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 (32 degrees Fahrenheit) 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 or below 0 degrees Celsius (32 degrees Fahrenheit) 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-square-meter (10.76-square-foot) area with snow 50 millimeters deep (1.97 inches deep). Newly fallen snow is porous and often has a density of less than 0.05 grams per cubic centimeter. Sublimation of the snow crystals occurs readily and the snow loses its porosity quickly; when it reaches a density of 0.8 grams per cubic centimeter, 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 are known to be exactly alike.

In the atmosphere, water droplets (water molecules) form around microscopic condensation nuclei. Each cubic centimeter of air contains approximately 10 to 10,000 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 (32 degrees Fahrenheit), 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 (14 degrees Fahrenheit) 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 (14 degrees Fahrenheit), only one in a million droplets freeze; at -30 degrees Celsius (-22 degrees Fahrenheit), one in a thousand, and at -40 degrees Celsius (-40 degrees Fahrenheit), all water droplets freeze simultaneously.

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 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 H-O-H 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 or 32 degrees Fahrenheit). 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 (28.56 degrees Fahrenheit). 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 (32 degrees Fahrenheit), 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 sufficient 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 (39.2 degrees Fahrenheit).

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 (9.8 inches) deep will allow only about 8 percent of the available light to pass through it; 50 centimeters (19.7 inches) of snow crystals transmit only 4 percent of the light that falls on it.

It requires about 80 gram-calories per gram 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 grams per cubic centimeter, so it floats on water. Water has a density of 1.0 gram per cubic centimeter.

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. More than twenty solid forms of water have been identified, with ordinary ice being form I_h.

Under different temperature and atmospheric pressure combinations, numerous additional ice forms (such as ice II through XIX and beyond) can be created; many of these are denser than normal ice (I_h). 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 Marvin 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 studied ice at high pressure. They used a diamond anvil cell, which generated a high-pressure atmosphere to form the predicted symmetrical ice crystal. However, other research groups have also worked on Ice X in parallel and have staked a claim to its identification and creation. As such, Ice X would be the first symmetric 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 have 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 regions of the world is governed to a 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 2 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, the 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 are thought to have entered Alaska from Siberia during the late Pleistocene, likely between about 15,000 and 20,000 years ago, although ongoing research in this area may lead to revisions to the timeline.

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 the last ice age remain 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 large stores of fresh water and have been considered as potential sources 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 global sea levels by about 70 meters (230 feet) 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. Overall, considering the seasonal fluctuations of sea ice, the sea ice (about 7 percent) covers a larger area than the area of terrestrial ice (about 3 percent): about 15 percent of the ocean’s surface and about 7 percent of the entire Earth’s surface. Researchers study 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

“How Does Sea Ice Affect Global Climate?” National Ocean Service, oceanservice.noaa.gov/facts/sea-ice-climate.html. Accessed 18 Apr. 2026.

“How Much Water Is in the Ocean?” NOAA’s National Ocean Service, 16 June 2024, oceanservice.noaa.gov/facts/oceanwater.html. Accessed 18 Apr. 2026.

Kingery, W. D. Ice and Snow: Properties, Processes, and Applications. MIT Press, 1963.

Kirk, Ruth. Snow. Morrow Quill Paperbacks, 1980.

Klug, D. D., et al. “Hydrogen-Bond Dynamics and Fermi Resonance in High-Pressure Methane Filled Ice.” The Journal of Chemical Physics, vol. 125, no. 15, 2006, doi:10.1063/1.2357954. Accessed 18 Apr. 2026.

Perla, Ronald I., and M. Martinelli, Jr. Avalanche Handbook. Government Printing Office, 1976.

Polian, A., and M Grimsditch. “New High-Pressure Phase of H20: Ice X.” Physical Review Letters, 1984, hal.science/hal-03085330/document. Accessed 18 Apr. 2026.

Pounder, Elton R. Physics of Ice. Pergamon Press, 1965.

Reinhardt, Aleks, et al. “Thermodynamics of High-Pressure Ice Phases Explored with Atomistic Simulations.” Nature Communications, vol. 13, 2022, article no. 4707, doi:10.1038/s41467-022-32374-1. Accessed 18 Apr. 2026.

“Sea Ice.” National Snow and Ice Data Center, nsidc.org/learn/parts-cryosphere/sea-ice. Accessed 18 Apr. 2026.

Tufnell, Lance. Topics in Applied Geography—Glacial Hazards. Longman, 1984.

U.S. Navy Hydrographic Office. A Functional Glossary of Ice Terminology. Author, 1952.

Weeks, Wilford F., and Serena Schroeter. “Sea Ice: Overview.” Encyclopedia of Ocean Sciences. vol. 6, 3rd ed., edited by John H. Steele et al., Elsevier, 2019, pp. 181–86, doi:10.1016/B978-0-12-409548-9.10819-X. Accessed 18 Apr. 2026.