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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℃, its melting point, to 4℃, 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.
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℃. 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℃ and 4℃, 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).
water in the solid phase; there are ten known crystal modifications of ice, as well as amorphous ice. Figure 1 shows the phase diagram of water, which indicates the temperatures and pressures at which the various modifications are stable.
The best-known modification is ice I (see Tables 1 and 2), which is the only modification found in nature. Ice is found in nature in the form of ordinary ice (continental, floating, subterranean, and other varieties), and as snow and frost. Natural ice is usually considerably purer than water, since the solubility of materials (except NH4F) in ice is exceptionally poor. Ice may contain physical impurities (solid particles, droplets of concentrated solutions, and gas bubbles). The saltiness of sea ice is due to the presence of salt crystallites and brine droplets. The total deposits of ice on earth are about 30 million cu km. There is evidence concerning the presence of ice on planets in the solar system and in comets. The main ice deposits on earth are concentrated in polar regions (mainly in Antarctica, where the thickness of the ice layer is as great as 4 km).
Because of the great abundance of water and ice on the earth’s surface, the sharp differences between certain properties of ice and of other materials are of great importance in natural processes. As a result of its lower density than water, ice forms a floating sheath on the surface of water, protecting rivers and reservoirs from freezing to the bottom. Polycrystalline ice has a hyperbolic dependence between the established rate of flow and the stress; when this relationship is approximately described by
|Table 1. Some properties of ice I|
|Note: 1 cal/(g.°C) = 4.186 kJ/(kg.°K); 1 ohm–1 . cm–1 =100 S/m; 1 dyne/cm = 10–3 N/m; 1 cal/icm-sec-°C) = 418.68 W/(m.°K); 1 poise = 10–1 N-sec/m2|
|Heat capacity, cal/(g.°C).........................||0.51 (0°C)||Decreases sharply with decreasing temperature|
|Heat of fusion, cal/g............................||79.69|
|Heat of vaporization, cal/g........................||677|
|Coefficient of thermal expansion, 1 /°C.................||9.1 × 10–5(0°C)|
|Thermal conductivity, cal/(cm.sec-°C).................||4.99 × 10–3|
|Index of refraction|
|ordinary ray ..............................||1.309 (–3°C)|
|extraordinary ray...........................||1.3104 (–3°C)|
|Specific electric conductivity, ohm –1 · cm –1 ..............||10–9 (0°C)||Apparent activation energy, 11 kcal/mole|
|Surface conductivity, ohm-1.......................||10-10 (–11°C)||Apparent activation energy, 32 kcal/mole|
|Young’s modulus, dynes/cm.......................||9 × 1010 (–5°C)||Polycrystalline ice|
|crushing ................................||Polycrystalline ice|
|Average effective viscosity, poises ...................||1014||Polycrystalline ice|
|Power index of the power law of flow .................||3|
|Activation energy during deformation and mechanical relaxation, kcal/mole ...............................||11.44–21.3||Increby 0.0361 kcal/(mole-°C) from 0° to 273.16°K|
a power equation, the power index increases with increasing stress. In addition, the rate of flow is directly proportional to the energy of activation and inversely proportional to the absolute temperature, so that ice approaches an absolutely rigid body with decreasing temperature. The average flow rate of ice at temperatures close to the melting point is 106 times higher than that of rock. Because of its fluidity, ice does not accumulate indefinitely but rather flows down from the areas of the earth’s crust in which the rate of accumulation exceeds that of melting. Because of the very high reflectivity of ice (0.45) and particularly of snow (up to 0.95), the surface covered by them—an annual average of 72 million sq km at high and medium latitudes of both hemispheres—receives 65 percent less than the normal quantity of solar energy. This is a powerful source of cooling of the earth’s surface and to a considerable degree is responsible for the latitudinal climatic zones. During the summer, solar radiation is more intense in the polar regions than in the equatorial belt, but the temperature remains low, since a significant quantity of absorbed heat is expended on the melting of ice, which has a very high latent heat of fusion.
Ice II, III, and V may be preserved for long periods of time at atmospheric pressure if the temperature does not exceed — 170°C. Heating to about — 150°C transforms these modifications into cubic ice (ice Ic), which is not shown in Figure 1 because it is not known whether or not it is a stable phase. Another method for the preparation of ice Ic consists in the condensation of water vapor on a substrate cooled to — 120°C. Condensation of vapor on colder surfaces leads to the formation of amorphous ice. Both of these forms of ice may undergo spontaneous transition to the hexagonal ice I, in which case the transition rate accelerates with increasing temperature.
Ice IV is a metastable phase in the range of stability of ice V. It forms readily and may be stable if heavy water is subjected to pressure. The melting curve of ice VII has been studied for pressures up to 20 giganewtons per sq m (GN/m2), or 200,000 kilograms-force per sq cm (kgf/cm2). Ice VII melts at 400°C at this pressure. Ice VIII is a low-temperature, more highly ordered form of ice VII. Ice IX is a metastable phase generated by supercooling of ice III and is essentially a low-temperature form of it. Phenomena of supercooling and metastable equilibrium are generally very characteristic of the phases formed by water. Some lines of metastable equilibriums are shown by dotted lines in Figure 1.
The polymorphism of ice was discovered by G. Tammann (1900) and studied in detail by P. Bridgman (beginning in 1912). The phase diagram of water developed by Bridgman has been revised and amended several times since the 1960’s. Data on the structure of modifications of ice, as well as some of the properties of the modifications, are given in Tables 3 and 4.
The crystals of all modifications of ice consist of water molecules (H2O) joined by hydrogen bonds into a three-dimensional framework (Figure 2). Each molecule participates in the formation of four such bonds, which are directed toward the apexes of a tetrahedron. In the structures of ice I, Ic, VII, and VIII the tetrahedron is regular—that is, the angle between the bonds is 109°28’. The high density of ice VII and VIII is explained by the fact that their structures contain two three-dimensional networks of hydrogen bonds (each of which is identical to the structure of ice Ic), one inside the other. The tetrahedrons in the structures of ice II, III, V, and VI are appreciably distorted. Two intersecting systems of hydrogen bonds may be identified in the structures of ice VI, VII, and VIII.
|Table 2. Quantity, distribution, and lifetime of ice I|
|Type||Weight||Area of distribution||Average concentration (g/cm2)||Rate of increase of mass (g/yr)||Average lifetime (years)|
|grams||percent||million sq km||percent|
|Glaciers.....||2.4 × 1022||98.95||16.1||10.9 (land)||1.48 × 105||2.5 × 1018||9,580|
|Subterranean ice||2 × 1020||0.83||21||14.1 (land)||9.52 × 103||6 × 1018||30–75|
|Sea ice .....||3.5 × 1019||0.14||26||7.2 (ocean)||1.34 × 102||3.3 × 1019||1.05|
|Snow cover . . .||1.0 × 1019||0.04||72.4||14.2 (earth)||14.5||2 × 1019||0.3–0.5|
|Icebergs.....||7.6 × 1018||0.03||63.5||18.7 (ocean)||14.3||1.9 × 1018||4.07|
|Atmospheric ice||1.7 × 1018||0.01||510.1||100 (earth)||3.3 × 10–1||3.9 × 1020||4 × 10–3|
|Table 3. Data on the structures of modifications of ice|
|Symmetry class||Fedorov group||Length of hydrogen bonds (A)||O angles in tetrahedrons (deg)|
|Note: 1 Å = 10–10 m|
Data on the positions of protons in various ice structures are less conclusive than data on the positions of oxygen atoms. It may be concluded that the configuration of water molecules characteristic of steam is retained in the solid state (the O—H distances apparently increase somewhat because of the formation of hydrogen bonds), whereas the protons tend to be positioned closer to the lines connecting the centers of oxygen atoms.
Thus, six more or less equivalent orientations of water molecules with respect to their neighbors are possible. Some of these orientations are excluded, since the simultaneous presence of two protons at the location of a hydrogen bond is improbable, but a definite uncertainty remains concerning the orientation of water molecules. This uncertainty applies to most of the modifications of ice—I, III, V, VI, and VII (and apparently, Ic)—so that according to the expression of J. Bernal, ice is crystalline with respect to the oxygen atoms but glasslike with respect to the hydrogen atoms. The water molecules are orientationally ordered in ice II, VIII, and IX.
|Table 4. Density and static dielectric constant of various types of ice|
|Temperature (°C)||Pressure (MN/m2)||Density (g/cm3)||Dielectric constant|
Ice in the atmosphere, in water, on the surface of land and water, and in the earth’s crust exerts a great influence on the environment and vital activity of plants and animals and on various types of human economic activity. Ice may generate various natural phenomena with harmful or destructive consequences (icing of aircraft, ships, structures, roads, and soil; hail, blizzards, and snowdrifts; river blockages and jams with flooding; avalanches; and breakage of plant roots resulting from the formation of layers of ice in the soil). The prediction, identification, and prevention of harmful phenomena, the control of their effects, and the use of ice for various purposes (snowdrift control, construction of ice crossings and isothermal storage areas, lining of warehouses, and ice-lining of mine shafts) are the subjects of various branches of hydrometeorological and engineering science (ice and snow engineering; engineering applications of soil freezing studies) and of various service activities (ice reconnaissance, icebreaker transport, snow-removal services, and artificial provocation of avalanches). Some types of sports use ice rinks with artificial refrigeration, which make possible the conduct of competitions on ice during warm seasons and in enclosed areas. Natural ice is used for the preservation and storage of foods, as well as biological and medicinal preparations; it is produced and prepared for these purposes.
REFERENCESShumskii, P. A. Osnovy strukturnogo ledovedeniia. Moscow, 1955.
Pounder, E. R. Fizika l’da. Moscow, 1967. (Translated from English.)
Eisenberg, D., and W. Kauzmann. The Structure and Properties of Water. Oxford, 1969.
Fletcher, N. H. The Chemical Physics of Ice. Cambridge, 1970.
G. G. MALENKOV
What does it mean when you dream about ice?
Ice often symbolizes the dreamer’s emotional state. The dreamer may not be conscious of being blocked or frozen emotionally. Falling through the ice suggests the dreamer may be “skating on thin ice” and should alter course to avoid mishap.
ICE(1) (Information and Content Exchange) A data syndication protocol that allows one website to obtain content from another website. Based on XML and meta tags, ICE provides a standard for subscribing to content. See XML, meta tag and syndication format.
(2) (In-Circuit Emulator) A chip used for testing and debugging logic circuits typically in embedded systems. The chip emulates a particular microprocessor and contains breakpoints and other debugging functions. See ROM emulator.
(3) (In Case of Emergency) A cellphone entry stored under the name of "ICE" that contains an emergency contact number and other medical information. It was recommended by a British paramedic, and a campaign for public awareness was launched in the U.K. in 2005. See emergency app.
(4) (Ice) A Lotus 1-2-3 add-on from Baler Software Corporation, Rolling Meadows, IL, that added extensions to Lotus macros.
(5) (Image Correction and Enhancement) See Digital ICE.
(6) (Internal Combustion Engine) A motor that explodes gasoline or diesel fuel to drive a piston within a cylinder. Dating back to the mid-1800s, ICE motors today come in two-stroke and four-stroke designs. Two-stroke engines (chainsaws, weed whackers, etc.) mix the fuel with the oil that lubricates the cylinders, whereas four-stroke engines (automobiles, high-power generators, etc.) have separate inputs for oil and fuel.