Structure Of Ice

Type of physical science: Chemistry

Field of study: Chemical compounds

Ice, basically a hexagonal crystal of water (H-O-H) molecules linked by hydrogen-bonded networks, occurs in many physical forms. It forms snow, hail, glaciers, and icebergs, and it covers the surface of freshwater lakes when temperatures are 0 degrees Celsius or colder. Ice's unique physical and chemical properties greatly influence human activity on earth and in the air.

Overview

Ice crystals begin to form when the temperature is at 0 degrees Celsius and the water molecule attaches to a nucleating agent. Ice crystals grow at the expense of water droplets. They grow in two ways. First, water vapor deposits directly onto the crystal. The vapor pressure over the ice crystal is less than that over a water droplet. By this vapor process, ice crystals grow into a hexagonal form. In the second mode of growth, ice crystals collide with the supercooled water droplets, and the droplets freeze onto the crystals. The crystals become coated with a layer of frozen droplets, called rime. Riming tends to obscure the hexagonal form of the parent crystal.

Many people know ice crystals as snowflakes. A wide variety of solid precipitation forms are observed in nature. A snowflake may be a single ice crystal or a coagulation of several crystals. Some 27 million snow crystals cover a 1-meter-square area with snow 50 millimeters deep. Newly fallen snow is porous and often has a density of less than 0.05. Sublimation of the snow crystals occurs readily and the snow loses its porosity quickly; when it reaches a density of 0.8, it technically becomes ice.

In 1951, the International Commission on Snow and Ice published a classification system for snow and ice crystals. It recognized seven basic forms of falling snow crystals plus ice pellets, hail, and graupel, crystals heavily coated with rime. The seven snow crystal types were star, plate, needle, column, column with a cap at each end, spatial dendrite, and irregular. In 1966, Choji Magono and Chung Woo Lee developed a detailed system of snow crystal classification. They described some eighty categories of these ice crystals. The crystal structure of snow is described by four intrinsic axes, three a-axes and a c-axis. The a-axes lie in the basal plane of the ice crystal; the c-axis is perpendicular to the basal plane of the crystal. Depending on meteorological conditions, crystal growth occurs either in the basal plane or perpendicular to the basal plane. Growth in the basal plane results in flat, platelike ice crystals. Growth along the c-axis results in columnlike structures. Wilson A. Bentley in 1931 published three thousand photographs of the artistic splendor of these ice crystals in his book SNOW CRYSTALS. The six sides of snow crystals are a result of the atomic structure of snow crystals. Form and growth rates of these ice forms are the product of environmental conditions. No two snowflakes have ever been found that were exactly alike.

In the atmosphere, water droplets (water molecules) form around microscopic condensation nuclei. Each cubic centimeter of air contains ten to ten thousand such nuclei. The condensation nuclei are microscopic dust, salt, or soil particles blown from the earth by air currents. Even if the air temperature is below 0 degrees Celsius, a droplet will not automatically freeze to form an ice crystal unless it contains another type of impurity, called a freezing nucleus (FN). These are much rarer than condensation nuclei; a cubic centimeter of air at -10 degrees Celsius may have only ten freezing nuclei. The colder the air, the more freezing nuclei will be found, and therefore the more frozen water droplets. At -10 degrees Celsius, only one in a million droplets freeze; at -30 degrees Celsius, one in a thousand, and at -40 degrees Celsius, all water droplets freeze simultaneously.

Three molecules of water (H-O-H) attach themselves to the agent, which produces the typical hexagonal, platelike crystal structure of ice. Additional water molecules join the crystal to form a chain of crystals; it grows until it meets another crystal or the end of its source of water molecules. The atoms of the ice crystal are arranged in a platelike pattern that allows easier slipping along planes parallel to the base of the hexagonal crystal. This allows glaciers to move and avalanches of snow to occur more easily.

After the surface of the ice crystal has frozen, the liquid center of the crystal then freezes. Freezing requires that heat be removed from the water to cool it to the freezing point (0 degrees Celsius). The heat passes through the ice layer by conduction, then the air above the ice's surface carries the heat away by either conduction or convection.

In chemically pure water, containing only HOH molecules, the temperature is lowest in the ice. At the meeting place of the ice crystals and the liquid, the temperature is at the freezing point of water (0 degrees Celsius). If there are other atoms, molecules, or impurities in the water, the freezing point will be lower. Seawater, because of its salt content, forms ice crystals at -1.91 degrees Celsius. When seawater freezes, the ice crystals are salt-free and consist only of water molecules.

As the water turns to ice at the freezing interface, any impurities are carried deeper into the unfrozen center of the crystal. This process is known as constitutional supercooling and causes patterns often seen in the ice. When constitutional supercooling proceeds at a slow rate, the projections are hexagonal cells of ice separated by water. At faster rates, projections resemble fern leaves that form a dendritic pattern. A sample of ice may contain many crystals initiated by many nucleating agents. Robert A. Lowdise and Robert L. Barnes used polarized light to see these structures in ice. The different crystals within the ice show up in various colors or gray areas, depending on the angle of the polarization.

Water can exist as a solid (ice), a liquid, or a vapor. Hydrogen bonding holds water molecules together; kinetic energy, the vibrational movement present in all atoms and molecules, tries to break this bonding. Below 0 degrees Celsius, the kinetic energy of water molecules is slight, so the hydrogen bonding holds the molecules in a rigid, hexagonal pattern, forming ice.

Heating the water molecule increases the amount of kinetic energy; when 8 percent of the hydrogen bonding of the water molecules forming ice is broken, the ice melts. The increased kinetic energy results in the water molecule reaching its maximum density at 4 degrees Celsius.

Ice is more or less transparent to both heat and light; it does not readily allow gases to pass through it. The percentage of light transmission through clear, colorless ice is about 99 percent. Absorption of light increases rapidly if the ice is stained with organic matter or contains air bubbles. Under these conditions, only 50 to 60 percent of the available light will be transmitted through the ice. Snow decreases abruptly the amount of light transmission allowed by the ice crystals. A layer of snow 25 centimeters deep will allow only about 8 percent of the available light to pass through it; 50 centimeters of snow crystals transmit only 4 percent of the light that falls on it.

It requires about 3,500 gram-calories per square centimeter to change solid ice to a liquid state. If ice is exposed to bright sunlight, it can melt internally, even if its surface remains frozen. Internal melting was first reported in 1858 by a British physicist, John Tyndall. Since ice decreases in volume when it melts, a small bubble of water vapor forms in the liquid. Light scattering by these bubbles makes the ice sparkle and dance with bright points of light. Tyndall found that these spots of internal melting can take a variety of designs. The commonest type is oval. Some figures may be several millimeters long; others may be too small to be seen without magnification. Symmetrical Tyndall figures lie in a plane parallel to the basal plane of the ice crystal in which they form. Fernlike figures lie in planes perpendicular to the basal plane of the crystal, parallel to the c-axis of the ice crystal. Tyndall figures probably develop where there are defects or impurities in the ice.

When an ice cube melts, it develops thin tubes, or "wormholes," along the fracture lines, or crystal boundaries, of the cube. Water and air bubbles move along these tubes from the interior of the cube to the surface of it; there the water spews out, and air gurgles through the meltwater to the surface. This produces the crackling sound often associated with melting ice.

One of the most distinctive features of water is that at normal atmospheric pressure its solid phase, ice, is less dense than its liquid phase. When frozen, water is one of the few substances that does not shrink when changed from a liquid to a solid; it expands about one-ninth in volume. Pure ice has a density of about 0.92, so it floats on water. Water has a density of 1.0.

Otherwise, water would freeze from the bottom up, rather than from the top down. If this were to happen, all life in aquatic communities where ice formed would die with each freeze.

The tendency of ice to float results from its remarkable, open-crystal structure. The water molecules in ordinary ice are joined by highly directional, obtuse-angled hydrogen bonds to form a regular hexagonal arrangement that leaves a considerable amount of empty space between the molecules. The density of ice can be increased by subjecting ice to pressures of more than 2,000 atmospheres, which reduces the amount of empty space in the crystal. Nine solid forms of water, designated I through IX, with ordinary ice being form I, have been identified.

Under different temperature and atmospheric pressure combinations, ice forms II through IX can be created; each is denser than normal ice (I). When the high pressure is released, each of the II through IX structures reverts to ordinary ice or liquid water, depending on the temperature.

The German physicist Wilfred B. Holzapfel proposed the existence of another solid form, Ice X. The density of this Ice X would be so high that the crystal structure would no longer consist of water molecules linked by hydrogen bonds. Instead, each oxygen atom would be surrounded by a tight cubic array of nearest-neighbor oxygen atoms; a hydrogen atom would be situated halfway between each oxygen atom, and so would be associated no more with one oxygen atom than with the other. The structure, known as symmetric ice, or Ice X, was predicted to form at pressures greater than 300,000 atmospheres.

The invention of the ingenious high-pressure device known as the diamond-anvil cell allowed Alain Polian, a visiting French investigator, and Marces H. Grimsditch, of the Argonne staff, actually to make Ice X at the Argonne National Laboratory.

Using the technique of Brillouin-scattering spectroscopy, in which the compressibility of a sample of matter is determined indirectly by measuring the reflection of laser light from highly directional sound waves in the sample, Polian and Grimsditch made Ice X. They created pressures above 440,000 atmospheres and made the predicted symmetrical ice crystal. As such, Ice X would be the first nonmolecular structure for water.

Applications

Ice has a dramatic impact on life's survival on earth. Its greatest impact is on biological activities, but it also changes and modifies the physical environment. Glaciers have ground down mountains, and their fragments have been carried many kilometers to new areas where they made soil. Materials carried by glaciers have formed rich outwash plains when the glaciers melted; these outwash plains now are some of the most fertile farmlands of the world. Glacial action has created spectacular scenery, such as that of Yosemite Valley in Yosemite National Park, California.

Glaciers flow to the sea, and large masses of ice break from them; these pieces of glacier are known as icebergs. Icebergs create serious navigation hazards, and collisions can sink ships like the TITANIC. Sea ice, formed by the freezing of seawater, locks many of the far northern and southern areas of the globe in impenetrable covers for several months each year, preventing shipping in these areas, at least seasonally.

The climate of some of the world is governed to large degree by the presence of ice.

Ice-covered areas reflect 60 to 98 percent of the sun's incoming rays. Land covered with vegetation reflects only 20 percent of this light. This reflection of the sun's energy cools the temperature but also limits climatic extremes in these areas. About 3 percent of the water on earth is held frozen by glaciers and the polar ice caps.

Water expands when it freezes, so it can break rocks to create soil or break water pipes in one's home. A frozen layer of ice effectively seals a lake or pond from any heat transfer or gas exchange. If a layer of snow falls on the ice, photosynthetic action of plants within the lake stops, as no light can reach the chlorophyll to trigger this chemical reaction. If photosynthesis stops, no oxygen is produced. Oxygen-breathing animals in the lake may die for lack of oxygen if the lake remains frozen over or snow-covered for too long a period.

Ice causes many accidents to people traveling the roads and sidewalks. Ice on the wings of aircraft has caused plane crashes, and ships have sunk because they became encrusted with ice. Airplane runways crafted from ice and frozen rivers have provided useful highways in winter. Loggers have stored their logs on ice-covered rivers; when the ice melted, the logs were floated down to sawmills for processing. Bridges of ice have been built, and scientists have lived on floating "ice islands" in the Arctic to learn about the world.

Context

The physical properties of ice make ice sculpture, ice skating, and ice boating familiar pastimes in cold lands. More important, ice has actually shaped human history and may alter greatly its future.

Early humans could not have crossed the Bering Sea from Siberia to North America if the water of this ocean area had not become locked in the ice sheet covering North America.

Humans first entered Alaska from Siberia during the Pleistocene, some forty thousand years ago.

The area between Siberia and Alaska became a marshland over which many animals emigrated to North America. Human hunters followed these animals across this corridor and discovered a new continent. Ice-covered rivers and tundra became their highways.

Remnants of this last ice age remain today in northern lands as permafrost, permanently frozen ground. If the ice within this permafrost melts, buildings built on it sag drunkenly and roads buckle and heave, developing a roller-coaster look. Insulation and even mechanical refrigeration have been used to keep this permafrost frozen and permanent under these structures.

Remote sensing from satellites monitors the area of the polar ice caps and sea ice boundaries. Icebergs contain a large store of fresh water and may be mined to produce freshwater supplies for arid and semiarid areas. If global warming raises the earth's temperature enough, however, the ice at the polar ice caps would melt. The release of this water would raise existing sea levels along U.S. coasts 15 to 60 meters and totally change the outline of Earth's land masses.

Antarctic and Greenland ice fields represent 99 percent of the earth's glacial ice. These glaciers cover more than 10 percent of the land's surface, equal in area to that being farmed or occurring in the tropical rain forest belt. Sea ice covers about twice the area of that of terrestrial ices: 23 percent of the ocean's surface, and 14 percent of the entire earth's surface. Researchers are studying the physical, chemical, and biological events that occur in these ice worlds.

If the climate of the earth were to cool, then more of the moisture in the atmosphere would fall as snow. As the snow accumulated, as it did during the last ice age, the weight of it would result in ice forming. Glaciers would then reoccupy much of the cold areas of the earth, as they did in past eons. A new ice age would dramatically alter humankind's history.

Principal terms

GRAM-CALORIE: the quantity of heat needed to raise one gram of water from 15 degrees Celsius to 16 degrees Celsius

CONDENSATION NUCLEI: a tiny particle of matter in the atmosphere around which the water (H-O-H) molecule forms; essential for the formation of water droplets, which may form ice crystals

SUBLIMATION: the process of going directly from a solid to a gas form, or from a gas to a solid, without having a liquid state

SUPERCOOLED: having a temperature below 0 degrees Celsius but still in liquid form; refers mainly to water molecules

Bibliography

Kingery, W. D. ICE AND SNOW: PROPERTIES, PROCESSES, AND APPLICATIONS. Cambridge, Mass.: MIT Press, 1963. Papers from one of a series of conferences dealing with ice and snow, varying from highly technical to readable reviews of the physical, chemical, and biological properties of ice and snow. This series may be of limited value for the general reader.

Kirk, Ruth. SNOW. New York: Morrow Quill Paperbacks, 1980. The many mysteries of snow and ice are clearly explained in this excellent book. How animals and people survive in snow, the awesome power of glaciers, continental ice sheets, blizzards, and avalanches all are documented. More than one hundred references on snow and ice are listed in the bibliography.

Perla, Ronald I., and M. Martinelli, Jr. AVALANCHE HANDBOOK. Washington, D.C.: Government Printing Office, 1976. For those who encounter ice and snow avalanches in their work or play and those interested in one of the most awesome of natural forces. Chapter 2, "Avalanche Meteorology," provides an excellent introduction to ice and snow physics, chemistry, and weather as it produces a dangerous event.

Pounder, Elton R. PHYSICS OF ICE. New York: Pergamon Press, 1965. A detailed review of the structure and properties of ice. Sea ice is discussed in detail, as is ice control. Individual chapters discuss crystallography of ice, mechanical properties, thermal and electrical properties, and the growth and decay of ice cover. A good background in science helps the reader understand some of this material.

Tufnell, Lance. TOPICS IN APPLIED GEOGRAPHY--GLACIAL HAZARDS. New York: Longman, 1984. An integrated treatment of a major climate-related hazard: the advance and retreat of glaciers. The first five chapters provide a very readable introduction to glacier dynamics; case histories of glacial action are presented in the last three chapters. An extensive listing of almost two hundred references is given in the bibliography section.

U.S. Navy Hydrographic Office. A FUNCTIONAL GLOSSARY OF ICE TERMINOLOGY. Washington, D.C.: Author, 1952. Thirty pages of terms relating to ice are clearly defined in this Navy publication. Another 110 photographs illustrate ice conditions, nicely supplementing the glossary definitions. Very readable; no extensive scientific background is needed to understand these clear definitions.

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