106 volts per meter. (Choi 2005)
The effect of such electric fields has been suggested as an explanation of cloud formation. The first time cloud ice forms around a clay particle, it requires a temperature of −10 °C, but subsequent freezing around the same clay particle requires a temperature of just −5 °C, suggesting some kind of "ice memory" (Connolly, P.J, et al, 2005)
Surface tension
Water drops are stable thanks to the high surface tension of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property is important for life. For example, when water is carried through xylem up stems in plants the strong intermolecular attractions hold the water column together. Strong cohesive properties hold the water column together, and strong adhesive properties stick the water to the xylem, and prevent tension rupture caused by transpiration pull. Other liquids with lower surface tension would have a higher tendency to "rip", forming vacuum or air pockets and rendering the xylem water transport inoperative.
Electrical properties
Pure water is actually a good insulator (poor conductor), meaning that it does not conduct electricity well. Because water is such a good solvent, however, it almost always has some solute dissolved in it, most frequently a salt. If water has even a tiny amount of such impurities, then it can conduct electricity much better, because impurities such as salt separate into free ions in aqueous solution by which an electric current can flow.
Water can be split into its constituent elements, hydrogen and oxygen, by passing a current through it. This process is called electrolysis. Water molecules naturally dissociate into H+ and OH- ions, which are pulled toward the cathode and anode, respectively. At the cathode, two H+ ions pick up electrons and form H2 gas. At the anode, four OH- ions combine and release O2 gas, molecular water, and four electrons. The gases produced bubble to the surface, where they can be collected. It is known that the theoretical maximum electrical resistivity for water is approximately 182 kilohm-meters (or 18.2 MΩ·cm) at 25 degrees Celsius. This figure agrees well with what is typically seen on reverse osmosis, ultrafiltered and deionized nanosiemens per meter of conductance).
Dipolar nature of water
An important feature of water is its polar nature. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge. A molecule with such a charge difference is called a dipole. The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules. This attraction is known as hydrogen bonding, and explains many of the properties of water.
Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself, it is responsible for a number of water's physical properties. One such property is its relatively high melting and boiling point temperatures; more heat energy is required to break the hydrogen bonds between molecules. The similar compound hydrogen sulfide (H2S), which has much weaker hydrogen bonding, is a gas at room temperature even though it has twice the molecular weight of water. The extra bonding between water molecules also gives liquid water a large specific heat capacity. This high heat capacity makes water a good heat storage medium.
Hydrogen bonding also gives water its unusual behavior when freezing. When cooled to near freezing point, the presence of hydrogen bonds means that the molecules, as they rearrange to minimize their energy, form the hexagonal crystal structure of ice that is actually of lower density: hence the solid form, ice, will float in water. In other words, water expands as it freezes, whereas virtually all other materials shrink on solidification.
An interesting consequence of the solid having a lower density than the liquid is that ice will melt if sufficient pressure is applied. With increasing pressure the melting point temperature drops and when the melting point temperature is lower than the ambient temperature the ice begins to melt. A significant increase of pressure is required to lower the melting point temperature by very much — the pressure exerted by an ice skater on the ice would only reduce the melting point by something like 0.09 °C.
Water as a solvent
Water is also a good solvent due to its polarity. When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends.
In general, ionic and polar substances such as acids, alcohols, and salts are relatively soluble in water, and nonpolar substances such as fats and oils are not. Nonpolar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with nonpolar molecules.
An example of an ionic solute is table salt; the sodium chloride, NaCl, separates into Na+ cations and Cl- anions, each being surrounded by water molecules. The ions are then easily transported away from their crystalline lattice into solution. An example of a nonionic solute is table sugar. The water dipoles make hydrogen bonds with the polar regions of the sugar molecule (OH groups) and allow it to be carried away into solution.
The solvent properties of water are vital in biology, because many biochemical reactions take place only within aqueous solutions (e.g., reactions in the cytoplasm and blood).
Amphoteric nature of water
Chemically, water is amphoteric — i.e., it is able to act as either an acid or a base. Occasionally the term hydroxic acid is used when water acts as an acid in a chemical reaction. At a pH of 7 (neutral), the concentration of hydroxide ions (OH-) is equal to that of the hydronium (H3O+) or hydrogen (H+) ions. If the equilibrium is disturbed, the solution becomes acidic (higher concentration of hydronium ions) or basic (higher concentration of hydroxide ions).
Water can act as either an acid or a base in reactions. According to the Brønsted-Lowry system, an acid is defined as a species which donates a proton (an H+ ion) in a reaction, and a base as one which receives a proton. When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid. For instance, it receives an H+ ion from HCl in the equilibrium:
- HCl + H2O ⇌ H3O+ + Cl-
Here water is acting as a base, by receiving an H+ ion.
In the reaction with ammonia, NH3, water donates an H+ ion, and is thus acting as an acid:
- NH3 + H2O ⇌ NH4+ + OH-
Acidity in nature
In theory, pure water has a pH of 7 at 298 K. In practice, pure water is very difficult to produce. Water left exposed to air for any length of time will rapidly dissolve carbon dioxide, forming a dilute solution of carbonic acid, with a limiting pH of about 5.7. As cloud droplets form in the atmosphere and as raindrops fall through the air minor amounts of CO2 are absorbed and thus most rain is slightly acidic. If high amounts of nitrogen and sulfur oxides are present in the air, they too will dissolve into the cloud and rain drops producing more serious acid rain problems.
Hydrogen bonding in water
Water molecule can form a maximum of four hydrogen bonds because it can accept two and donate two hydrogens. Other molecules like hydrogen fluoride, ammonia, methanol form hydrogen bonds but they do not show anomalous behaviour of thermodynamic, kinetic or structural properties like those observed in water. The answer to the apparent difference between water and other hydrogen bonding liquids lies in the fact that apart from water none of the hydrogen bonding molecules can form four hydrogen bonds either due to an inability to donate/accept hydrogens or due to steric effects in bulky residues. In water local tetrahedral order due to the four hydrogen bonds gives rise to an open structure and a 3-dimensional bonding network, which exists in contrast to the closely packed structures of simple liquids. There is a great similarity between water and silica in their anomalous behaviour, even though one (water) is a liquid which has a hydrogen bonding network while the other (silica) has a covalent network with a very high melting point. One reason that water is well suited, and chosen, by life-forms, is that it exhibits its unique properties over a temperature regime that suits diverse biological processes, including hydration.
It is believed that hydrogen bond in water is largely due to electrostatic forces and some amount of covalency. The partial covalent nature of hydrogen bond predicted by Linus Pauling in 1930s is yet be to proven unambiguously by experiments and theoretical calculations.
Quantum properties of molecular water
Although the molecular formula of water is generally considered to be a stable result in molecular thermodynamics, recent work, started in 1995 [2] has shown that at certain scales, water may act more like H3/2O than H2O at the subatomic quantum level. This result could have significant ramifications at the level of, for example, the hydrogen bond in biological, chemical and physical systems. The experiment shows that when neutrons and protons collide with water, they scatter in a way that indicates that they only are affected by a ratio of 1.5:1 of hydrogen to oxygen respectively. However, the time-scale of this response is only seen at the level of attoseconds, and so is only relevant in highly resolved kinetic and dynamical systems. For more references see [3] and [4].
History
In 1742, Anders Celsius defined the Celsius temperature scale with the freezing point of water at 100 degrees and the boiling point at standard atmospheric pressure at 0 degrees. The scale was reversed in 1744.
The first decomposition of water into hydrogen and oxygen, by electrolysis, was done in 1800 by William Nicholson, an English chemist.
Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933.
Polywater was a hypothetical polymerized form of water that was the subject of much scientific controversy during the late 1960s. The consensus now is that it does not exist.
Systematic naming
The accepted IUPAC name of water is simply "water", although there are two other systematic names which can be used to describe the molecule.
The simplest and best systematic name of water is hydrogen oxide. This is analogous to related compounds such as hydrogen peroxide, hydrogen sulfide, and deuterium oxide (heavy water). Another systematic name that has been accepted by IUPAC is oxane. This name, however, has the problem of already being the name of a cyclic ether also known as tetrahydropyran (similar compounds include dioxane and trioxane).
Systematic nomenclature and humor
- Main article: dihydrogen monoxide hoax
Chemists sometimes refer to water as dihydrogen monoxide or DHMO, an overly pedantic systematic covalent name of this molecule, especially in parodies of chemical research that call for this "lethal chemical" to be banned. In 2004, the town of Aliso Viejo, California nearly banned foam cups after learning that DHMO was used in their production (see [5]). In reality, a more realistic systematic name would be hydrogen oxide, since the "di-" and "mon-" prefixes are superfluous. Hydrogen sulfide, H2S, is never referred to as "dihydrogen monosulfide", and hydrogen peroxide, H2O2, is never called "dihydrogen dioxide".
Some overzealous material safety data sheets for water list the following: Caution: May cause drowning!
The systematic acid name of water is hydroxic acid or hydroxilic acid. Likewise, the systematic alkali name of water is hydrogen hydroxide – both acid and alkali names exist for water because it is able to react both as an acid or an alkali, depending on the strength of the acid or alkali it is reacted with (it is amphoteric). None of these names are used widely outside of DHMO sites.
See also
- dihydrogen monoxide hoax
- double distilled water
- heavy water
- hydrodynamics
- Mpemba effect
- polywater theory
- water dimer
External links
- Water Structure and Behaviour A comprehensive and up-to-date NPOV resource maintained by Prof Martin Chaplin of South Bank University, UK
- A spoof site on the "dangers" of dihydrogen monoxide
- Stockholm International Water Institute (SIWI)
- Explanation of the anomalous properties of water
- Computational Chemistry Wiki
- This article focuses on water as it is experienced in everyday life. See Water (molecule) for information on the chemical and physical properties of pure water (H2O, hydrogen oxide), and Water (film) for the movie of the same name.
Water (from the Old English waeter; c.f German "Wasser" or Weniger, from PIE*wod-or, "water"), in its pure form, is a tasteless, odorless substance that is essential to all known forms of life and is known also as the most universal solvent. Without it, life as we know it would not exist. It appears colorless to the naked eye in small quantities, though it can be seen to be blue in large quantities or with scientific instruments [1]. An abundant substance on Earth, it exists in many places and forms. It appears mostly in the oceans and polar ice caps, but also as clouds, rain water, rivers, freshwater aquifers, and sea ice. On the planet, water is continuously moving through the cycle involving evaporation, precipitation, and runoff to the sea.
Water fit for
human consumption is called
drinking water or "potable
water". Water that is not
specifically made for drinking,
but is not harmful for humans
when used for food preparation
are called
safe water.
This natural resource is becoming more scarce 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 have 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 [2]. 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 supplies and over 1 billion without access to adequate sanitation facilities. Poor water quality and bad sanitation are killers; some 5 million deaths a year are caused by polluted drinking water.
Hardly surprising, since 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 . The strain affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.
Astronomical position of Earth and impact on its water
Scientists theorize that most of the universe's water is produced as a byproduct of star formation. Gary Melnick, a scientist at the Harvard-Smithsonian Center for Astrophysics, explains: "For reasons that aren't entirely understood, when stars are born, their birth is accompanied by a strong outward wind of gas and dust. When this outflowing material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water we observe is rapidly produced in this warm dense gas." [3]
The coexistence of the solid, liquid, and gaseous phases of water on Earth is vital to existence of life on Earth. However, if the Earth's location in the solar system were even marginally closer or further from the Sun (ie, a million miles or so), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.
Earth's mass allows gravity to hold an atmosphere. Water vapor and carbon dioxide in the atmosphere provide a greenhouse effect which helps maintain a relatively steady surface temperature. If Earth were less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in polar ice caps (as on Mars).
The distance between Earth and the Sun, the combination of solar radiation received and the greenhouse effect of the atmosphere ensure that Earth's surface is neither too cold nor too hot for liquid water. If Earth were more distant from the Sun, most water would be frozen. If Earth were nearer to the Sun, its higher surface temperature would limit the formation of ice caps, or cause water to exist only as vapor.
It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature of Earth has been relatively constant through geologic time despite varying levels of incoming solar radiation (insolation), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. This proposal is known as the Gaia hypothesis.
Human uses of water
All known forms of life depend on water. (Note however that some bacteria and plant seeds can enter a cryptobiotic state for an indefinite period when dehydrated, and "come back to life" when returned to a wet environment). Water is a vital part of many metabolic processes within the body, and significant quantities of water are used during the digestion of food.
About 72% of the fat free mass of the human body is made of water. To function properly the body requires between one and seven liters of water per day to avoid dehydration, the precise amount depending on the level of activity, temperature, humidity, and other factors. However, most of this is ingested through other foods or beverages (hot tea being often used in deserts to avoid dehydration, etc.) It is not clear how much water intake is needed by healthy people. However, for those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) 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 [4]. There are other myths such as the effect of water on weight loss and constipation that have been dispelled [5].
The latest dietary reference intake report by the US National Research Council recommended 2.7 liters of water total (including food sources) for women and 3.7 liters for men [6]. Water is lost from the body in urine and feces, through sweating, and by exhalation of water vapor in the breath.
Humans require water that does not contain too much salt or other impurities. Common impurities include chemicals and/or harmful bacteria, such as Vibrio. Some solutes are acceptable and even desirable for perceived taste enhancement and to provide needed electrolytes.
Water as a precious resource: politics of water
- See water resources for information about fresh water supplies; see also Category:Water and politics for articles treating about water politics
Because of the growth of world population, mass consumption and pollution, the availability of drinking water per capita is shrinking. 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", making Canada, with this resource in abundance, possibly the richest country in the world. 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 [7]. UNESCO's World Water Development Report (WWDR, 2003) from its 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 due to easily preventable water-related diseases. 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.
Water in the OECD countries
With nearly 2,000 cubic metres of water per person and per year, the United States leads the world in water consumption per capita (a large quantity of golf fields and car washing partly explain this massive consumption). In the Organisation for Economic Co-operation and Development (OECD) countries, the U.S. comes first for water consumption, then Canada with 1,600 cubic metres 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) [8] [9].
Ninety-five percent of the United States' fresh water is underground. One crucial source is a huge underground reservoir, the 800-mile (1,300 km) 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 metres a 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 irrigated agriculture as they become aware of the hazards of overpumping [10].
In Mexico City, an estimated 40% of the city's water is lost through leaky pipes built at the turn of the century [11].
Water in the Middle East
The Middle East region has only 1% of the world's available fresh water, which is shared between 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 [12]. According to a report by the Arab League, two-thirds of Arab countries have less than 1,000 cubic meters water per person per year, which is considered the limit [13].
Jordan, for example, has little water and dams in other countries have reduced its available water over the years. The 1994 Israel-Jordan Treaty of Peace stated that Israel would give 50 million cubic meters of water 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 [14]. 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 Amman. The groundwater project, in the south of Jordan, will cost at least $250 million to bring out water. Along with the Syria. This "Unity Dam" still hasn't been implemented because of Israel's opposition, Jordan and Syrian conflictual relations and refusal of world investors. However, Jordan's reconciliation with Syria following the death of King Hussein would make the project envisionable again [15].
Both Israel and Jordan rely on the Jordan river, but Israel controls it, as well as 9/10 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. According to the BBC, "with the Israeli army in control prohibiting Palestinians from pumping water, and settlers using much more advanced pumping equipment, Palestinians complain of "daily theft" of as much as 80% of their underground water" [16]. Israelis in the West Bank use four times as much water as their Palestinian neighbours [17]. According to the World Bank, 90% of the West Bank's water is used by Israelis [15]. 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 Syrian Golan also provides 770 million cubic meters of water per year to Israel, which represents a third of its annual consumption. The Golan's table water goes to the Sea of Galilee, which is Israel's largest reserve, which is afterward redistributed throughout the country by the National Water Carrier. Occupied since 1967, the Golan thus represents for Israel a strategic territory because of its water resources. [15]. 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 building desalination plants [18].
On the other hand, 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 [19]. 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).
Water in Asia
In Asia, Vietnam and Cambodia 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 one of the world's largest dams, 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.
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 and deforestation in the Himalayas causing subsoil streams flowing into the Ganges river to dry up. Downstream, India controls the flow to Bangladesh with the Calcutta to stop the city's port drying up during the dry season. This denied Bangladeshi farmers water and silt, and 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 huge problem, with high levels of arsenic and untreated sewage in the river water [20].
Privatisation of water companies
Privatisation of water companies has been contested on several occasions, due to bad quality of the water, increasing prices, etc. In Bolivia for example, the proposed privatization of water companies by the IMF were 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, due to similar protests (in Buenos Aires in Argentina, as well as in Santa Fe; in Córdoba, consumers took to the streets to protest water rate hikes of as much as 500 percent 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 [21]. South Africa also made moves to privatize water, provoking an outbreak of cholera killing 200 [22].
Regulating water distribution
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 surface 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. Governments 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, that is 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 pollutor. As other types of pollution, this doesn't enter standard accounting of market costs, being conceived as externalities for which the market can't account for. Thus other people pay the price of this 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.
The impact of water on religion and philosophy
Water is considered a purifier in most religions, including Hinduism, Christianity, Islam, Judaism, and Shinto. For instance, baptism in Christian churches is done with water. 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 only be done after completing washing the body with clean water (wudu). In Shinto, water is used in almost all rituals to cleanse a person or an area. 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)
Water is often believed to have spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganga 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, river or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids.
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 stuff of the universe. Water was considered cold and moist. In the theory of the four bodily humours, water was associated with phlegm. Water was also one of the Five Elements in traditional Chinese philosophy, along with earth, fire, wood, and metal.
Notes
- ^ The Color of Water: Visibility Under Water
- ^ G8 "Action plan" decided upon at the 2003 Evian summit
- ^ "Discover of Water Vapor Near Orion Nebula Suggests Possible Origin of H20 in Solar System", The Harvard University Gazette, April 23, 1998.
- ^ "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 × 8"? by Heinz Valdin, Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire
- ^ Drinking Water - How Much?, Factsmart.org web site and references within
- ^ Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate, Food and Nutrition Board
- ^ A Chronology of Water-Related Conflicts
- ^ Water consumption indicator in the OECD countries
- ^ "Golf 'is water hazard'", BBC News, March 17, 2003.
- ^ "Ogallala aquifer - Water hot spots", BBC News, ?.
- ^ "Mexico City - Water hot spots", BBC News, ?.
- ^ "Water shortages 'foster terrorism'", BBC News, March 18, 2003.
- ^ "Major aspects of scarce water resources management with reference to the Arab countries", Arab League report published for the International Conference on water gestion and water politics in arid zones, in Amman, Jordan, December 1-3, 1999. Quoted by French journalist Christian Chesnot in "Drought in the Middle East", Monde diplomatique, February 2000. - French original version freely available here. Compare with the 1,600 cubic meters of water used per person and per year in Canada, for example
- ^ See 1994 Israel-Jordan Treaty of Peace, annex II, article II, first paragraph
- ^ a b c See Christian Chesnot in "Drought in the Middle East", Monde diplomatique, February 2000. - French original version freely available here.
- ^ "Analysis: Middle East water wars, by Abel Darwish", BBC News, May 30, 2003.
- ^ "Israel - water hot spots", BBC News, ?.
- ^ "Israel - water hot spots", BBC News, ?.
- ^ "Turkey - water hot spots", BBC News, ?.
- ^ "Ganges river - water hot spots", BBC News, ?.
- ^ "Bolivia's water wars coming to end under Morales", Mercury News, February 26, 2006.
- ^ "Water privatisation: ask the experts", BBC News, December 10, 2004.
- Atmospheric water generator
- Air to Water Harvest
- Bioswale
- Dehydration (hypohydration) vs. hyperhydration
- Desalination
- Dihydrogen monoxide hoax
- Distilled water
- Double distilled water
- Drought
- Ecohydrology
- Evapotranspiration
- Flood
- Flume
- Fountain
- Fresh water
- Heavy water
- Holy water
- Hydrography
- Hydrology
- Hydrosphere
- Ice
- Irrigation
- Mineral water
- Origin of water on Earth
- Pond
- Precipitation (meteorology)
- Rain
- Safe water
- Sea water
- Spring water
- Tide
- Transvasement
- Vienna Standard Mean Ocean Water (VSMOW)
- Wastewater
- WaterAid
- Water Air Extraction Devices
- Water (data page)
- Water (molecule)
- Water industry
- Water intoxication
- Water ionizer
- Water memory
- Water park
- Water purification
- Water quality
- Water quality modelling
- Water resources
- Water tank
- Water therapy
- Water torture
- World Ocean Day
- World Water Day
References
- OA Jones, JN Lester and N Voulvoulis, Pharmaceuticals: a threat to drinking water? TRENDS in Biotechnology 23(4): 163, 2005
- Franks, F (Ed), Water, A comprehensive treatise, Plenum Press, New York, 1972-1982
- Property of Water and Water Steam w Thermodynamic Surface
- PH Gleick and associates, The World's Water: The Biennial Report on Freshwater Resources. Island Press, Washington, D.C. (published every two years, beginning in 1998.)
Water as a natural resource
- Gleick, Peter H.. The World's Water: The Biennial Report on Freshwater Resources. Washington: Island Press. (Produced every two years; data available here)
- Postel, Sandra (1997, second edition). Last Oasis: Facing Water Scarcity. New York: Norton Press.
- Anderson (1991). Water Rights: Scarce Resource Allocation, Bureaucracy, and the Environment.
- Marq de Villiers (2003, revised edition). Water: The Fate of Our Most Precious Resource.
- Diane Raines Ward (2002). Water Wars: Drought, Flood, Folly and the Politics of Thirst.
- Miriam R. Lowi (1995). Water and Power: The Politics of a Scarce Resource in the Jordan River Basin. (Cambridge Middle East Library)
- Worster, Donald (1992). Rivers of Empire: Water, Aridity, and the Growth of the American West.
- Reisner, Marc (1993). Cadillac Desert: The American West and Its Disappearing Water.
- Maude Barlow, Tony Clarke (2003). Blue Gold: The Fight to Stop the Corporate Theft of the World's Water.
- Vandana Shiva (2002). Water Wars: Privatization, Pollution, and Profit. ISBN 0745318371.
- Anita Roddick, et al (2004). Troubled Water: Saints, Sinners, Truth And Lies About The Global Water Crisis.
External links
- UNESCO's World Water Assessment Program
- UNESCO Water Portal
- UNESCO-IHE Institute for Water Education
- United Nations' World Water Development Report
- United Nations GEMS/Water Programme
- The World's Water, data, historical information on conflict
- Phase diagrams of water
- Oceans and Water Issues Page
- Scientific Facts on Water Disinfectants A faithful summary by GreenFacts of a leading scientific consensus report on Drinking Water Disinfectants published by the International Programme on Chemical Safety of the WHO.
- Residential water problems and markets Study paper from Helmut Kaiser Consultancy
- Water markets worldwide Study paper from Helmut Kaiser Consultancy
- World Water Forum
- Water Structure and Behaviour
- WaterAid
- SAHRA—Global Water Newswatch
- Stockholm International Water Institute (SIWI)
- California Water Impact Network (C-WIN)
- BBC: The water debate
- BBC News (International): The Water Debate
- Tap Water Vs Bottled Water - Interesting site providing facts about tap and bottled water.
- E the Environmental Magazine piece on bottled water (Oct 2003).
- International Association for the Properties of Water and Steam
- US Geological Survey: Comprehensive discussion of the water cycle, in many languages
- Why is water blue?
- Water requirements in adults
- Climate change raises markets for environmental technology, drinking water and clean energies
- 'Unfreezable water', bound water and water of hydration
- MSN Encarta - Multimedia - Phase Diagram for Water
