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Because of its scarcity, water plays a central role in Middle Eastern politics and society.

Nowhere in the world is water more important than in the Middle East and North Africa. In no other region do so many people strive so hard for economic growth on the basis of so little water: here is found 5 percent of the world's population but only 1 percent of its fresh water. Of the ten nations with the least water per capita, six are in this region. No wonder that both Jewish and Muslim scriptures are full of references to water.

Role of Climate

The more heavily populated parts of the Middle East are semiarid, with rainfall of 10 to 29 inches (250 to 750 mm) per year. However, low rainfall is less of a problem than variability in rainfall. The great bulk of the rain falls in four winter months, with none falling during the rest of the year. Rainfall also changes rapidly with distance, from more than 20 inches (500 mm) on the coast of Lebanon to 8 inches (200 mm) in the Biqa, only an hour away by road but across the Lebanon mountains.

Seasonal and spatial variations in rainfall are sharp but predictable. What makes planning difficult is the sharp variation from one year to the next. Reliable flow in the rivers (the flow that can be expected nine years out of ten) is only 10 percent of the average. In northern latitudes, water planning can be built around statistical averages; here, it must be built around extremes.

This already difficult water situation will likely get worse. Population growth rates are high, and most climate change models suggest higher temperatures, lower rainfall, and more frequent droughts for the region.

Role of History

Development in the Middle East and North Africa has always been more dependent on water than on any other resource, including oil. By the fourth millennium b.c.e., the Sumerians had built a paradise in what is now Iraq through intricate canals for irrigating crops; two millennia later it had largely collapsed because of salinization of the soil. Ancient cities, such as Palmyra in Syria, were possible only because of carefully engineered tunnels, called qanats (foggaras in Iran), to bring water from springs tens of kilometers away.

Over the years, the peoples of the Middle East have made water a preoccupation, and each nation has a central agency, typically a full ministry, to deal with water. Many of the principles for good water management were worked out in the Middle Eastalthough just as often they were ignored for political, financial, or social reasons.

Water Sources

The Middle East includes two of the mightiest river systems in the world. The Nile has two main branches: The White Nile originates in Uganda, and the larger Blue Nile (together with the Atbara) originates in Ethiopia; they join near Khartoum and flow northward through Egypt to the Mediterranean. The Tigris and Euphrates both originate in Turkey and flow south-southeastward through Syria and Iraq before joining and flowing into the Persian Gulf via the Shatt al-Arab, at the Iranian border.

The region also includes numerous mediumsized rivers, such as the Jordan, which flows from three springs through the Sea of Galilee (one of the few natural lakes in the region) and into the Dead Sea, 415 meters below sea level. Only Turkey has an
abundance of river water, but its big rivers are only found in the eastern part of the nation. Finally, there are small coastal rivers (many of them ephemeral), and a few major wetlands, such as the marshlands in southern Iraq and the Sudd swamp in southern Sudan.

The construction of new dams and pipelines to deliver water from major rivers in the Middle East will cost two or three times as much per unit of water as current supplies, and if construction occurs in upstream countries, such as Ethiopia and Turkey, it will reduce flows downstream. Therefore, the region will increasingly shift toward the use of underground water, which has the great advantage of not evaporating. (Lakes and reservoirs in the region lose meters of water per year to evaporation.)

Historically, underground water was tapped by shallow wells dug in unconsolidated materials to get small flows of water. Today, much larger volumes of water are extracted from wells drilled tens to hundreds of meters into aquifers, which are rock layers with pores that contain water. Renewable aquifers are replenished (generally slowly) by rainfall; non-renewable, or fossil, aquifers contain water trapped in sediments laid down millions of years ago.

Just more than 10 percent of the water supply for the region comes from aquifers, but in Israel and Jordan the share approaches 50 percent, and in Kuwait and parts of the Arabian Peninsula it approaches 100 percent (apart from desalination). Libya's Great Man-Made River pumps water from fossil aquifers in the south of the country and moves it 930 miles (1,500 km) to farms and cities in the north.

The third most important source of water in the Middle East is recycled sewage, which is treated and reused, mainly for irrigation. Despite common
belief (shared by both Muslims and non-Muslims), there is no objection in Islamic law to the reuse of sewer water provided it is properly treated.

More than half of the world's desalination capacity is found in the region, mainly in the oil-producing nations of the Arabian peninsula with lots of by-product natural gas that was formerly flared. (Desalination is an energy-intensive process.) Costs for desalination have fallen to a level that makes it feasible as a source of potable water but still too expensive for irrigation.

Other sources of water are individually small but collectively provide sizable amounts of water. Water harvesting gathers rain that falls over a wide area and directs it to one field through small channels and micro-barrages. The technique can allow crops in areas where rainfall is only 4 inches (100 mm) per year. Rainwater is also collected from rooftops and stored in cisterns. If handled carefully, rooftop water can be used for drinking.

Uses of Water

By far the largest share of water in the region goes to agricultureas much as 90 percent of total water use in some countries, and 60 percent in the more industrialized countries.

Drinking requires only a relatively small volume of water, but it must meet higher standards than that used for irrigation. Thirty liters of potable water per person-day is generally regarded as the minimum for drinking, cooking, and washing.

Industrial water use is low. Food and beverage processing are the largest industrial consumers. More is withdrawn for cooling but most of this water is recycled or returned to the watercourse.

A hidden but critical amount of water must be left in place to support fisheries and hydropower, as well as to protect habitat. This use is typically neglected by governments when they drain swamps, canalize rivers, or extend land. As a result, not only has the environment been degraded, but fish catches have declined and the salinity of groundwater has increased.


The nations of the Middle East all face three overlapping sources of stress in their water management: 1) quantity, which has been a source of stress since history began; 2) quality, which is a newer stress but increasingly important; and 3) equity, which occurs when the same water is subject to competing demands.

Quantity. Iran, Iraq, Lebanon, Sudan, Syria, and Turkey are fairly well endowed with water, with more than 1 million cubic meters (Mcm) per capita; Algeria, Egypt, Israel, Morocco, and Palestine form a middle group; and Jordan, Libya, Tunisia, and the countries of the Arabian Peninsula are least well endowed, with less than 500 Mcm per capita. However, water availability is declining in every nation, which means that current patterns of water use are not sustainable. Some projections for the Jordan River basin suggest that by 2025 household and industrial uses will require all the fresh water, leaving none for farmers. Most nations are also drawing down their renewable aquifers and mining fossil ones. Some have annual water deficits of several thousand Mcm.

Water quantity problems in the region can be resolved in small part by exploiting additional

freshwater withdrawals by country and sector
estimates for 2000
        (cubic km/a)  
country total per capita use (%) domestic industry % with safe (cubic m/p) drinking water
* percentage by sector adjusted by author on basis of estimates by the planning department of the israeli water commission. all data for israel based on estimates by the author.
source: Gleick, Peter, et al, ed. The World's Water: The Biennial Report in Freshwater Resources, 20022003 (Washington, D.C., Island Press, 2002).
table by ggs information services, the gale group.
afghanistan 26.1 1,020 1 0 99 13
algeria 4.5 142 25 15 60 94
bahrain 0.2 387 39 4 56 100
egypt 55.1 809 6 8 86 95
iran 70.0 916 6 2 92 95
iraq 42.8 1,852 5 3 92 85
israel 1.7 280 36 11 51* 100
jordan 1.0 155 22 3 75 96
lebanon 1.3 393 28 4 68 100
libya 4.6 720 11 2 87 72
morocco 11.1 381 5 3 92 82
oman 1.2 450 5 2 94 39
saudi arabia 17.0 786 9 1 90 95
sudan 17.8 597 4 1 94 75
syria 14.4 894 4 2 94 80
tunisia 3.1 313 32 8 60 99
turkey 31.6 481 16 11 72 83
yemen 2.9 162 7 1 92 69

sources of supply but in much larger part by better use of the water that is already available. People in the region use less water than those elsewhere in the world, but as a result of poor management and misguided economic policies conservation here (as in most other parts of the world) remains far short of its potential. Many nations lose half the water put into municipal systems to leaks, and they typically deliver piped water at low (or no) price. Cost-effective savings of 25 to 50 percent are possible in most uses.

Moreover, every country in the region provides water to farmers at highly subsidized prices. Under the influence of higher prices, Israeli scientists developed drip irrigation systems that have cut water use per hectare by 40 percent. However, drip irrigation is expensive and not appropriate for all crops. Lower-cost sprinkler systems, used at night to minimize evaporation, can also increase irrigation efficiency, as can irrigating only at times critical to plant growth.

Most analysts find that water is tens of times more valuable in industrial or household uses than in agriculture. Therefore, crops grown in the region will gradually be replaced by imports. It takes roughly a thousand tons of water to produce one ton of wheat. Using that ratio, Middle Eastern nations already import grains with a virtual water content equal to the flow of the Nile.

Quality. Much of the limited fresh water in the Middle East is polluted from growing volumes of human, industrial, and agricultural waste. Three problems stand out: 1) Overpumping of wells causes a decline in the water tableby as much as a meter a year in some areas. This decline adds to pumping costs and permits lower-quality water (or, if near the coast, seawater) to flow inward and contaminate the aquifer. The only way to avoid the problem is to match pumping rates to inflow. 2) Agricultural runoff is the major non-point source of water pollutionmainly sediment, phosphorus, nitrogen, and pesticides. Better farming methods, such as conservation tillage, contour planting, and terracing can control soil erosion and cut pollution by half or more. 3) Urban sewage systems have either begun to deteriorate or cannot handle the growing loads placed on them. Large investments are needed to improve their physical infrastructure.

Equity. Most of the larger rivers in the region cross an international bordersome cross several bordersor form a border. No tabulation exists for aquifers that underlie national borders, but there are many.

Despite many statements suggesting that the next war in the Middle East will be over fresh water, there is little evidence for this. Not a single war has been fought over water for hundreds of years, but many treaties dealing with water have been signed. Water will be a source of conflict, but the conflicts will mainly be intranational rather than international. Likely sources of conflict include rural and urban users contending for the same water and rising demands from poor farmers, who are often disadvantaged in their access to water, and from women, who typically want more water for their households while men prefer to use it to grow cash crops. Israeli control of water in the West Bank is contentious, but even here experts have shown that compromise is feasible.

None of the three stresses on water in the Middle East will be easily resolved. Most of the nations in the region have already reached or are fast approaching the limits of their indigenous water supplies. Although higher prices for water and technological advances may defer the crisis, the only long-term solutions involve much greater efficiency in use, full reuse of wastewater, and gradual shifts of water from agriculture to other sectors. All of the nations of the Middle East and North Africa must revise their water policies to provide for a sustainable future, and they must find equitable ways to share water within and between nations.


Amery, Hussein A., and Wolf, Aaron T., eds. Water in the Middle East: A Geography of Peace. Austin: University of Texas Press, 2000.

Beaumont, Peter. "Water Policies for the Middle East in the Twenty-first Century: The New Economic Realities." International Journal of Water Resources Development 18, no. 2 (2002): 315334.

Brooks, David B., and Mehmet, Ozay, eds. Water Balances in the Eastern Mediterranean. Ottawa: International Development Research Centre, 2000.

Kolars, John. "The Spatial Attributes of Water Negotiation: The Need for a River Ethic and River Advocacy in the Middle East." In Water in the Middle East: A Geography of Peace, edited by Hussein A. Amery and Aaron T. Wolf. Austin: University of Texas Press, 2000.

Lonergan, Stephen C., and Brooks, David B. Watershed: The Role of Fresh Water in the Israeli-Palestinian Conflict. Ottawa: International Development Research Centre, 1994.

Postel, Sandra. Pillar of Sand: Can the Irrigation Miracle Last? New York: Norton, 1999.

Rogers, Peter, and Lydon, Peter, eds. Water in the Arab World: Perspectives and Prognoses. Cambridge, MA: Division of Applied Sciences, Harvard University, 1994.

Shapland, Greg. Rivers of Discord: International Water Disputes in the Middle East. New York: St. Martin's Press; London: Hurst, 1997.

Waterbury, John. The Nile Basin: National Determinants of Collective Action. New Haven, CT: Yale University Press, 2002.

Wolf, Aaron T. "Transboundary Fresh Water Database." Department of Geosciences, Oregon State University. Available from <http://www.transboundarywaters.orst.edu>.

david b. brooks

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Water is a chemical compound needed by most plants and animals on Earth in order to sustain life. Pure water is a tasteless, odorless, transparent liquid. In small amounts it is colorless, but it takes on a bluish tint in larger amounts. Water is an excellent solvent and as a result it usually contains a wide variety of dissolved minerals and other chemicals. It can also carry and support bacteria. Most of the water distributed through municipal water systems is treated to remove harmful substances. Some bottled waters undergo even further treatment to remove almost all impurities. The English word water is derived from the German word wasser, which in turn is derived from an ancient Indo-European word meaning to wet or wash.

The controlled use of water dates to at least 8,000 b.c. when farmers in Egypt and parts of Asia trapped floodwaters for crop irrigation. The concept of using irrigation canals to bring water to crops, rather than waiting for a flood, was first developed about 2,000 b.c. in Egypt and Peru. By about 1,000 b.c., the city of Karcho, in what is now Jordan, built two aqueducts to bring an adequate supply of water for the city's population. This is the first recorded instance of a planned municipal water supply.

Early water treatment was surprisingly advanced, although rarely practiced. An ancient Sanskrit manuscript, from what is now India, advises that drinking water should be kept in copper vessels, exposed to sunlight, and filtered through charcoal. Ancient Egyptian inscriptions give similar advice. Many of these methods are still used today. In about 400 b.c., the Greek medical practitioner Hippocrates suggested that water should be boiled and strained through a piece of cloth. Despite these early references, most people drank untreated water from flowing streams or subterranean wells. As long as there were no sources of contamination nearby, this was a satisfactory solution.

As the population of Europe and other parts of the civilized world grew, their sources of water became increasingly contaminated. In many cities, the rivers that served as the primary sources of drinking water were so badly contaminated with sewage that they resembled open cesspools. Cholera, typhoid, and many other water-borne diseases took their toll. In 1800, William Cruikshank of England demonstrated that small doses of chlorine would kill germs in water. By the 1890s, several municipalities found that slowly filtering water through beds of sand could also significantly reduce the incidence of disease. The public outcry for safe drinking water reached such a crescendo that by the early 1900s most major cities in the United States had installed some sort of water treatment system.

Even with water treatment, water contamination remained a serious concern as an increasing amount of industrial wastes poured into the nation's rivers and lakes. As the adverse health effects of lead, arsenic, pesticides, and other chemicals became known, the United States federal government was obliged to pass the Water Pollution Control Act of 1948. This was the first comprehensive legislation to define and regulate water quality. It was followed by a series of increasingly tougher requirements, culminating in the current Environmental Protection Agency (EPA) water quality standards. In addition to the federal standards, most states have their own water quality laws, and some state laws are more stringent than those specified by the EPA.

Types of Water

Pure water is an almost non-existent entity. Most water contains varying amounts of dissolved minerals and salts, plus an abundance of suspended particles such as silt and microscopic organic material. Different types of water are classified by the presence or absence of these impurities.

Tap water, or municipal water, has under-gone a series of treatments to kill harmful bacteria, remove sediments, and eliminate objectionable odors. It may also have had one or more chemicals added for a variety of reasons.

Hard water contains high amounts of calcium and magnesium salts. This causes soap to form curds. Hard water is further divided into temporarily hard water and permanently hard water. Temporarily hard water contains bicarbonates of calcium and/or magnesium, which react to form a hard substance called scale when the water is heated. Scale can clog hot water heaters and pipes and leave deposits on cooking utensils. Permanently hard water contains sulphates, chlorides, or nitrates of calcium and/or magnesium, which are not affected by heating. Soft water contains relatively low amounts of calcium and magnesium salts, although the definition of "low" varies. The term "softened water" refers to hard water that has had enough salts chemically removed to avoid forming soap curds. It is high in sodium chloride.

If water contains a large quantity of dissolved minerals, it is called mineral water. Mineral waters can be divided into five main classes: saline, alkaline, ferrunginous, sulphurous, and potable. Saline water has a high level of sodium or magnesium sulphate or sodium chloride. Alkaline water has a high concentration of salts which give it a pH in the range of about 7.2-9.5, where a pH of 7 is neutral and a pH of 14 is highly alkaline. Ferrungious water is rich in iron, which gives it a rusty color. Sulphurous water is rich in sulphur compounds and is distinguished by its rotten egg smell. Potable water has a mineral content of less than 500 parts per million and is most commonly bottled and sold as a specialty drinking water.

Carbonated water, soda water, and sparkling water all contain dissolved carbon dioxide. This may occur naturally where limestone or other carbonate rocks are present, or the carbon dioxide may be added artificially under pressure.

Spring water and artesian water are distinguished only by the fact that they flow from the ground naturally without the aid of drilling or pumping. Otherwise, there is nothing that makes them different than water from other sources.

Distilled water has been purified by an evaporation-condensation process that removes most, but not all, impurities. Deionized water has been purified by an ion-exchange process, which removes both positive ions, such as calcium and sodium, and negative ions, such as chlorides and bicarbonates. It is sometimes called de-mineralized water. Purified water is municipal water that has undergone carbon filtration, distillation, deionization, reverse osmosis, ultraviolet sterilization, or some combination of these processes to remove almost all minerals and chemical elements, both good and bad.

Raw Materials

A water molecule consists of two atoms of hydrogen bonded to one atom of oxygen. The chemical symbol is H2O. Water usually also contains a wide range of organic and inorganic materials in solution or suspension.

In the process of treating water for use in a municipal system, several chemicals may be added. These include disinfectants like chlorine, chloramine, or ozone; coagulantants like aluminum sulfate, ferric chloride, and various organic polymers; acidity neutralizers like caustic soda or lime; and chemicals to help prevent tooth decay in the form of various fluoride compounds.

The Treatment Process

The specific water treatment process depends on the intended application. Some water, such the water used to irrigate crops, receives no treatment. Other water, such as the water used to make pharmaceuticals, is highly purified.

Here is a typical series of operations used to treat municipal water for distribution to homes and businesses.


  • 1 Most municipal water comes from two sources: ground water and surface water. Most ground water is tapped by drilling wells into the underground water-bearing layer called the aquifer. Some ground water rises naturally in the form of springs. Surface water is tapped by impounding rivers behind dams. The surrounding area that drains into the rivers is called the watershed. In many cases, access to and use of the watershed is limited to prevent contamination of the runoff water.
  • 2 From the well or dam, the water is carried to the water treatment plant in open canals or closed pipes. In some cases, the water supply is close to the municipality. In other cases, the water has to be transported many hundreds of miles (km) to reach its destination. Sometimes the water is stored in intermediate reservoirs along the way to ensure that there will always be an adequate supply available to meet a city's fluctuating needs.


  • 3 In some water treatment plants, the water is initially disinfected by contact with ozone-rich air in a series of chambers. This step is used by most plants in Europe, but only a few plants in the United States. Ozone (03) is formed by passing compressed air through a high-voltage electric arc. This causes some of the oxygen (02) molecules in the air to split in half and reattach themselves to other oxygen molecules to form ozone. Ozone effectively kills most germs and also destroys compounds, which cause unpleasant tastes and odors. It has a relatively short life, however, and does not remain in the water to protect it during storage and distribution. For this reason, a small dose of chlorine or chloramine is added to the water at the end of the treatment process.


  • 4 The water then passes through a flash mixer where chemicals known as coagulants are rapidly mixed with the water. The coagulants alter the electric charge around any suspended particles in the water and make them attract each other and clump together, or coagulate.
  • 5 The water moves slowly through a series of chambers where it is gently mixed by the swirling flow. As the water mixes, the charged particles continue to bump into each other and form even larger particles called flocs.


  • 6 The water flows into a settling basin or tank where the heavy flocs sink to the bottom. Some settling basins have two levels to double their capacity. The material that settles to the bottom is vacuumed out of the basin with a device like a pool vacuum and is deposited in a solids holding basin. The trapped material from the filter (step 7) is also added to the solids holding basin. These combined materials are sent through a gravity thickener and then a press where most of the water is squeezed out. The remaining solids are loaded into trucks and transported to a landfill for disposal.


  • 7 The partially cleaned water passes through several layers of sand and pulverized coal, which trap any very small particles that remain in the water. Some harmful organisms are also trapped this way in those water treatment plants that do not use ozone as an initial disinfectant. The filter layers are back-flushed periodically to remove the trapped material.


  • 8 In some plants, the water is passed through a bed of activated charcoal granules. Chemical contaminants in the water stick to the surface of the charcoal in a process known as carbon adsorption.


  • 9 In some areas where the water contains undesirable amounts of iron and manganese or certain dissolved gases, the water is sprayed into the air from large basins to aerate it. When the water mixes with the air, it picks up oxygen, which causes some of the contaminants to settle out. Other contaminants are removed by evaporation.


  • 10 In some water treatment plants, a fluoes ride compound is added to the water to help prevent tooth decay. Fluoride occurs naturally in some water supplies and additional amounts are not required. In the past, fluoridation has been a hotly debated subject, and not every municipality adds fluoride to their water.


  • 11 Other chemicals may be added to the water to help reduce corrosion in pipes and plumbing fixtures. This is done by adding controlled amounts of certain chemicals to adjust the pH factor to a neutral level.


  • 12 As the water leaves the treatment plant, it receives a small dose of chlorine or chloramine to kill any harmful bacteria that may have found their way into the distribution system. If the plant does not use ozone as an initial disinfectant, a larger amount of chlorine or chloramine is added to the water.
  • 13 After the water leaves the plant, it is usually stored in covered tanks or reservoirs to protect it from contamination. In some areas, these storage facilities are located at a higher elevation than the surrounding terrain, and the water is pumped up into the tank or reservoir. This elevated storage position provides the pressure necessary for adequate flow through the water mains and pipes within the city. In other cases, the water is stored in ground-level facilities, and the pressure is supplied by electric pumps that run on demand.

Quality Control

The federal and state water quality standards set maximum contamination levels for more than 90 organic, inorganic, microbiological, and radioactive materials that may be found in water. These standards are further divided into primary standards, which cover materials that may be harmful to humans, and secondary standards, which cover materials and properties that may affect aesthetic qualities such as taste, odor, and appearance. A typical water district may perform more than 50,000 chemical and bacteriological analyses of the water supply each year to ensure the standards are being met.

The Future

The public's concern over safe drinking water is expected to result in even more stringent water quality standards in the future. Ironically, one of the most recent concerns is not about outside contamination, but about the effects of one of the substances commonly used to disinfect water—chlorine. Studies within the last 30 years have shown that chlorine forms certain compounds with the organic materials found in water. The most common compounds are called trihalomethanes, or THMs, which have a 1-in-10,000 risk of causing cancer when ingested or inhaled over a long period. One alternative to using chlorine is chloramine, which is a combination of ammonia and chlorine that does not form THMs as readily. Many water treatment plants have already switched to chloramine. Other alternative disinfectants include ozone, ultraviolet light, chlorine dioxide, and a hybrid of ozone and hydrogen peroxide called peroxone.

Where to Learn More


von Wiesenberger, Arthur. H2O: The Guide to Quality Bottled Water. Woodbridge Press, 1988.

Water Quality Standards Handbook, 2nd edition. United States Environmental Protection Agency, 1994.


Arrandale, T. "A Guide to Clean Water." Governing (December 1995): 57-60.

Wasik, J. F. "How Safe is Your Water?" Consumers Digest (May/June 1996): 63-69.


"Alameda County Water District Water Treatment Facility." Pamphlet. Alameda County Water District, 1993.

"Layperson's Guide to Drinking Water." Pamphlet. Water Education Foundation, 1995.

Los Angeles Department of Water and Power. http://www.ladwp.com.

—Chris Cavette

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If a person knows nothing else about chemistry, he or she will likely know that water is H2O. The chemical formula for water is common knowledge, used in advertisements, elementary school science classes, and casual conversation. But more than just a conversation piece, the formula H2O can tell a chemist a great deal of information.

For starters, H2O indicates that water is composed of two hydrogen atoms and one oxygen atom. That this is so can be demonstrated using very simple apparatusa couple of pieces of wire, a battery, and some tap water. Electrolysisthe decomposition of water molecules with electricitywill result when the wires are connected to the ends of the battery and the other ends are immersed in the water with a small gap between them. One electrode releases bubbles of pure oxygen and the other pure hydrogen. Measuring the volume of gases released reveals that twice as much hydrogen is produced as oxygen. Twice as much gas means two hydrogen atoms for every one oxygen atom. Of course, this is our modern understanding of water. When these experiments were first tried, around 1800, an explanation for the results was not available. But the experiments did force scientists to think about the nature of water.

From our modern understanding of both the formula of water and the Periodic Table, we also know that the hydrogen atoms in water are bound to the oxygen. That is, water is "HOH" and not "HHO." We know that atoms can form either covalent or ionic bonds to give molecules. In water, the interaction of hydrogen and oxygen is a polar covalent bond, meaning that the two elements share a pair of electrons and that each atom contributes one of the electrons in the pair. Since hydrogen is the first element of the Periodic Table, it has only one electron and can form just one covalent bonding interaction. In the case of water, hydrogen bonds by sharing its electron with the oxygen. If hydrogen shared its electron with the other hydrogen atom in this instance, there would be no electron available to interact with the oxygen. Indeed, hydrogen gas, H2, results when two hydrogen atoms form a covalent bond, and hydrogen gas is very different from water.

In a pure (nonpolar) covalent bond, both atoms have possession of the electron pair exactly the same amount of time. In a polar covalent bond, there is unequal sharing that results from an inequity in the distribution of the electrons due to the effective nuclear charge on the atoms. This polarization of the O-H interaction is critical to explaining all of the properties of water. It results in water having a dipole with the hydrogens having a slight positive charge and the oxygen having a slight negative charge. (More precisely, the advanced explanation is that the molecular orbital that describes the oxygen-hydrogen interaction has more oxygen character, resulting in a skewed electron distribution.)

If we consider oxygen's position in the Periodic Table, we know that it starts with six valence electrons, and since it has two bonds with hydrogen, two of its electrons are involved in bonding pairs. This means that the oxygen has four electrons remaining. These electrons are organized into two "non-bonding" pairs. That is, the oxygen of water has four pairs of electrons around ittwo that are interacting in polar covalent bonds with hydrogen and two that are not interacting when water is in the gaseous state. Four electron pairs means that the atoms adopt a tetrahedral arrangement with the two hydrogens occupying two corners and the electron pairs occupying the other two.

In a perfect tetrahedron, the angle between the hydrogens would be 109.5°, but because the lone pairs occupy a little more space, the experimentally measured angle in water is actually 104.5°.

The presence of two lone pairs plays a very important role in "hydrogen bonding," which is one of the most critical properties of water. The positively charged hydrogen of one water molecule can be attracted to the lone pair of an adjacent molecule, resulting in a weak hydrogen bonding interaction. This bonding is much weaker than the polar covalent bond that holds a water molecule together, but it is a substantial inter-molecular interaction resulting in two water molecules being attracted to one another. Water forms an extended network of hydrogen bonding interactions, with each water molecule capable of both creating and accepting two hydrogen bonds. As a result each and every water molecule in the liquid or solid state is surrounded by four hydrogen bonded neighbors. The presence of hydrogen bonding interactions means:

  • that water has an anomalously high melting and boiling point;
  • that solid water or ice is less dense than liquid; and
  • that water has a high surface tension.

Melting and Boiling Point of Water

Phase changes in matter result because of a change in the translational motion of molecules. A solid is a solid because its molecules are stuck in place. In a liquid, molecules can move past one another but are still closely associated. In a gas, molecules move independent of one another and only occasionally collide. It would make sense then that lighter molecules would shift from a solid to a liquid to a gas at lower temperatures than heavy molecules because they require less energy to get moving. Consider the molecular substances in Table 1.

Hydrogen gas, being the lightest and smallest molecule in the list, has the lowest melting point ( 259°C or 14.15 K or 434°F) and boiling point ( 253°C or 20.15 K or 423°F). Similarly, of the second row compounds with hydrogen, methane (CH4) has the lowest melting and boiling points. However, water does not follow this trend. Its melting point is 0°C or 32°F. Its boiling point is 100°C or 212°F. Compared to the other molecules around it or its heavier cousin, hydrogen sulphide (H2S), water has melting and boiling points that are anomalously high. This is due to the fact that the hydrogen bonds between water molecules must be broken for a phase transition to occur. The extra energy required results in more heat being necessary and a higher temperature.

Density of Ice

Hydrogen bonding interactions between water molecules hold the molecules in place in the solid state. The O-H....O interaction spaces all of thewater molecules in an orderly array, much like students sitting in rows of desks. This spacing provides an open structure. When water is in the liquid state, the water molecules hold on to each other through hydrogen bonding interactions, but individual molecules can occupy the space between rows. The result is that at a molecular level, more liquid water molecules can occupy a given volume than when water is in the solid state. More students will fit in a classroom if they are allowed to stand than if they are arranged in nice neat rows. More molecules or more mass in a given volume means a higher density.

Substance Molecular Weight (g/mol) Melting Point ˚C Boiling Point ˚C
source: Adapted from Jones, Loretta, and Atkins, Peter (1999). Chemistry: Molecules, Matter, and Change. New York: W. H. Freeman.
H2 2.016 259 253
N2 28.01 210 196
O2 32.00 218 183
CH4 16.04 182 162
NH3 17.03 78 33
H2O 18.02 0 100
HF 20.01 93 20
H2S 34.08 86 60

The decrease in density between water and ice has a number of important implications for the world around us. Ice floats because it is less dense than liquid water. This is not true of any other liquid/solid equilibrium. Solid methane sinks in liquid methane and solid ammonia sinks in liquid ammonia. Floating ice means that ponds and lakes freeze from the top down, allowing fish and other biota to live protected from the cold weather of winter. If water froze from the bottom up, life as we know it would not have evolved on Earth.

Surface Tension

Surface tension is a bulk property of matter and results in liquid water trying to contract to the smallest possible surface area for a given volume. Surface tension explains why water beads up on the surface of a freshly waxed car and droplets of water in a fog are spherical. The sphere is the shape with the minimum area for a given volume. Surface tension results because of the asymmetry of forces at the surface of liquid water. Water molecules at the surface are missing their hydrogen bonding interactions on one side. They are being "tugged" back into the bulk of solution.

Of course, occasionally water molecules have sufficient energy to leave the surface, resulting in evaporation. Conversely, sometimes water molecules in the gaseous state strike the surface of a drop of water and have insufficient energy to leave again. The result is condensation. The competing rates of evaporation and condensation lead to the formation of clouds and fog, to cloudy mirrors after a shower and iced-up windows on a winter's day.

Surface tension is also critical to capillary action. A surface that is covered in suitable molecules or functional groups, such as a glass surface or the cellulose of paper, will interact with water molecules and can actually draw the molecules out of the bulk. In this case, the interaction with the surface is stronger than the hydrogen bonding interaction between adjacent water molecules. As a result, water will creep up a glass tube or adsorb into paper. The latter is critically important in mopping up a spill or mess.

Universal Solvent

Water is often called the "universal solvent," as it is capable of dissolving a wide range of compoundsfrom sugars to salt, from DNA to hydrogen. Again, hydrogen bonding interactions play a role. For example, sugar dissolves because of the hydrogen bonding interactions between the hydroxyl groups on the sugar molecules (-OH groups) and the water molecules. But of equal importance to the dissolution of substances in water is water's capacity to act as a dipole. Water's negatively charged oxygen binds to sodium ions in salt while the positively charged hydrogens interact with the chloride ions. The result is that sodium chloride or table salt dissolves into ionic species that are more energetically stable with the sodium and chloride ions surrounded by water.

The ability of water to dissolve a wide variety of substances makes it the ideal medium for living organisms. Water's great solvency is also the reason that water pollution is so pervasive. Almost any substance will dissolve in water, including pesticides, herbicides, industrial waste, household byproducts, and a wide variety of other potentially harmful compounds. Indeed, we rely on the dissolving properties of water to get our clothes clean.

Water Source Water Volume(cubic miles) Mass (kg) Percent of Total Water
source: Adapted from the U.S. Geological Survey at http://ga.water.usgs.gov/edu/waterdistribution.html.
Oceans 317,000,000 1.335 1021 97.24%
Icecaps, glaciers 7,000,000 2.949 1019 2.14%
Ground water 2,000,000 8.424 1018 0.61%
Fresh-water lakes 30,000 1.264 1017 0.009%
Saline lakes and inland seas 25,000 1.053 1017 0.008%
Soil moisture 16,000 6.740 1016 0.005%
Atmosphere 3,100 1.306 1016 0.001%
Rivers and streams 300 1,264 1015 0.0001%

But the dirt and grime from our clothes must end up somewhere, and that somewhere is in the water that we discharge from our homes. The Law of Conservation of Matter says that matter can be neither created nor destroyed. The atoms and molecules that we dissolve into the water in our washing machines are only being removed to another location.

Dealing with the pollution of water is a huge task, and for too long the philosophy was "the solution to pollution is dilution." Dilution is no longer an acceptable approach, as it just shifts the problem instead of addressing it. Significant effort is being spent in both addressing the real problems of water pollution and in ensuring that we have access to clean water sources. There are many techniques for purifying water, with distillation providing the cleanest and purest water. Unfortunately, distillation requires a lot of energy as it is difficult to overcome the hydrogen bonds between water molecules. Distillation also leaves behind the polluting material, which must be disposed of in a manner that does not allow it to come in contact with water and thus simply dissolve again. The difficulties of maintaining clean water is one of the major challenges facing us in the twenty-first century. For without water, life as we know it would not exist. It is because of the shape and the interactions of that very simple molecule, H2O, that water is the most essential of all chemical compounds.

see also Green Chemistry; Molecular Geometry; Valence Bond Theory; Water Pollution; Water Quality.

Todd W. Whitcombe


Atkins, Peter W. (1987). Molecules. New York: Scientific American Library.

Leopold, Luna B.; Davis, Kenneth S.; and the Editors of Life (1966). Water. New York: Time Incorporated New York.

Stanitski, Conrad L.; Eubanks, Lucy Pryde; Middlecamp, Catherine H.; and Pienta, Norbert J. (2003). Chemistry in Context : Applying Chemistry to Society. New York: McGraw-Hill.

Internet Resources

United States Geological Survey Water-Resources Investigations Report 984086. "Water Science for Schools." Available from <http://ga.water.usgs.gov/edu/index.html>.

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Water is an odorless, tasteless, transparent liquid that appears colorless but is actually very pale blue. The color is obvious in large quantities of water such as lakes and oceans. Water is the most abundant liquid on Earth. In its liquid and solid (ice) form, it covers more than 70 percent of Earth's surfacean area called the hydrosphere.

Earth's supply of water is constantly being replaced through a natural cycle called the hydrologic cycle. Water is continually evaporating from the surface of the planet, condensing in the atmosphere, and falling back to the surface as precipitation.

It is impossible to overstate the importance of water to almost every process on Earth, from the life processes of the lowest bacteria to the shaping of continents. Water is the most familiar of all chemical compounds known to humans. In fact, the human body is composed mainly of water.

Chemical properties of water

Water is a single chemical compound whose molecules consist of two hydrogen atoms attached to one oxygen atom. The chemical formula of this compound is H2O. Considering that a hydrogen atom weighs only about one-sixteenth as much as an oxygen atom, most of the weight in water is due to oxygen: 88.8 percent of the weight is oxygen and 11.2 percent is hydrogen. This percentage remains the same from a single water molecule to a lake full of water molecules.

Water can be made (synthesized) from hydrogen and oxygen, both of which are gases. When these two gases are mixed, however, they do not react unless the reaction is started with a flame or spark. Then they react with explosive violence. The tremendous energy that is released is a signal that water is an extremely stable compound. It is hard to break a water molecule apart into its components.

The normal boiling point of water is 212°F (100°C) and its freezing point is 32°F (0°C). As water is cooled to make ice, it becomes slightly denser, like all liquids. But at 39.2°F (4°C), it reaches its maximum density. When cooled below that temperature, it becomes less dense. At 32°F (0°C), water freezes and expands. Since ice is less dense than water, ice floats on it.

In pure water, 1 out of every 555 million molecules is broken down into a hydrogen ion and a hydroxide ion (an ion is an electrically charged atom or group of atoms). These ions are enough to make water a slight conductor of electricity. That is why water is dangerous when there is electricity around.

Because water dissolves so many substances (it is called the universal solvent), all of the water on Earth is in the form of solutions.

Words to Know

Aquifer: Underground layer of sand, gravel, or spongy rock that collects water.

Estuary: Lower end of a river where ocean tides meet the river's current.

Hydrologic cycle: Continual movement of water from the atmosphere to Earth's surface through precipitation and back to the atmosphere through evaporation and transpiration.

Ion: A molecule or atom that has lost one or more electrons and is, therefore, electrically charged.


The oceans contain more than 97 percent of all the water on Earth. However, seawater is unsuitable for drinking because of the large amount of dissolved salts in it. The six most abundant elements making up these salts in seawater are chlorine, sodium, sulfur, magnesium, calcium, and potassium. Chlorine and sodium, the most abundant of these elements, combine to form sodium chloride, more commonly known as table salt.

These elements are deposited in seawater through various means. Volcanic activity (on land and on the seafloor) releases chlorine and sulfur. Other elements reach the oceans through runoffs from land. Rain and other precipitation weathers and erodes rocks and soil on land, dissolving the minerals (salts) they contain. This material is then transported to the oceans by rivers.

Salinity is the measure of the amount of dissolved salts in seawater. This measurement is usually the mass of material dissolved in 1,000 grams (35 ounces) of water. The average salinity of seawater is about 35 grams (1.2 ounces) of salts in 1,000 grams (35 ounces) of seawater, or 3.5 percent of the total.

Hard Water

Hard water is water that contains large amounts of ions (electrically charged particles) of calcium, magnesium, or iron. Hard water often has an unpleasant taste, interferes with the ability of soaps to dissolve, and can cause scaling (the building up of insoluble material) in pipes and hot water systems.

Water hardness is most commonly the result of acidic water containing carbon dioxide passing through limestone or dolomite and dissolving the minerals these rocks contain. The dissolved minerals lead to an increase in the amounts of calcium and magnesium ions in the water.

Hard water can be treated by boiling the water, but this method is effective only for small quantities. A more efficient method is to use ion-exchangers, in which the unwanted calcium and magnesium ions are exchanged or traded for sodium ions that do not cause scaling. Most water softeners work by the ion-exchange method. The soft water that is produced is not free of ions, only of undesirable ions.

Desalination. Desalination is the process of removing salt from sea-water to provide essential water for drinking, irrigation, and industry, especially in desert regions or areas where freshwater is scarce. In the almost 4,000 desalination plants worldwide, most desalination takes place through two methods: distillation and reverse osmosis.

At its simplest, distillation consists of boiling seawater to separate it from dissolved salt. Once the seawater boils, water vapor rises, leaving the salt on the bottom of the tank. The water vapor is then transferred to a separate, cooler tank where it condenses as pure liquid water. Heat for distillation usually comes from burning fossil fuels (oil and coal). Distillation is widely used in the Middle East, where fossil fuel is plentiful but freshwater is scarce.

Reverse osmosis uses high pressure to force pure water out of saltwater. Pressures up to 60 atmospheres (800 to 1,200 pounds per square inch) are applied to saltwater, forcing it through a special membrane that allows only pure water to flow through, trapping the salt on the other side. Reverse osmosis is widely used to desalinate brackish water, which is less salty than seawater and therefore requires pressures only about one-half as great.

Brackish water

Brackish water has a salinity between that of freshwater and seawater. Brackish waters develop through the mixing of saltwater and freshwater. This occurs mostly near the coasts of the oceans in coastal estuaries (the lower course of a river where it flows into an ocean) or salt marshes that are frequently flooded with ocean currents due to the rising and falling of tides.

Most species can tolerate either saltwater or freshwater, but not both. Organisms that live in brackish habitats must be tolerant of a wide range of salt concentrations. The small fish known as killifish are common residents of estuaries, where within any day the salt concentration in tidal pools and creeks can vary from that of freshwater to that of the open ocean. During their spawning migrations, salmon and eels experience a range of salt concentration as they move through all three water environments: seawater, brackish water, and freshwater.


Freshwater is chemically defined as water that contains less than 0.2 percent dissolved salts. Of all the water on Earth, less than 3 percent is freshwater. About two-thirds of all freshwater is locked up in ice, mainly in Greenland and the Antarctic.

The remaining freshwaterless than 1 percent of all the water on Earthsupports most plants and animals that live on land. This freshwater occurs on the surface in lakes, ponds, rivers, and streams and underground in the pores in soil and in subterranean aquifers in deep geological formations. Freshwater also is found in the atmosphere as clouds and precipitation.

Worldwide, agricultural irrigation uses about 80 percent of all freshwater. The remaining 20 percent is used for domestic consumption, as cooling water for electrical power plants, and for other industrial purposes. This figure varies widely from place to place. For example, China uses 87 percent of its available water for agriculture. The United States uses 40 percent for agriculture, 40 percent for electrical cooling, 10 percent for domestic consumption, and 10 percent for industrial purposes.

[See also Hydrologic cycle; Lake; Ocean; River ]

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Solar System, Water in the

Solar System, Water in the

Liquid water is an essential ingredient for all life on Earth. With 70 percent of its planetary surface covered by water, Earth is unique in the solar system. Earth is the only planet known to have indigenous life; yet water, mostly as water ice, is found throughout the solar system.

Why is water so abundant on Earth? The answer lies in the materials from which the planets were formed, the conditions under which the planets formed, and the properties of water. Hydrogen and lesser amounts of helium were the most abundant ingredients of the "chemical stew" from which the solar system emerged. Oxygen, one of the most common products of nuclear fusion, is found throughout the molecular clouds from which new stars and new solar systems are born. Oxygen combines with hydrogen to make H2O, or water.

The Inner Terrestrial Planetary Bodies

In the solar system, inward of the asteroid belt , oxygen combined with elements such as silicon, aluminum, and calcium to make minerals and rocks that formed the inner terrestrial planetary bodies (Mercury, Venus, Earth, the Moon, and Mars). Beyond the asteroid belt, in the colder conditions of the outer solar system, water condensed to form ice grains that were incorporated in the giant planets (Jupiter, Saturn, Uranus, and Neptune) and their moons.


Mercury, the closest planet to the Sun with an average distance to the Sun of 0.387 astronomical unit , or about 58 million kilometers, is in a very unusual elliptical orbit around the Sun. It spins three times around its own rotational axis for every two times that it orbits the Sun. This results in the greatest range of temperature extremes in the solar system. Daytime surface temperatures on Mercury can rise to 800 kelvin, then fall to 90 kelvin during the night. In other words, temperatures can range from about 525°C (980°F) down to 180°C (297°F).

Although Mercury is one of the Earth's closest neighbors in the solar system, it has been studied very little and the majority of detailed information on the closest planet to the Sun was gathered during three Mariner flyby missions during 1974 and 1975. Mercury's closeness to the Sun makes it particularly difficult to observe with conventional telescopes, but advances in radar techniques in the 1990s revealed large deposits of possible water ice (equivalent to thousands of cubic kilometers) in permanently shaded craters near the poles of Mercury.


The cloud layers that permanently shroud Venus are largely composed of carbon dioxide and sulfur gases, but include water vapor. In a runaway greenhouse effect , the clouds reflect the thermal energy radiated from the surface of Venus back onto the surface, resulting in the high surface temperatures of around 450°C (842°F). However, this may not always have been the case. Studies of the decomposition of hydrous minerals (minerals that contain water) at extreme temperatures provide tentative indirect evidence that Venus may have had surface water in the past. The hot and dry Venus of today may have been a wet planet in the past, like Earth and ancient Mars.


Water that was present during the formation of Earth and the other planets was incorporated into the bedrock of the planets or on their surfaces. Much of the water on Earth today may have been delivered by cometary and asteroidal bombardment several hundred million years after the planet formed. That includes the water in the atmosphere, the oceans, the polar caps and glaciers, in human bodies, and in all other living things in the biosphere.

The Moon.

The existence of water ice in polar regions of the Moon was first suggested in the late 1800s. The Lunar Prospector mission, launched in January 1998, carried an instrument that detected possible indications for water ice at both the north and south poles of the Moon. While some scientists contemplated deep frozen reservoirs in permanently shaded craters, the widespread nature of the signals suggest the water may be bound up in clays. The possible presence of water on the Moonin any formwould be a great natural resource for future human missions (for propulsion as well as drinking water) and would provide valuable information about earlier conditions in the EarthMoon system.


Water is certainly present on Mars. There is water vapor in the Martian atmosphere that forms into white clouds, fog, and frost. Water ice exists in a residual polar cap at its north pole. Images provided by the Mars Global Surveyor spacecraft suggested the existence of subsurface water on Mars, and the Mars Odyssey spacecraft confirmed the presence of significant amounts of subsurface water on Mars via gamma-ray spectrometer measurements of subsurface hydrogen. These measurements indicate the presence of a considerable amount of water a few meters below the Mars surface. Further discussions of water on Mars may be found elsewhere in this encyclopedia.

Asteroids and Comets

Many planetary scientists believe that cometary impacts early in Earth's history could have supplied much of the water for its oceans. Further discussions of asteroidal and cometary water may be found elsewhere in this encyclopedia.

The Outer Solar System

The outer solar system is locked in a permanent ice age. Even so, water is present in the atmospheres of Jupiter, Saturn, Uranus, Neptune, and Pluto. Uranus and Neptune contain much larger proportions of ice-forming and rock-forming constituents than Jupiter or Saturn. Density and gravity field data for Uranus and Neptune, together with theories of the formation of those gas giants, suggest that both have massive global oceans of water ice.

Water condenses in the atmosphere of Jupiter. When the Galileo spacecraft arrived at Jupiter in 1995, it released a probe that measured levels of atmospheric water significantly lower than the expected abundance; however, the probe unexpectedly descended through a very dry part of the Jovian atmosphere.

Satellites of the Outer Solar System Planets.

The low bulk density of most of the outer solar system satellites (moons) indicates that they are composed from 30 percent to 70 percent of ice, mixed with higher density rock. The ice component includes water ice, carbon dioxide ice, carbon monoxide ice, and nitrogen ice.

Spectroscopic observations and magnetic field measurements taken by the Galileo spacecraft found evidence for the presence of liquid water under the frozen surfaces of three of Jupiter's four Galilean satellites: Callisto, Europa, and Ganymede. In the case of Ganymede, the water appears to be between 145 and 193 kilometers (90 and 120 miles) below the surface. Ganymede, the largest satellite in the solar system, is larger than either Mercury or Pluto.

Water ice has also been detected on all of the major satellites of Saturn and Uranus. The exterior layers of Pluto and Triton are composed of nitrogen, carbon monoxide, carbon dioxide, and water ices.

see also Astrobiology: Water and the Potential for Extraterrestrial Life;Comets and Meteorites, Water in; Earth: The Water Planet; Earth's Interior, Water in the; Mars, Water on; Volcanoes and Water.

Pat Dasch


Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System, 4th ed. New York: Cambridge University Press, 1998.

Cattermole, Peter. Venus. Baltimore, MD: Johns Hopkins University Press, 1994.

Dasch, Pat, ed. Space Sciences. New York: Macmillan Reference USA, 2002.

Spudis, Paul D. The Once and Future Moon. Washington, D.C. and London, U.K.:Smithsonian Institution Press, 1996.


The term "water ice" may seem redundant to most people, because frozen water is commonly encountered on Earth. Yet within the solar system, gases such as carbon dioxide, carbon monoxide, and nitrogen can exist as ice on the surface of planetary bodies. Hence, the descriptor "water ice" is necessary to distinguish it from carbon dioxide ice or other icy forms.

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Water is a colorless and odorless liquid made up of molecules containing two atoms of hydrogen and one atom of oxygen . Water is essential for all life to exist, as it makes up more than 70 percent of most living things. While a human can survive more than a week without food, a person will die within a few days without water.


Water serves as a solvent for nutrients and delivers nutrients to cells, while it also helps the body eliminate waste products from the cells. Both the spaces between cells (intercellular spaces) and the spaces inside cells (intracellular spaces) are filled with water. Water lubricates joints and acts as shock absorbers inside the eyes and spinal cord. Amniotic fluid, which is largely water, protects the fetus from bumps and knocks.

Water also helps the body maintain a constant temperature by acting as a thermostat. When a person is too hot, whether from being in a hot environment or from intense physical activity, the body sweats. When sweat evaporates, it lowers the body temperature and restores homeostasis .

Sources of Water

About 70 percent of the earth's surface is covered with water. The amount of water in a human body depends on age, gender, body type, and level of physical activity. The bodies of infants up to about twelve months of age contain about 58 percent water; the bodies of children six to seven years of age are 62 percent water; teenage boys are about 59 percent water; and teenage girls are about 45 percent water. The body of an adult male is approximately 62 percent water, while an adult female is 51 percent water. Physically active individuals generally have more water in their bodies than those who are less physically active. Because they sweat more, active people need to replenish water more often, thus raising their water level. A trained male runner may have up to 71 percent water in his body, while a female gymnast may have 70 percent. Obese individuals, on the other hand, have a lower percentage of water in their bodies (about 48%). Morbidly obese individuals are only about 36 percent water. In addition, the older one gets, the less water is retained in one's cells. As a result, old skin looks drier and wrinkles appear.


The most efficient way for the body to get water is for a person to drink water. It is recommended that an adult drink eight to ten eight-ounce glasses of water a day. Athletes and active teens should drink at least ten to twelve glasses of water daily. However, many foods and beverages contain water, which can make up part of this daily intake. Fresh fruits and vegetables, cooked vegetables, canned and frozen fruits, soups, stews, juices, and milk are all sources of water. Most fruits and vegetables contain up to 90 percent water, while meats and cheeses contain at least 50 percent. Metabolic processes in the human body generate about 2.5 liters of water daily.

Water Balance

Water balance refers to the balance between the amount of water consumed and the amount of water excreted. The body's water content needs to be constant for optimal functioning. Cells are bathed in interstitial fluids (fluids from between cells) that contain nutrients. These fluids also carry metabolic wastes away from the cells. Intracellular fluids facilitate chemical reactions inside the cells, and they help maintain cell structure by adhering to the cell's larger molecules, such as proteins and glycogen . Body fluids contain solutes (chemical compounds that are soluble in water), which separate into charged particles, or ions, when dissolved in water. Intracellular fluids are high in potassium and phosphate ions, while interstitial fluids are high in sodium and chloride ions. These ions help to maintain the amount of fluids both within and outside the cells. Water molecules follow the solutes moving across cell membranes from a lower to higher solute concentration to maintain homeostasis.

Water Intake Regulation

When the body has lost a lot of water, the concentration of solutes in the blood becomes too high. The solutes attract water from the salivary glands, making the mouth dry and causing a person to feel thirsty. The sense of thirst is a craving for water or other fluids. When water loss is slow, a person may have time to feel thirsty enough to replenish the water loss. In cases where the water loss is excessive and acute , however, and replenishment is not adequate, a state of dehydration can occur. Dehydration is a state in which the body has lost so much water that normal physiologic functions cannot take place, resulting in symptoms such as fainting and nausea .

Heat, intense physical activity (profuse sweating), diarrhea, vomiting, and excessive urination can all cause excessive fluid loss. A runner can sweat off six cups of fluid in an hour. Mild dehydration occurs with a loss of 5 percent or less of a person's bodily fluids, moderate dehydration is a loss of 5 to 10 percent of a person's bodily fluids, and severe dehydration is a loss of 10 to 15 percent of fluids. Severe dehydration can cause death. Some clinical signs of dehydration include dry skin, less frequent urination, fatigue , light-headedness, dark-colored urine, dry mouth, and lack of skin elasticity. Often, increased fluid intake and replacement of lost electrolytes are sufficient oral rehydration therapy for mild dehydration. However, the cause of dehydration has to be addressed for further improvement. In cases of severe dehydration, it may be necessary to hospitalize the person and restore fluid balance through intravenous fluid replacement.

Water Excretion Regulation

The brain and kidneys regulate the amount of water excreted by the body. When the blood volume is low, the concentration of solutes in the blood is high. The brain responds to this situation by stimulating the pituitary gland to release an antidiuretic hormone (ADH), which signals the kidneys to reabsorb and recirculate water. When the individual needs more water, the kidneys will excrete less and even reabsorb some.

When excessive fluid loss occurs, the blood volume will fall, as will blood pressure . The kidneys respond by secreting an enzyme called rennin. Rennin activates the blood protein angiotensinogen to convert to angiotensin, which causes the blood vessels to constrict and blood pressure to rise. Angiotensin also activates the adrenal glands to release a hormone called aldosterone. Aldosterone causes the kidneys to retain sodium and water. When the body needs water, less is excreted and more is retained.

Water Intoxication

Water intoxication occurs when there is too much fluid in the body. Excess fluid may collect in bodily tissue, particularly in the feet and legs, a condition called edema . Excess consumption of fluids, as well as kidney disorders that reduce urine output, may contribute to water intoxication. The symptoms of water intoxication are confusion, convulsions, and, in extreme cases, death.

see also Dehydration; Diarrhea; Nutrients; Oral Rehydration Therapy.

Kweethai C. Neill


Whitney, Eleanor N., and Rolfes, Sharon R. (2002). Understanding Nutriton, 9th edition. Belmont, CA: Wadsworth/Thomson Learning.

Shils Maurice, E., and Young, Vernon R. (1988). Modern Nutrition in Health and Disease, 7th edition. Philadelphia, PA: Lea and Febinger.

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water, odorless, tasteless, transparent liquid that is colorless in small amounts but exhibits a bluish tinge in large quantities. It is the most familiar and abundant liquid on earth. In solid form (ice) and liquid form it covers about 70% of the earth's surface. It is present in varying amounts in the atmosphere. Most of the living tissue of a human being is made up of water; it constitutes about 92% of blood plasma, about 80% of muscle tissue, about 60% of red blood cells, and over half of most other tissues. It is also an important component of the tissues of most other living things.

Chemical and Physical Properties

Chemically, water is a compound of hydrogen and oxygen, having the formula H2O. It is chemically active, reacting with certain metals and metal oxides to form bases, and with certain oxides of nonmetals to form acids. It reacts with certain organic compounds to form a variety of products, e.g., alcohols from alkenes. Because water is a polar compound, it is a good solvent. Although completely pure water is a poor conductor of electricity, it is a much better conductor than most other pure liquids because of its self-ionization, i.e., the ability of two water molecules to react to form a hydroxide ion, OH-, and a hydronium ion, H3O+. Its polarity and ionization are both due to the high dielectric constant of water.

Water has interesting thermal properties. When heated from 0°C, its melting point, to 4°C, it contracts and becomes more dense; most other substances expand and become less dense when heated. Conversely, when water is cooled in this temperature range, it expands. It expands greatly as it freezes; as a consequence, ice is less dense than water and floats on it. Because of hydrogen bonding between water molecules, the latent heats of fusion and of evaporation and the heat capacity of water are all unusually high. For these reasons, water serves both as a heat-transfer medium (e.g., ice for cooling and steam for heating) and as a temperature regulator (the water in lakes and oceans helps regulate the climate).

Structure of the Water Molecule

Many of the physical and chemical properties of water are due to its structure. The atoms in the water molecule are arranged with the two H-O bonds at an angle of about 105° rather than on directly opposite sides of the oxygen atom. The asymmetrical shape of the molecule arises from a tendency of the four electron pairs in the valence shell of oxygen to arrange themselves symmetrically at the vertices of a tetrahedron around the oxygen nucleus. The two pairs associated with covalent bonds (see chemical bond) holding the hydrogen atoms are drawn together slightly, resulting in the angle of 105° between these bonds. This arrangement results in a polar molecule, since there is a net negative charge toward the oxygen end (the apex) of the V-shaped molecule and a net positive charge at the hydrogen end. The electric dipole gives rise to attractions between neighboring opposite ends of water molecules, with each oxygen being able to attract two nearby hydrogen atoms of two other water molecules. Such hydrogen bonding, as it is called, has also been observed in other hydrogen compounds. Although considerably weaker than the covalent bonds holding the water molecule together, hydrogen bonding is strong enough to keep water liquid at ordinary temperatures; its low molecular weight would normally tend to make it a gas at such temperatures.

Various other properties of water, such as its high specific heat, are due to these hydrogen bonds. As the temperature of water is lowered, clusters of molecules form through hydrogen bonding, with each molecule being linked to others by up to four hydrogen bonds, each oxygen atom tending to surround itself with four hydrogen atoms in a tetrahedral arrangement. Hexagonal rings of oxygen atoms are formed in this way, with alternate atoms in either a higher or lower plane than their neighbors to create a kinked three-dimensional structure.

Liquid Water

According to present theories, water in the liquid form contains three different molecule populations. At the highest temperatures single molecules are the rule, with little hydrogen bonding because of the high thermal energy of the molecules. In the middle range of temperatures there is more hydrogen bonding, and clusters of molecules are formed. At lower temperatures aggregates of clusters also form, these aggregates being the most common arrangement below about 15°C. On the basis of these three population types and the transitions between them, many aspects of the anomalous behavior of water can be explained. For example, the tendency of water to freeze faster if it has been cooled rapidly from a relatively warm temperature than if it has been cooled at the same rate from a lower temperature is explained in terms of the greater number of irregularly shaped cluster aggregates in the cooler water that must find a suitable means of fitting together with a neighboring aggregate.

The discovery in the late 1960s of "superwater," or "polywater," helped to shed light on some aspects of the structure of water. This substance was thought by some to be a giant polymer of water molecules, 40 times denser and 15 times more viscous than ordinary water. Studies showed, however, that these new and unexplained properties were connected with the presence of contaminants in the water. Even so, the interaction of the water molecules with these other substances may be helpful in understanding the way in which water molecules interact with each other.


In ice, each molecule forms the maximum number of hydrogen bonds, resulting in crystals composed of open, hexagonal columns. Because these crystals have a number of open regions and pockets, normal ice is less dense than water. However, other forms of ice also exist at conditions of higher pressure, each of these different forms (designated ice II, ice III, etc.) having greater density and other distinct physical properties that differ from those of normal ice, or ice I. As many as eight different forms of ice have been distinguished in this manner. The higher pressures creating such forms cause rearrangements of the hexagonal columns in ice, although the basic kinked hexagonal ring is common to all forms.

When ice melts, it is thought that the fragments of these structures fill many of the gaps that existed in the crystal lattice, making water denser than ice. This tendency is the dominant one between 0°C and 4°C, at which temperature water reaches its maximum density. Above this temperature, expansion due to the increased thermal energy of the molecules is the dominant factor, with a consequent decrease in density.


See D. Eisenberg and W. Kauzmann, The Structure and Properties of Water (1969); A. K. Biswas, History of Hydrology (1970); C. Hunt and R. M. Garrels, Water: The Web of Life (1972); P. Ball, Life's Matrix: A Biography of Water (2000).

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Comets and Meteorites, Water in

Comets and Meteorites, Water in

An understanding of the earliest composition of the solar system is derived from analyses of the Sun, comets, and the little-altered carbonaceous chondrites . Most of these measurements have come from analyzing the spectra of light that originates from the Sun or that is reflected by bodies such as comets and asteroids , the presumed source of most meteorites . Many chemical analyses of meteorites have been made in laboratories on Earth and, increasingly, from spacecraft that encounter comets, asteroids, and the Sun's atmosphere.

Solar Components of Water

Because the great mass of matter in the solar system (nearly 99.9 percent) resides in the Sun, solar hydrogen (H), oxygen (O), and the water molecule (H20) must be considered when discussing extraterrestrial water.

The Sun, with an average surface temperature of about 6,000 kelvin (approximately 5,727°C or 10,340°F), would seem to be a poor place to look for water. However, scientists know that the constituents of water are enormously abundant in the Sun: hydrogen is its most abundant component, and oxygen also is a major component. Although tiny in amount, traces of the water molecule have been reported in cooler parts of the Sun's atmosphere.


Earth's solar system is believed to be about 4.6 billion years old, based in part on the age-dating of many chondrites . The Sun is slightly older. Comets, consisting of ice and grains of minerals and rocksthe "dirty snowballs" of astronomer Fred Whippleare thought to be the oldest, least altered of any of the components of the solar system. They contain mainly water ice, but other icy components have been measured as well, such as carbon monoxide, carbon dioxide, sulfur, hydrogen sulfide, methane, and hydrogen cyanide. (The term "water ice" is not redundant because in space, some gases can change to ice on the surface of planetary bodies.) Comets may have rocky cores as well as interspersed grains of material. The interspersed granular character extends to comet tails, which form by ablation as comets approach the Sun and the ice heats up.

These materials are minor components, however, compared to water ice. Comets thus account for a very significant fraction of water in the solar system: although small, there are many of them. The Oort Cloud of comets and the more recently described Kuiper Belt of comets account for untold numbers of comets and therefore comprise a huge reservoir of water. Many planetary scientists believe that cometary impacts early in Earth's history could have supplied much of the water for its oceans.


Though small in amount compared to cometary water, meteoritic and asteroidal water is important in understanding water-rich Earth. Earth accumulated early in the history of the solar system by the sweeping up and accumulating of enormous amounts of debris as the early Earth orbited the Sun. These materials contributed their contained water to the growing Earth.

As Earth grew, it evolved into a core, an intermediate mantle , and an external crust. Water played a critical part, especially through the process of melting of the mantle and crust, with resulting volcanism and other igneous activity. Owing to volcanism, water and other volatile components were expelled to the surface, forming, along with possibly significant additions of cometary water, Earth's hydrosphere .

Meteorites fall to Earth continuously and are studied intensively for the information they provide about the earliest solar system and Earth. Until the return of rocks from the Moon, meteorites were the only known samples of extraterrestrial materials. Most meteorites are believed to come from the asteroid belt , which contains numerous rocky objects that orbit the Sun, mainly between the orbits of Mars and Jupiter. Some asteroids may have originated as comets, with icy exteriors that were vaporized by the Sun, leaving the rocky core.

Meteorites and asteroids have been extensively studied for their chemical composition, including water. Carbonaceous chondrites, which are the most fundamental and least modified type of meteorites, and are chemically most like the composition of the Sun, have several percent of water, primarily chemically combined in their mineral structures. The amount of contained water may decrease or increase as meteorites are altered, such as through metamorphism , melting, or weathering or impacts.

see also Astrobiology: Water and the Potential for Extraterrestrial Life; Earth: The Water Planet; Earth's Interior, Water in the; Mars, Water on; Solar System, Water in the; Volcanoes and Water.

E. Julius Dasch


Chaisson, Eric J. Astronomy. 2nd ed. Upper Saddle River, NJ: Prentice Hall, 1997.


Where water is found on Earth, life is also found, even in the most extreme physical and chemical environments. Although there is no direct evidence that life was transported to Earth by way of comets or meteorites, cometary and meteoritic transport certainly delivered the components required for the development of life: water and organic (carbon-containing) compounds.

The most common surficial features of most planets and moons are impact craters, so water and organic compounds also have been transported to all other solar system bodies via the impacts of comets and asteroids. Where heat or other sources of energy exist, along with water and organic compounds, the potential for the development of life is high.

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Water is a chemical compound composed of a single oxygen atom bonded to two hydrogen atoms (H2O) that are separated by an angle of 105°. Because of their polar covalent bonds and this asymmetrical bent arrangement, water molecules have a tendency to orient themselves in an electric field, with the positively charged hydrogen toward the negative pole and the negatively charged oxygen toward the positive pole. This tendency results in water having a large dielectric constant, which is responsible for making water an excellent solvent. Water is therefore referred to as the universal solvent. Water can be reused indefinitely as a solvent because it undergoes almost no modification in the process.

Because mineral salts and organic materials can dissolve in water, it is the ideal medium for transporting products of geochemical weathering as well as life-sustaining minerals and nutrients into and through animal and plant bodies. Brackish and ocean waters may contain large quantities of sodium chloride as well as many other soluble compounds leached from Earth's crust .

The concentration of mineral salts in ocean water is about 35,000 parts per million. Water is considered to be potable (drinkable) only if it contains less than 500 parts per million of salts.

Hydrogen bonding, which joins water molecule to water molecule, is responsible for other properties that make water a unique substance. These properties include its large heat capacity, which causes water to act as a moderator of temperature fluctuations due to variations in solar illumination, its high surface tension (due to cohesion among water molecules), and its adherence to other substances, such as the walls of a vessel (due to adhesion between water molecules and the molecules of a second substance). The high surface tension makes it possible for surface-gliding insects and broad, flat objects to be supported on the surface of water. Adhesion of water molecules to soil particles is the primary mechanism by which water moves through unsaturated soils.

Hydrogen bonding is also responsible for ice being less dense than water. If ice did not float, all bodies of water would freeze from the bottom up, becoming solid masses of ice and destroying all life in them. In addition, from season to season, frozen water bodies would remain frozen, resulting in large changes in climate and weather , such as decreased precipitation due to reduced evaporation . Ice floats because as the temperature of water is lowered the tendency of water to contract as its molecular motion decreases is overcome by the strength of hydrogen bonding between molecules. At 4°C (39°F), water molecules start to structure themselves directionally along the lines of the hydrogen bonds, at angles of 105° As the temperature drops toward 0°C (32°F), spaces develop between the lines until the open, crystalline form characteristic of ice develops. Its openness produces a density slightly less than that of liquid water, and ice floats on the surface, with approximately nine-tenths submerged.

Water is the only common substance that occurs naturally on earth in three different physical states. The solid state, ice, is characterized by a rigid crystalline structure occurring at or below 0°C (32°F) and occupying a definite volume (found as glaciers and ice caps, as snow, hail, and frost, and as clouds formed of ice crystals ). At sea level atmospheric pressure , the liquid state exists over a definite temperature range 0°C to 100°C (32 to 212°F), but is not rigid nor does it have a particular shape. Liquid water has a definite volume but assumes the shape of its container. Liquid water covers three-fourths of Earth's surface in the form of swamps, lakes, rivers , and oceans as well as found as rain clouds, dew, and ground water. The gaseous state of water (water vapor) neither occupies a definite volume nor is rigid because it takes on the exact shape and volume of its container. Water vapor (liquid water molecules suspended in the air) occurs in steam, humidity, fog , and clouds.

During phase changes, one phase does not suddenly replace its predecessor as the temperature changes, but for a time at the melting or boiling point, two phases will coexist. As water changes from the gaseous form to the liquid form, it gives off heat at about 540 calories per gram, and as it changes from the liquid form to the solid form, it gives off about 80 calories per gram. The turbulence of thunderstorms is in large part due to the release latent heat of water especially as water condenses into water droplets or into crystals of ice (i.e., hail).

Pressure affects the transition temperature between phases. For example, at pressures below atmospheric, water boils at temperatures under 100°C (212°F), therefore food takes longer to cook at higher elevations.

Water is a major geologic agent of change for modifying Earth's surface through erosion by water and ice.

See also Acid rain; Atmospheric chemistry; Chemical bonds and physical properties; Chemical elements; Clouds and cloud types; Condensation; El Nino and La Nina phenomena; Erosion; Evaporation; Freezing and melting; Freshwater; Rate factors in geologic processes

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Earth's Interior, Water in the

Earth's Interior, Water in the

Water is found in different physical states at different levels below the Earth's surface. At shallower depths, it is a liquid occurring in fractures and in the pore spaces of rocks and unconsolidated materials. At greater depths, it is found as a gas within pore spaces, at minerals boundaries, and even locked within a part of the crystalline structure of the minerals themselves.

Water States

The major source of trapped water at shallow depths is rain and melted snow that infiltrates the ground and moves through the interconnected pore spaces as groundwater . Groundwater found at greater depths, about 300 meters (approximately 1,100 feet), generally has been below the surface for hundreds to thousands of years. Below a depth of about 750 meters (approximately 2,500 feet), the amount of groundwater gradually, though irregularly, diminishes.

Water that is present in pore spaces or around mineral-grain boundaries at greater depths will occur as a vapor due to the high temperatures. The pore spaces found at this depth are usually small and are no longer well interconnected. At that depth, water may also be found locked in the crystal structure of minerals as fluid inclusions , or in hydrous minerals as molecular water, hydroxyl anions, or protons.

Many scientists believe that the existence of the Low Velocity Zone (LVZ) is due to the presence of partially molten rock. The LVZ is described as a region approximately 60 to 150 kilometers (37 to 94 miles) deep beneath oceanic plates where earthquake wave velocities are noticeably slower. The LVZ occurs because the presence of water lowers the melting temperature of mantle rock, producing a partially molten region.

Influence of Water

The presence of water in the pore spaces at great depths within the Earth plays a role in metamorphism . Water enhances metamorphism by increasing the rate of chemical reactions, by increasing the distribution of heat, and by forming water-bearing minerals. Under dry conditions, most minerals react slowly, but when water is present, reaction rates increase, mainly because ions can move readily with water through the rock, thereby increasing the rate of chemical reactions and the formation of new minerals.

Metamorphism can also release water. This can occur either through a chemical reaction that produces water, or when water molecules previously locked in the crystalline structure of the minerals are released. This water, along with dissolved ions from the host rock, is sometimes carried away through systems of rock fractures. As this extremely hot, ion-rich water travels these cracks, it reacts chemically with the surrounding rock. This produces a distinct type of metamorphism called metasomatism ("meta" meaning change and "soma" derived from the Sanskrit word for juice ). During metasomatism, water temperatures can be 250°C (482°F) or higher. Such fluids are called hydrothermal solutions. ("Hydro" is the Greek word for water and "thermal," from "therme," is the Greek word for heat ).

Metamorphism may also have its own "seismic signature." A different type of earthquake activity has been noted in subduction zones. The activity is more like that associated with volcanoes; the signals may record the effect of metamorphism and associated fluids in subducting plates.

Hydrothermal solutions form veins by depositing dissolved minerals as the solution flows through the cracks in the rocks. By altering existing rocks, hydrothermal solutions play an important role in the formation of ore deposits. Probably more mineral deposits have been formed by deposition from hydrothermal solution than by any other way. Examples of rich hydrothermal ore deposits include the famous tin-copper-lead-zinc veins of Cornwall, England; the gold-quartz veins of Kalgoorlie, Australia; and the tin-silver veins of Potsi, Bolivia.

see also Groundwater; Hot Springs and Geysers; Plate Tectonics; Springs; Volcanoes and Water.

Alison Cridland Schutt


Monroe, James S., and Reed Wicander. Physical Geology: Exploring the Earth, 2nd ed.St. Paul, MN: West Publishing Company, 1995.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology, 4th ed., New York: John Wiley & Sons, 2000.


Earthquakes result from rock deformation, typically faulting. Regions of rock around an active fault are stretched, whereas other regions are squeezed. This squeezing and stretching causes the rock volume and therefore its pore spaces to be increased or decreased.

A decrease in pore space size will squeeze confined groundwater into a nearby water well, causing the water level in the well to rise. An increase in pore space size will cause water to be drawn from the well into the surrounding rock, thereby yielding a water-level decrease in the well.

Consequently, careful measurement of water well levels near an earthquake can indicate how stress fields change after seismic events. This is important for understanding how kinematic stresses change, and it helps geologists measure the probability of future earthquakes in that region.

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wa·ter / ˈwôtər; ˈwä-/ • n. 1. a colorless, transparent, odorless, tasteless liquid that forms the seas, lakes, rivers, and rain and is the basis of the fluids of living organisms. ∎  this as supplied to houses or commercial establishments through pipes and taps: each bedroom has a washbasin with hot and cold water | [as adj.] water pipes. ∎  one of the four elements in ancient and medieval philosophy and in astrology (considered essential to the nature of the signs Cancer, Scorpio, and Pisces): [as adj.] a water sign. ∎  (usu. the waters) the water of a mineral spring, typically as used medicinally for bathing in or drinking: resorts where southerners came to take the waters. ∎  a solution of a specified substance in water: ammonia water. ∎  urine: drinking alcohol will make you need to pass water more often. ∎  (waters) the amniotic fluid surrounding a fetus in the womb, esp. as discharged in a flow shortly before birth: I think my waters have broken. 2. (the water) a stretch or area of water, such as a river, sea, or lake: the lawns ran down to the water's edge. ∎  the surface of such an area of water: she ducked under the water. ∎  [as adj.] found in, on, or near such areas of water: a water plant. ∎  (waters) the water of a particular sea, river, or lake: the waters of Hudson Bay | fig. the government is taking us into unknown waters with these changes in the legislation. ∎  (waters) an area of sea regarded as under the jurisdiction of a particular country: Japanese coastal waters. 3. the quality of transparency and brilliance shown by a diamond or other gem. 4. Finance capital stock that represents a book value greater than the true assets of a company. • v. 1. [tr.] pour or sprinkle water over (a plant or an area of ground), typically in order to encourage plant growth: I went out to water the geraniums. ∎  give a drink of water to (an animal): they stopped to water the horses and to refresh themselves. ∎  [intr.] (of an animal) drink water. ∎  (usu. be watered) (of a river) flow through (an area of land): the valley is watered by the Pines River. ∎  take a fresh supply of water on board (a ship or steam train): the ship was watered and fresh livestock taken aboard. ∎  Finance increase (a company's debt, or nominal capital) by the issue of new shares without a corresponding addition to assets. 2. [intr.] (of the eyes) become full of moisture or tears: Rory blinked, his eyes watering. ∎  (of the mouth) produce saliva, typically in response to the sight or smell of appetizing food: the smell of frying bacon made Hilary's mouth water. 3. [tr.] dilute or adulterate (a drink, typically an alcoholic one) with water: staff at the club had been watering down the drinks. ∎  (water something down) make a statement or proposal less forceful or controversial by changing or leaving out certain details: the army's report of its investigation was considerably watered down. PHRASES: by water using a ship or boat for travel or transport: at the end of the lake was a small gazebo, accessible only by water.cast one's bread upon the waterssee bread.like water in great quantities: George was spending money like water.make water 1. urinate. 2. (of a ship or boat) take in water through a leak. of the first water (of a diamond or pearl) of the greatest brilliance and transparency. ∎  (typically of someone or something perceived as undesirable or annoying) extreme or unsurpassed of their kind: she was a bore of the first water. under water submerged; flooded.the water of life whiskey.water off a duck's backsee duck1 . water on the brain inf. hydrocephalus.water under the bridge (or water over the dam) used to refer to events or situations that are in the past and consequently no longer to be regarded as important or as a source of concern.DERIVATIVES: wa·ter·er n.wa·ter·less adj. ORIGIN: Old English wæter (noun), wæterian (verb), of Germanic origin; related to Dutch water, German Wasser, from an Indo-European root shared by Russian voda (compare with vodka), also by Latin unda ‘wave’ and Greek hudōr ‘water.’

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414. Water

See also 36. BATHING ; 107. DAMPNESS ; 234. LAKES ; 345. RAIN ; 356. RIVERS ; 360. SEA ; 375. SNOW .

an abnormal fear of floods.
hydroponics. aquicultural , adj.
the science dealing with the behavior of water vapor. atmologist , n. atmologic, atmological , adj.
the treatment of illness or disease by bathing.
the skill or talent of water divining.
1. the process of dehydrating or removing the water from a substance.
2. the state of being dehydrated.
the property of a substance to attract and absorb moisture, especially from the air. Cf. efflorescence . deliquescent , adj.
a form of divination involving a rod or wand, especially the art of finding underground supplies of water, ores, etc. Also called rhabdomancy .
the property of a substance to yield up water through evaporation. Cf. deliquescence . See also 329. PROCESSES . efflorescent , adj.
the science of watercourses, especially rivers. fluviologist , n.
Obsolete. the moving of water by an artificval channel.
an abnormal love of drinking water.
1. the study, description, and mapping of oceans, lakes, and rivers, especially with reference to their use for navigational purposes.
2. those parts of the map, collectively, that represent surface waters. hydrographer , n. hydrographic, hydrographical , adj.
the science that studies the occurrence, circulation, distribution, and properties of the waters of the earth and its atmosphere. hydrologist , n. hydrologie, hydrological , adj.
a form of divination involving observations of water or of other liquids.
an excessive love of water.
the water cure, first developed in Germany in 1825. Also called hydriatrics . hydropathist , n. hydropathic , adj.
Botany. the capacity of a plant to be pollinated through the agency of water. hydrophilous , adj.
an abnormal fear of water.
a device for viewing things below the surface of a body of water.
the treatment of disorders by the use of water externally, especially in the form of exercises in a pool, etc. hydrotherapist , n. hydrotherapeutic , adj.
growth or movement in response to water as a stimulus. hydrotropic , adj.
the branch of physics that studies atmospheric humidity.
1. hydrophobia
2. an abnormal fear of water.
a form of divination involving the examination of water in a basin.
a form of self-hypnotism involving staring at water in a basin.
the scientific study of bodies of fresh water, as lakes or rivers, with reference to their physical, geographical, and biological features. limnologist , n. limnologic, limnological , adj.
the branch of hydrography that studies the drainage phenomena of mountains. orohydrographic , adj.
the tendency of some plants to respond to a current of water by growing with it (positive rheotaxis ) or against it (negative rheotaxis ).
the tendency of certain living things to move in response to the mechanical stimulus of a current of water.
sympesiometer, sympiezometer
an instrument for measuring the pressure exerted by currents of water. See also 226. INSTRUMENTS .
an instrument for measuring the turbidity of water or other fluids. turbidimetric , adj.
the measurement of the turbidity of water or other fluids, as with a turbidimeter. turbidimetric , adj.

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water one of the four elements in ancient and medieval philosophy and in astrology (considered essential to the nature of the signs Cancer, Scorpio, and Pisces).

The word is recorded from Old English (in form wæter) and is of Germanic origin; it comes from an Indo-European root shared by Lain unda ‘wave’ and Greek hudōr ‘water’.
don't go near the water until you learn how to swim proverbial saying, mid 19th century; advising against overconfidence when inexperienced.
of the first water referring to the highest grade of diamond. The three highest grades of diamond were formerly known as first, second, and third water, and the phrase of the first water is used generally to indicate the finest possible quaility. The usage may come ultimately from Arabic, where this sense of water is a particular application of ‘lustre, splendour’ (e.g., of a sword).
Water Bearer the zodiacal sign or constellation Aquarius, also called the Water Carrier.
water under the bridge referring to events or situations that are in the past and consequently no longer to be regarded as important or as a source of concern (in North American usage, water over the dam).

See also blood is thicker than water, bread and water, clear blue water, dead in the water, dirty water will quench fire, fish out of water, fish in troubled waters, come hell or high water, you can take a horse to water, still waters run deep, don't throw out your dirty water.

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ELIZABETH KNOWLES. "water." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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water A colourless liquid, H2O. In the gas phase water consists of single H2O molecules in which the H–O–H angle is 105°. The structure of liquid water is still controversial; hydrogen bonding of the type H2O…H–O–H imposes a high degree of structure (see hydrogen bond) and current models supported by X-ray scattering studies have short-range ordered regions, which are constantly disintegrating and re-forming. This ordering of the liquid state is sufficient to make the density of water at about 0°C higher than that of the relatively open-structured ice; the maximum density occurs at 3.98°C. This accounts for the well-known phenomenon of ice floating on water and the contraction of water below ice, a fact of enormous biological significance for all aquatic organisms. Water is a powerful solvent for both polar and ionic compounds; molecules or ions in solution are frequently strongly hydrated. Pure liquid water is very weakly dissociated into H3O+ (hydroxonium) and OH (hydroxyl) ions by self-ionization and consequently any compound that increases the concentration of the positive ion, H3O+, is acidic and compounds increasing the concentration of the negative ion, OH, are basic (see acid). The phenomena of ion transport in water and the division of materials into hydrophilic (water loving) and hydrophobic (water hating) substances are central features of almost all biological chemistry.

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"water." A Dictionary of Biology. 2004. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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Water (H2O) is vital for all living organisms, and it is no exaggeration to say that life could not occur without it. The central feature of the water molecule is the bond between the strong electron attractor oxygen and the weak attractor hydrogen. This creates a polar covalent bond, with a weak positive charge on each hydrogen and a weak negative charge on the oxygen. The polar water molecule dissolves ions (such as sodium, essential for membrane transport) and polar molecules (including sugars) while excluding large nonpolar molecules such as fats. This selective solvation forms the basis of cell structure and function, in which large insoluble membranes enclose aqueous solutions of nutrients and other small molecules.

Water is liquid between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit), a temperature range that is high enough to promote random mixing in aqueous solutions (necessary for biochemical reactions) but low enough to prevent random breaking of most covalent bonds, which would make stable life forms impossible. Most organisms must live in the low end of this range. Finally, its high heat capacity moderates temperature changes, especially in organisms with large bodies.

see also LipidsMembrane Structure

Richard Robinson

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water transparent liquid forming the material of seas, lakes, and rivers OE.; (prob. after Arab. mā' water, lustre, splendour) transparency and lustre of a gem (whence in popular phr. of the first w.) XVII. OE. wæter = OS. watar (Du. water), OHG. wazzar (G. wasser):- WGmc. *watar (ON. vatn, Goth. watō, g. watins, show a var. with n-formative), f. Gmc. *wat- :- IE. *wod-, repr. by OSl. (Russ.) voda; the var. *wēd- is repr. by WET, the var. *ud- (sometimes with nasal infix) by L. unda (cf. UNDULATE), Gr. húdōr, g. húdatos (:- udṇtos), hudro- HYDRO-, Skr. udán.
So water vb. tr. OE.; intr. XIV. OE. (ġe)wæterian, corr. to MLG. wateren, weteren (Du. wateren), MHG. wezzern (G. wässern). watery OE. wæteriġ = MLG. waterich, etc.; see -Y1. Comps. waterfall XIV (cf. OE. wæterġefeall). watershed line separating waters flowing into different river basins XIX. See SHED2.

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T. F. HOAD. "water." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

T. F. HOAD. "water." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (May 29, 2016). http://www.encyclopedia.com/doc/1O27-water.html

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water (H2O) Odourless, colourless liquid that covers about 70% of the Earth's surface and is the most widely used solvent. Essential to life, it makes up from 60–70% of the human body. It is a compound of hydrogen and oxygen, with the two H-O links of the molecule forming an angle of 105°. This asymmetry results in polar properties and a force of attraction (hydrogen bond) between opposite ends of neighbouring water molecules. These forces maintain the substance as a liquid, in spite of its low molecular weight, and account for its unusual property of having its maximum density at 4°C (39.2°F). Besides its everyday uses, water is used to manufacture sulphuric acid, nitric acid, sodium carbonate, and ammonia; it is also employed to extract oil and sulphur from the ground. Properties: r.d. 1.000; m.p. 0°C (32°F); b.p. 100°C (212°F).

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"water." World Encyclopedia. 2005. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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689. Water

  1. Adad storm god; helped cause the Flood. [Babyl. Myth.: Benét, 7]
  2. Adams ale water; only drink in Paradise. [Folklore: Brewer Dictionary, 9]
  3. Alpheus river god. [Gk. Myth.: Zimmerman, 18]
  4. Apsu personification of fresh water. [Babyl. Myth.: Benét, 4]
  5. Arethusa changed into stream by Artemis to save her from Alpheus. [Gk. Myth.: Zimmerman, 29]
  6. Cyane turned into a fountain by Hades. [Gk. Myth.: Kravitz, 70]
  7. Dirce turned into a fountain at death. [Gk. Myth.: Kravitz, 8283]
  8. Galatea grieving, turned into a fountain. [Gk. Myth.: Metamorphoses ]
  9. Jupiter Pluvius dispenser of rain. [Rom. Myth.: Espy, 22]
  10. Neptune in allegories of the elements, personification of water. [Art: Hall, 128]
  11. undine female water spirit. [Medieval Hist.: Brewer Dictionary, 1115]

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"Water." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. 1986. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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water The water molecule is composed of two hydrogen atoms, each of atomic weight 1.00797, combined with an oxygen atom of atomic weight 15.9994. It can occur as liquid, solid, and gas phases. It is a very powerful solvent which is responsible for the transfer of material on and below the Earth's surface.

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AILSA ALLABY and MICHAEL ALLABY. "water." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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water. Classical ornament such as the Vitruvian scroll may represent waves, while the Ancient Egyptians used parallel zig-zag lines to suggest water. Sculpted representations of flowing water are associated with grottoes, nymphaea, etc., and are found in rustication, often frozen, or congelated.

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JAMES STEVENS CURL. "water." A Dictionary of Architecture and Landscape Architecture. 2000. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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This entry contains three subentries:
Water as a Beverage and Constitutent of Food
Water as a Resource
Safety of Water

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"Water." Encyclopedia of Food and Culture. 2003. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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wateraorta, daughter, exhorter, exporter, extorter, Horta, importer, mortar, porter, quarter, slaughter, snorter, sorter, sporter, supporter, three-quarter, torte, transporter, underwater, water •altar, alter, assaulter, defaulter, falter, Gibraltar, halter, Malta, palter, psalter, salter, vaulter, Walter •flaunter, haunter, saunter, taunter, vaunter •exhauster, Forster •fraudster • granddaughter •stepdaughter • manslaughter •ripsnorter • pole-vaulter • backwater •headquarter • freshwater •breakwater • rainwater • seawater •dishwater • tidewater • Whitewater •saltwater • rosewater • shearwater •firewater •doubter, grouter, outer, pouter, scouter, shouter, spouter, touter •counter, encounter, mounter •jouster, ouster •revcounter •bloater, boater, Botha, Dakota, doter, emoter, floater, gloater, iota, Kota, Minnesota, motor, promoter, quota, rota, rotor, scoter, voter •bolter, coulter (US colter), Volta •boaster, coaster, poster, roaster, toaster •roadster • oldster •bolster, holster, pollster, soulster, upholster •billposter

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"water." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 29 May. 2016 <http://www.encyclopedia.com>.

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