Water

I

Introduction

Water, common name applied to the liquid state of the hydrogen-oxygen compound H₂O. The ancient philosophers regarded water as a basic element typifying all liquid substances. Scientists did not discard that view until the latter half of the 18th century. In 1781 the British chemist Henry Cavendish synthesized water by detonating a mixture of hydrogen and air. However, the results of his experiments were not clearly interpreted until two years later, when the French chemist Antoine Laurent Lavoisier proved that water was not an element but a compound of oxygen and hydrogen. In a scientific paper presented in 1804, the French chemist Joseph Louis Gay-Lussac and the German naturalist Alexander von Humboldt demonstrated jointly that water consisted of two volumes of hydrogen to one of oxygen, as expressed by the present-day formula H₂O.

Almost all the hydrogen in water has an atomic weight of 1. The American chemist Harold Clayton Urey discovered in 1932 the presence in water of a small amount (1 part in 6,000) of so-called heavy water, or deuterium oxide (D₂O); deuterium is the hydrogen isotope with an atomic weight of 2. In 1951 the American chemist Aristid Grosse discovered that naturally occurring water contains also minute traces of tritium oxide (T₂O); tritium is the hydrogen isotope with an atomic weight of 3. See Atom.

II

Properties

Pure water is an odorless, tasteless liquid. It has a bluish tint, which may be detected, however, only in layers of considerable depth. Under standard atmospheric pressure (760 mm of mercury, or 760 torr); the freezing point of water is 0°C (32°F) and its boiling point is 100°C (212°F). Water attains its maximum density at a temperature of 4°C (39°F) and expands upon freezing. Like most other liquids, water can exist in a supercooled state; that is, it may remain a liquid although its temperature is below its freezing point. Water can easily be cooled to about -25°C (-13°F) without freezing, either under laboratory conditions or in the atmosphere itself. Supercooled water will freeze if it is disturbed, if the temperature is lowered further, or if an ice crystal or other particle is added to it. Its physical properties are used as standards to define the calorie and specific and latent heat (see Heat) and in the metric system for the original definition of the unit of mass, the gram.

Water is one of the best-known ionizing agents (see Ionization). Because most substances are somewhat soluble in water, it is frequently called the universal solvent. Water combines with certain salts to form hydrates. It reacts with metal oxides to form acids (see Acids and Bases). It acts as a catalyst in many important chemical reactions.

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III

Occurrence

Water is the only substance on Earth that occurs at ordinary temperatures in all three states of matter, that is, as a solid, a liquid, and a gas. As a solid, or ice, it is found as glaciers and ice caps, on water surfaces in winter, as snow, hail, and frost, and as clouds formed of ice crystals. It occurs in the liquid state as rain clouds formed of water droplets, and on vegetation as dew; in addition, it covers three-quarters of the surface of Earth in the form of swamps, lakes, rivers, and oceans. As gas, or water vapor, it occurs as fog, steam, and clouds. Atmospheric vapor is measured in terms of relative humidity, which is the ratio of the quantity of vapor actually present to the greatest amount possible at a given temperature. See Atmosphere; Cloud; Fog; Humidity; Rain.

Astronomers have also found water in outer space. Within our solar system, water in the form of ice has been identified on Mars; on some of the moons of Jupiter, Saturn, Uranus, and Neptune; on dwarf planets such as Pluto and Haumea; and in comets. Water has been detected in the atmospheres of the giant planets Jupiter and Saturn, and in the atmospheres of some distant extrasolar planets. Water molecules have also been observed in interstellar gas clouds. So far, liquid water that could support life has not been found on any bodies beyond Earth.

On Earth, water occurs as moisture in the upper portion of the soil profile, in which it is held by capillary action to the particles of soil. In this state, it is called bound water and has different characteristics from free water (see Soil; Soil Management). Under the influence of gravity, water accumulates in rock interstices beneath the surface of Earth as a vast groundwater reservoir supplying wells and springs and sustaining the flow of some streams during periods of drought.

IV

Water in Life

Water is the major constitutent of living matter on Earth. Scientists think that any life elsewhere in the universe would also need liquid water to exist (see Astrobiology). From 50 to 90 percent of the weight of living organisms is water. Protoplasm, the basic material of living cells, consists of a solution in water of fats, carbohydrates, proteins, salts, and similar chemicals. Water acts as a solvent, transporting, combining, and chemically breaking down these substances. Blood in animals and sap in plants consist largely of water and serve to transport food and remove waste material. Water also plays a key role in the metabolic breakdown of such essential molecules as proteins and carbohydrates. This process, called hydrolysis, goes on continually in living cells.

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