In distillation saltwater is heated in one container to make the water evaporate, leaving the salt behind. The desalinated vapor is then condensed to form water in a separate container. Although long known, distillation has found limited application in water supply because of the fuel costs involved in converting saltwater to vapor. Representative of the early attempts in this direction were the solar distillation methods employed (c.49 B.C.) by the legions of Julius Caesar for using water from the Mediterranean. Modern technological advances led to the development of more efficient distillation units using solar energy; however, since these units have small capacities, their utility is restricted.
Distillation plants having high capacities and using combustible fuels employ various devices to conserve heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a spray or thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly, yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000 desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.
Another method of desalination is by electrodialysis. When salt dissolves in water, it splits up into charged particles called ions. Placed in a container with a negative electrode at one end and a positive electrode at the other, the ions are filtered by the membranes as they are attracted toward the electrodes; they become trapped between semipermeable membranes, leaving outside the membranes a supply of desalinated water that can be tapped. The first large installation using this process began operating in South Africa in 1958, but its electrical demands make it impractical except where such energy is abundant.
By far the most promising approach is the reverse osmosis process, in which pressure is applied to saltwater to force it through a special membrane. Only pure water passes, leaving concentrated seawater behind. Where multistage flash distillation costs about $4 per 1,000 gallons, reverse osmosis costs about half that amount. This process is used by a plant in the Tampa Bay area, Florida, that produces 25 million gallons of drinking water a day. Another type uses an empty hollow sphere of semipermeable material that is lowered into the sea. The water flowing into the sphere is fresh, since the salt is excluded by the membrane that covers the entire sphere and is its guard.
One final approach is under development in Hawaii, where different layers of seawater display a large temperature differential. Here an Ocean Thermal Energy Conversion plant is being built which will use steam produced by the flash method to produce energy, then condense the steam into freshwater. Three such plants could produce a hundred megawatts of power, as well as supply 30% of Hawaii's water needs.
For emergency use, i.e., in lifeboats, various systems are available in addition to solar or fuel-heated distillation devices. One device made of flexible plastic is worn around the waist of the user to employ body heat for evaporation.
In the United States, the average residential daily water supply demand is 100 gal (380 liters) per person, although it can go as high as 500 gal (1900 liters) per person. The stringency of the requirements that a supply of water must meet depends on the use to be made of it. For example, water used to wash semiconductor material from which transistors are made must be extraordinarily pure. The more usual requirements, however, are that water be free enough of harmful bacteria, chemicals, and other contamination to be drinkable; free of substances that make its taste or appearance unpleasant; and if the water is to be used for washing, free of salts of calcium and magnesium that will interfere with the action of soap.
The basic source of water is rainfall, which collects in rivers and lakes, under the ground, and in artificial reservoirs. Water from under the ground is called groundwater and is tapped by means of wells. Most often water must be raised from a well by pumping. In some cases a well will draw water from a permeable rock layer called an aquifer in which the water is under pressure; such a well needs little or no pumping (see artesian well). Water that collects in rivers, lakes, or reservoirs is called surface water. Most large water supply systems draw surface water through special intake pipes or tunnels and transport it to the area of use through canals, tunnels, or pipelines, which are known as mains or aqueducts. These feed a system of smaller conduits or pipes that take the water to its place of use. The California Water Plan, initiated in 1957, eventually entailed twenty dams, seven power plants, and more than 700 mi (1100 km) of canals, tunnels, and pipelines to meet the needs of southern California residents—a total of more than five billion cubic meters of water per year.
A complete water supply system is often known as a waterworks. Sometimes the term is specifically applied to pumping stations, treatment stations, or storage facilities. Storage facilities are provided to reserve extra water for use when demand is high and, when necessary, to help maintain water pressure. Treatment stations are places in which water may be filtered to remove suspended impurities, aerated to remove dissolved gases, or disinfected with chlorine, ozone, ultraviolet light, or some other agent that kills harmful bacteria and microorganisms. Sometimes hard water is softened through ion exchange, by which dissolved calcium and magnesium salts are replaced by sodium salts, which do not interfere with soap. Salts of iodine and fluorine, which are considered helpful in preventing goiter and tooth decay, are sometimes added to water in which they are lacking.
Not all water supply systems are used to deliver drinking water. Systems used for purposes such as irrigation and fire fighting operate in much the same way as systems for drinking water, but the water need not meet such high standards of purity. In most municipal systems hydrants are connected to the drinking water system except during periods of extreme water shortage. Because many cities draw water from the same body into which they discharge sewage, proper sewage treatment has become increasingly essential to the preservation of supplies of useful water (see sewerage; water pollution).
See A. Tyll, Water Skiing (1966).
See S. Bhatt, Environmental Laws and Water Resources Management (1986).
Water polo was devised in England in the 1870s and became popular in the United States in the early 20th cent. It is played mainly by club teams, although it is also popular in collegiate competition. Water polo has been an men's Olympic event since 1900; women's water polo was first played at the games in 2000. A far rougher version of the game, played with a soft rubber ball in a larger pool and known as American or softball water polo, was formerly popular in the United States. However, its extreme violence brought it into disfavor, and today only the international or hardball game is played throughout the world.
In the United States industry is the greatest source of pollution, accounting for more than half the volume of all water pollution and for the most deadly pollutants. Some 370,000 manufacturing facilities use huge quantities of freshwater to carry away wastes of many kinds. The waste-bearing water, or effluent, is discharged into streams, lakes, or oceans, which in turn disperse the polluting substances. In its National Water Quality Inventory, reported to Congress in 1996, the U.S. Environmental Protection Agency concluded that approximately 40% of the nation's surveyed lakes, rivers, and estuaries were too polluted for such basic uses as drinking supply, fishing, and swimming. The pollutants include grit, asbestos, phosphates and nitrates, mercury, lead, caustic soda and other sodium compounds, sulfur and sulfuric acid, oils, and petrochemicals.
In addition, numerous manufacturing plants pour off undiluted corrosives, poisons, and other noxious byproducts. The construction industry discharges slurries of gypsum, cement, abrasives, metals, and poisonous solvents. Another pervasive group of contaminants entering food chains is the polychlorinated biphenyl (PCB) compounds, components of lubricants, plastic wrappers, and adhesives. In yet another instance of pollution, hot water discharged by factories and power plants causes so-called thermal pollution by increasing water temperatures. Such increases change the level of oxygen dissolved in a body of water, thereby disrupting the water's ecological balance, killing off some plant and animal species while encouraging the overgrowth of others.
Towns and municipalities are also major sources of water pollution. In many public water systems, pollution exceeds safe levels. One reason for this is that much groundwater has been contaminated by wastes pumped underground for disposal or by seepage from surface water. When contamination reaches underground water tables, it is difficult to correct and spreads over wide areas. In addition, many U.S. communities discharge untreated or only partially treated sewage into the waterways, threatening the health of their own and neighboring populations.
Along with domestic wastes, sewage carries industrial contaminants and a growing tonnage of paper and plastic refuse (see solid waste). Although thorough sewage treatment would destroy most disease-causing bacteria, the problem of the spread of viruses and viral illness remains. Additionally, most sewage treatment does not remove phosphorus compounds, contributed principally by detergents, which cause eutrophication of lakes and ponds. Excreted drugs and household chemicals also are not removed by present municipal treatment facilites, and can be recycled into the drinking water supply.
Rain drainage is another major polluting agent because it carries such substances as highway debris (including oil and chemicals from automobile exhausts), sediments from highway and building construction, and acids and radioactive wastes from mining operations into freshwater systems as well as into the ocean. Also transported by rain runoff and by irrigation return-flow are animal wastes from farms and feedlots, a widespread source of pollutants impairing rivers and streams, groundwater, and even some coastal waters. Antibiotics, hormones, and other chemicals used to raise livestock are components of such animal wastes. Pesticide and fertilizer residues from farms also contribute to water pollution via rain drainage.
Large and small craft significantly pollute both inland and coastal waters by dumping their untreated sewage. Oil spilled accidentally or flushed from tankers and offshore rigs (900,000 metric tons annually) sullies beaches and smothers bird, fish, and plant life. In 1989 in one of the world's worst single instances of water pollution, the Exxon Valdez spilled 11 million gallons of oil in Prince William Sound, Alaska, causing great environmental destruction. In 1997, the 22 oil spills reported worldwide involved a total of 15 million gallons (57 million liters) of oil. In addition to its direct damage to wildlife, oil takes up fat-soluble poisons like DDT, allowing them to be concentrated in organisms that ingest the oil-contaminated water; thus such poisons enter the food chains leading to sea mammals and people (see ecology).
Both DDT, which has been banned in the United States since 1972, and PCBs are manufactured in many parts of the world and are now widespread in the Atlantic and Pacific oceans. In addition, tarry oil residues are encountered throughout the Atlantic, as are styrofoam and other plastic rubbish. Plastic bits litter sections of the Pacific as far north as Amchitka Island near Alaska. Garbage, solid industrial wastes, and sludge formed in sewage treatment, all commonly dumped into oceans, are other marine pollutants found worldwide, especially along coastal areas.
Virtually all water pollutants are hazardous to humans as well as lesser species; sodium is implicated in cardiovascular disease, nitrates in blood disorders. Mercury and lead can cause nervous disorders. Some contaminants are carcinogens. DDT is toxic to humans and can alter chromosomes. PCBs cause liver and nerve damage, skin eruptions, vomiting, fever, diarrhea, and fetal abnormalities. Along many shores, shellfish can no longer be taken because of contamination by DDT, sewage, or industrial wastes.
Dysentery, salmonellosis, cryptosporidium, and hepatitis are among the maladies transmitted by sewage in drinking and bathing water. In the United States, beaches along both coasts, riverbanks, and lake shores have been ruined for bathers by industrial wastes, municipal sewage, and medical waste. Water pollution is an even greater problem in the Third World, where millions of people obtain water for drinking and sanitation from unprotected streams and ponds that are contaminated with human waste. This type of contamination has been estimated to cause more than 3 million deaths annually from diarrhea in Third World countries, most of them children.
The United States has enacted extensive federal legislation to fight water pollution. Laws include the Federal Water Pollution Control Act (1972), the Marine Protection, Research, and Sanctuaries Act (1972), the Safe Drinking Water Act (1974), and the Federal Insecticide, Fungicide, and Rodenticide Act, as amended in 1988. In the United States in 1996, nearly $10 billion was spent on water and wastewater treatment alone. International cooperation is being promoted by the Inter-Governmental Maritime Consultive Organization (IMCO), a UN agency. Limitation of ocean dumping was proposed at the 80-nation London Conference of 1972, and in the same year 12 European nations meeting in Oslo adopted rules to regulate dumping in the North Atlantic. An international ban on ocean dumping in 1988 set further restrictions.
The chytrids live in salt- and freshwater and in moist soil. They live as saprobes, obtaining their metabolic energy from decaying plant and animal material, or as parasites, attacking plants, fungi, and algae. They typically take the form of small coenocytic (multinucleated) masses, called sporangia, from which many hairlike rhizoids protrude. Like roots, the rhizoids absorb nutrients. Reproduction can be by simple division of a sporangium into individual motile, flagellated spores or by more complicated sexual processes that yield flagellated gametes. There are approximately 900 species of chytrids.
The oomycetes resemble fungi, taking the form of coenocytic filaments (hyphae). They differ from fungi, however, in that cellulose is present in their cell walls. The hyphae of oomycetes have specialized regions that can produce distinct male and female gametes. Oomycetes can also reproduce asexually. Many oomycetes are aquatic. Many of the others live in water in certain stages of the life cycle. Most of the 800 species of oomycetes are saprobes, but those that are parasitic are of great significance: they cause downy mildew, a disease often affecting grapes; late blight of potatoes, an outbreak of which led to the Great Potato Famine in Ireland (1845-49); sudden oak death syndrome (also known as ramorum leaf blight or ramorum dieback; redwoods, Douglas firs, and other plants also are harmed by the same water mold); and blue mold of tobacco. Other parasitic water molds cause diseases of fish and fish eggs.
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).
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.
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).
Facilities for the collection, treatment, storage, and distribution of water. Ancient systems included wells, storage reservoirs, canals and aqueducts, and water-distribution systems. Highly advanced systems appeared circa 2500 BC and reached their peak in the Roman aqueduct system. In the Middle Ages, water supplies were largely neglected and epidemics caused by waterborne organisms were common. In the 17th–18th century, distribution systems utilizing cast-iron pipes, aqueducts, and pumps began to be installed. The link between polluted water and disease came to be understood in the 19th century, and treatment methods such as slow sand filtration and disinfection with chlorine were introduced. Modern reservoirs are formed usually by constructing dams near the collection point of mountain-water runoff or across rivers. After the water reaches collection points, it is treated to improve its quality; it is then pumped either directly into a city or town's distribution system or to an elevated storage location, such as a water tank. Seealso plumbing.
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Principle that subatomic particles possess some wavelike characteristics, and that electromagnetic waves, such as light, possess some particlelike characteristics. In 1905, by demonstrating the photoelectric effect, Albert Einstein showed that light, which until then had been thought of as a form of electromagnetic wave (see electromagnetic radiation), must also be thought of as localized in packets of discrete energy (see photon). In 1924 Louis-Victor Broglie proposed that electrons have wave properties such as wavelength and frequency; their wavelike nature was experimentally established in 1927 by the demonstration of their diffraction. The theory of quantum electrodynamics combines the wave theory and the particle theory of electromagnetic radiation.
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Gently sloping rock ledge that extends from the high-tide level at a steep cliff base to below the low-tide level. It develops as a result of wave abrasion; beaches protect the shore from abrasion and therefore prevent the formation of platforms. A platform is broadened as waves erode a notch at the base of the sea cliff, causing overhanging rock to fall. As the sea cliffs are attacked, weak rocks are quickly eroded, leaving the more resistant rocks as protrusions.
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Variable quantity that mathematically describes the wave characteristics of a particle. It is related to the likelihood of the particle being at a given point in space at a given time, and may be thought of as an expression for the amplitude of the particle wave, though this is strictly not physically meaningful. The square of the wave function is the significant quantity, as it gives the probability for finding the particle at a given point in space and time. Seealso wave-particle duality.
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Imaginary surface that represents corresponding points of waves vibrating in unison. As identical waves from the same source travel through a homogeneous medium, corresponding crests and troughs are in phase at any instant; that is, they have completed the same fraction of their periodic motion. Any surface drawn through all points of the same phase constitutes a wave front.
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In oceanography, a ridge or swell on the surface of a body of water, normally having a forward motion distinct from the motions of the particles that compose it. Ocean waves are fairly regular, with an identifiable wavelength between adjacent crests and with a definite frequency of oscillation. Waves result when a generating force (usually the wind) displaces surface water and a restoring force returns it to its undisturbed position. Surface tension alone is the restoring force for small waves. For large waves, gravity is more important.
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Vibration generated by an earthquake, explosion, or similar phenomenon and propagated within the Earth or along its surface. Earthquakes generate two principal types of waves: body waves, which travel within the Earth, and surface waves, which travel along the surface. Seismograms (recorded traces of the amplitude and frequency of seismic waves) yield information about the Earth and its subsurface structure; artificially generated seismic waves are used in oil and gas prospecting.
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Any fish-eating bird of the family Anhingidae (order Pelecaniformes), sometimes considered a single species (Anhinga anhinga) with geographical variants. Anhingas are about 35 in. (90 cm) long, slender, and long-necked. They are mostly black, with silvery wing markings. Males, glossed with green, develop pale head plumes and a dark “mane” in breeding season. Anhingas live in small colonies along lakes and rivers in tropical to warm temperate regions except in Europe. They swim nearly submerged; the head and neck show above water, darting snakelike from side to side.
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Seasonal variations in groundwater levels.
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Any of about 200 snake species of about 38 genera, especially Natrix and Nerodia, classified within the family Colubridae, found worldwide except in South America. Most species have a stout body with dark blotches or streaks and ridged scales. Some are similar in appearance to venomous species. They kill fishes and amphibians with a nonvenomous bite. The New World species live in or near water and bear live young; European species are less water-dependent and lay eggs. In defense they inflate the head, strike, and release a foul secretion. Average length is about 3 ft (1 m); some Old World species reach 6 ft (1.8 m).
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Sport of planing and jumping on water skis, broad skilike runners that a rider wears while being towed by a motorboat. The sport originated in the U.S. in the 1920s. International competitions have been held since 1946. Single-ski slalom competition is held on a course consisting of a specified number of buoys around which the skier must negotiate. Jumping competitions employ a ramp; skiers are judged for distance and style. Barefoot and trick skiing are also part of some competitions. A later development in waterskiing, wakeboarding began in the U.S. in the 1980s when surfers began riding their boards as they were pulled behind boats. Since the mid 1990s wakeboarding has been an event at both the Gravity Games and the X Games (see extreme sports) and has become the fastest-growing water sport worldwide.
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Machine for raising water, said to have been invented by Archimedes for removing water from the hold of a large ship. One form consists of a circular pipe enclosing a helix and inclined at an angle of about 45°, with its lower end dipped in the water; rotation of the device lifts the water in the pipe. Other forms consist of a helix revolving in a fixed cylinder or a helical tube wound around a shaft.
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Any of the entire range of natural waters (vapour, liquid, or solid) that occur on the Earth and that are of potential use to humans. These resources include the waters of the oceans, rivers, and lakes; groundwater and deep subsurface waters; and glaciers and permanent snowfields. Continuing increase in water use has led to growing concern over the availability and quality of water supplies.
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Power produced by a stream of water as it turns a wheel or similar device. The waterwheel, probably invented in the 1st century BC, was widely used throughout the Middle Ages and into modern times for grinding grain, operating bellows for furnaces, and other purposes. The more compact water turbine, which passes water through a series of fixed and rotating blades, was introduced in 1827. Water turbines, used originally for direct mechanical drive for irrigation, now are used almost exclusively to generate hydroelectric power.
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Sport played in a swimming pool by teams of seven with a buoyant ball resembling a football (soccer ball). The ball may be carried or thrown, and a point is scored when the ball is placed in the opposing team's goal. The name derives from a mid-19th-century version of the sport in which players rode barrels and struck the ball with sticks. A rough and demanding game, it is played by both men and women. Modern water polo was introduced as an Olympic sport in 1900.
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Eurasian dipper (Cinclus cinclus)
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Accumulation of cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain. Hydrocephalus is caused by overproduction of CSF, congenital blockage that prevents drainage (see neural tube defect), or complications of head injuries or infections. Normally, CSF circulates through the brain and spinal cord and drains into the circulation. In infants and young children, hydrocephalus causes the brain and skull to enlarge because the fontanels have not yet closed. Without surgery to divert the excess fluid into the blood or abdomen, accumulating fluid eventually compresses the brain, causing convulsions, intellectual disability, and death.
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Alcoholic beverage obtained by distillation from wine or other fermented fruit juice or from various cereal grains that have first been brewed. The essential ingredient is usually a natural sugar or a starchy substance that may be easily converted into a sugar. The distillation process is based on the different boiling points of water (212 °F [100 °C]) and alcohol (173 °F [78.5 °C]). The alcohol vapours that arise while the fermented liquid boils are trapped and recondensed to create a liquid of much greater alcoholic strength. The resultant distillate is matured, often for several years, before it is packaged and sold. Seealso aquavit; brandy; gin; liqueur; rum; vodka; and whiskey.
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Coarse annual grass (Zizania aquatica) of the family Poaceae (or Gramineae) whose grain, now often considered a delicacy, has long been an important food of American Indians. Despite its name, the plant is not related to rice. Wild rice grows naturally in shallow water in marshes and along the shores of streams and lakes in northern central North America. Cultivated varieties are now grown in Minnesota and California. The plant, about 3–10 ft (1–3 m) tall, is topped with a large, open flower cluster. The ripened grains, dark brown to purplish-black, are slender rods 0.4–0.8 in. (1–2 cm) long.
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Machine for tapping the energy of running or falling water (hence a prime mover) by means of a set of paddles or buckets mounted around a wheel. The force of the moving water against the paddles, or the weight of water poured into the buckets, rotates the wheel. The resulting power is transmitted to machinery via the shaft of the wheel. The waterwheel was perhaps the earliest source of mechanical energy to replace that of humans and animals, and it was first used for such tasks as raising water and grinding grain. Seealso waterpower.
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Santa Cruz water lily (Victoria cruziana)
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Common water hyacinth (Eichhornia crassipes)
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Semiaquatic rodent (Hydrochoerus hyrdrochaeris) of Central and South America. Classified as the only species in its family, it is related to the cavy and the guinea pig. Capybaras are the largest living rodents, growing as large as 50 in. (1.25 m) long and weighing 110 lbs (50 kg) or more. They are sparsely haired and brownish, with a blunt snout, short legs, small ears, and almost no tail. Capybaras are shy and associate in groups along the banks of lakes and rivers. Herbivorous, they can become pests when they eat cultivated melons, grain, and squash. They swim and dive readily and commonly enter water to elude predators.
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Water flea of the genus Daphnia (magnified about 30×)
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Cycle that involves the continuous circulation of water in the Earth-atmosphere system. Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.
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Ancient device for measuring time by the gradual flow of water. One form, used by North American Indians and some African peoples, consisted of a small boat or floating vessel that shipped water through a hole until it sank. In another form, water escaped through a hole in a vessel marked with graduated lines; specimens from Egypt date from the 14th century BC. The Romans invented a clepsydra consisting of a cylinder into which water dripped from a reservoir; a float provided readings against a scale on the cylinder wall. Galileo used a mercury clepsydra to time his experimental falling bodies. Seealso clock.
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Any of several perennial water plants of the genus Trapa (family Trapaceae), native to Europe, Asia, and Africa, or their edible, nutlike fruit. The water caltrop (T. natans) has two sets of leaves—long, feathery, rootlike, submerged leaves and a loose rosette of floating leaves attached to leafstalks 2–4 in (5–10 cm) long. The small fruit usually has four spiny angles. The Chinese water chestnut (Eleocharis tuberosus or E. dulcis) is a member of the sedge family.
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Any of three subspecies of oxlike bovid (species Bubalus bubalis). Two have been domesticated in Asia since the earliest recorded history. The animal is named for its ability to work on waterlogged land and in humid climates. The largest breeds stand 5–6 ft (1.5–1.8 m), is up to 9 ft (2.8 m) long, and may weigh over 2,000 lb (900 kg). The dull black or dark gray body has little hair. The horns spread outward and upward, measuring up to 7 ft (2 m) across. One subspecies, the swamp buffalo, is the principal draft animal of southern China and South and Southeast Asia. Another, the river buffalo, is used for dairy and meat production and draft work in southern and South Asia and Egypt. The third subspecies is the wild water buffalo, of which only a few dozen herds remain. It is larger than domestic buffaloes and is sometimes referred to as a separate species (B. arnee).
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Dense aquatic accumulation of microscopic organisms produced by an abundance of nutrients in surface water coupled with adequate sunlight for photosynthesis. The microorganisms or the toxic substances they release may discolour the water, exhaust its oxygen content, poison aquatic animals and waterfowl, and irritate the skin and respiratory tract of humans. Single species of algae, diatoms, or dinoflagellates, reproducing every few hours, may dominate a bloom's population; the number of individuals per quart (litre) of water, normally about 1,000, can increase to 60 million. Blooms of the dinoflagellate genus Gymnodinium cause red tides. The Red Sea is named for the occasional blooms of the alga Trichodesmium erythraeum. Seealso water pollution.
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Polar bear (Ursus maritimus).
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Swimming sport in which the movements of one or more swimmers are synchronized with a musical accompaniment. The sport developed in the U.S. in the 1930s and was admitted as an Olympic event (solo and duet only) in 1984; in 1996 the rules were changed to allow only teams of eight women. Teams are judged on compulsory and optional routines.
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Inorganic compound composed of hydrogen and oxygen (H2O), existing in liquid, gas (steam, water vapour), and solid (ice) states. At room temperature, water is a colourless, odourless, tasteless liquid. One of the most abundant compounds, water covers about 75percnt of Earth's surface. Life depends on water for virtually every process, its ability to dissolve many other substances being perhaps its most essential quality. Life is believed to have originated in water (the world's oceans or smaller bodies), and living organisms use aqueous solutions (including blood and digestive juices) as mediums for carrying out biological processes. Because water molecules are asymmetric and therefore electric dipoles, hydrogen bonding between molecules in liquid water and in ice is important in holding them together. Many of water's complex and anomalous physical and chemical properties (high melting and boiling points, viscosity, surface tension, greater density in liquid than in solid form) arise from this extensive hydrogen bonding. Water undergoes dissociation to the ions H+ (or H3O+) and OH−, particularly in the presence of salts and other solutes; it may act as an acid or as a base. Water occurs bound (as water of hydration) in many salts and minerals. It has myriad industrial uses, including as a suspending agent (papermaking, coal slurrying), solvent, diluting agent, coolant, and source of hydrogen; it is used in filtration, washing, steam generation, hydration of lime and cement, textile processing, sulfur mining, hydrolysis, and hydraulics, as well as in beverages and foods. See also hard water; heavy water.
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Water that makes up the oceans and seas. Seawater is a complex mixture of 96.5percnt water, 2.5percnt salts, and small amounts of other substances. Much of the world's magnesium is recovered from seawater, as are large quantities of bromine. In certain parts of the world, sodium chloride (table salt) is obtained by evaporating seawater. In addition, desalted seawater can theoretically furnish a limitless supply of drinking water, but the high processing costs are prohibitive. Large desalination plants have been built in dry areas along seacoasts in the Middle East and elsewhere to relieve shortages of fresh water.
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Discoloration of seawater caused by dinoflagellates during periodic blooms (population increases). Toxic substances released by these organisms into the water may be lethal to fish and other marine life, and they irritate the human respiratory system. Coastal resorts sometimes close when breaking waves release the toxic substances into the air. The causes of red tide are uncertain; it may require the confluence of several natural phenomena, in which human influence may or may not play a part.
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Water composed of two atoms of deuterium (D; a heavy isotope of hydrogen) and one atom of oxygen (O), chemical formula D2O. Water from most natural sources contains about 0.015percnt deuterium oxide; this can be enriched or purified by distillation, electrolysis, or chemical processing. Heavy water is used as a moderator in nuclear power plants, slowing down the fast neutrons so that they can react with the fuel in the reactor. Heavy water is also used in research as an isotopic tracer for chemical reactions and biochemical pathways. Water with tritium (T2O) rather than deuterium may also be called heavy water.
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Water that contains mineral salts of calcium and magnesium, principally as bicarbonates, chlorides, and sulfates, and sometimes iron. Hardness caused by calcium bicarbonate is known as temporary, because boiling converts the bicarbonate to the insoluble carbonate; hardness from the other salts is called permanent. The calcium and magnesium in hard waters form a hard, adherent scale on boiler plates, increasing fuel consumption and leading to deterioration through overheating. Home water softeners consist of tanks containing zeolite minerals or ion-exchange resins, which contain sodium ions that change places with the calcium and magnesium.
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Lowermost layer of ocean water that can be distinguished by its characteristic low temperature, high density, and low oxygen content compared with surface waters. Most bottom waters are formed near Antarctica during the southern winter. The partial freezing of seawater over the Antarctic continental shelf produces salt-free ice and residual brine with a high density, which causes it to sink; it then flows northward along the seafloor. The Arctic Ocean is less important as a source of bottom water because it is isolated by barriers such as the Bering Sill and submarine ridges and banks between Greenland and the British Isles.
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Lake, Cumbria, England. Located in the Lake District, it is about 3 mi (5 km) long and 0.5–1.25 mi (0.8–2 km) wide, with a maximum depth of 72 ft (22 m). The River Derwent enters its southern end and leaves the lake's northern end near the market town of Keswick. Several sites on its shores are National Trust property and are frequented by tourists. Lords Island, one of several in the lake, was once the residence of the earls of Derwentwater.
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