Ice

For other uses, see Ice (disambiguation).

This article is about water ice. For the use of "ice" in the planetary sciences, see volatiles.

"Icy" redirects here. For the penguin that is similar to the iDog, see iCy.

A fountain in Boise, Idaho, February 2007

Snowflakes (ice crystals) by Wilson Bentley, 1902

Ice is a solid phase, usually crystalline, of a non-metallic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice. However, the word "ice" normally means water ice, technically restricted to one of the 15 known crystalline phases of water. In non-scientific contexts, it usually describes ice I_h, which is known to be the most abundant of these phases. It can appear transparent or an opaque bluish-white color, depending on the presence of impurities such as air. The addition of other materials such as soil may further alter the appearance.

The most common phase transition to ice I_h occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. It can also deposit from a vapour with no intervening liquid phase, such as in the formation of frost.

Ice appears in nature in forms as varied as snowflakes and hail, icicles, glaciers, pack ice, and entire polar ice caps. It is an important component of the global climate, particularly in regard to the water cycle. Furthermore, ice has numerous cultural applications, from the ice cooling one's drink to winter sports and ice sculpture.

The word is from Old English ís, in turn derived from Proto-Germanic *isaz.

Characteristics

String of needle ice found in the Adirondack Region of New York State

Ice crystals at refrigerator window

As a naturally occurring crystalline solid, ice is considered a mineral consisting of hydrogen oxide.

An unusual property of ice frozen at a pressure of one atmosphere is that the solid is some 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. Ice has a density of 0.9167 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is densest, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C. (In fact, the word "crystal" derives from Greek word for frost.) This is due to hydrogen bonds forming between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result of this is that ice floats on liquid water, which is an important factor in Earth's climate (if ice had sunk instead of floating, any body of water would have frozen from the bottom to the surface, killing any fish and other creatures not resistant to freezing temperatures). Density of ice increases slightly with decreasing temperature (density of ice at −180 °C (93 K) is 0.9340 g/cm³).

When ice melts, it absorbs as much heat energy (the heat of fusion) as it would take to heat an equivalent mass of water by 80 °C, while its temperature remains a constant 0 °C.

It is also theoretically possible to superheat ice beyond its equilibrium melting point. Simulations of ultrafast laser pulses acting on ice show it can be heated up to room temperature for an extremely short period (250 ps), without melting.

Light reflecting from ice can appear blue, because ice absorbs more of the red frequencies than the blue ones. Also, icebergs containing impurities (e.g. sediments, algae, air bubbles) can appear green.

Slipperiness

Until recently, it was widely believed that ice was slippery because the pressure of an object in contact with it caused a thin layer to melt. For example, the blade of an ice skate, exerting pressure on the ice, melted a thin layer, providing lubrication between the ice and the blade.

This explanation is no longer accepted. There is still debate about why ice is slippery. The explanation gaining acceptance is that ice molecules in contact with air cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semiliquid state, providing lubrication regardless of pressure against the ice exerted by any object.

brrrrrrr cold

Natural occurrence

Ice appears seasonally in many parts of the world in the winter, in the form of snowflakes and hail, icicles, and frozen bodies of water. Ice is also present year-round as part of glaciers (which may be found at very high altitudes or high latitudes), permafrost, pack ice, and the polar ice caps, which extend across the Arctic and Antarctica. Ice that is found at sea may be in the form of sea ice, pack ice, or icebergs. The term that collectively describes all of the parts of the Earth's surface where water is in frozen form is the cryosphere.

Ice is an important component of the global climate, particularly in regard to the water cycle. Glaciers and snowpacks are an important storage mechanism for fresh water; over time, they may sublimate or melt. Snowmelt is often an important source of seasonal fresh water.

Production

Ice is now mechanically produced on an large scale, but before appropriate coolants were developed ice was harvested from natural sources for human use.

Ice harvesting

Harvesting ice on Lake Saint Clair in Michigan, circa 1905

Ice being transported by cart in Mumbai, India

Ice has long been valued as a means of cooling. Until recently, the Hungarian Parliament building used ice harvested in the winter from Lake Balaton for air conditioning. Icehouses were used to store ice formed in the winter, to make ice available all year long, and early refrigerators were known as iceboxes, because they had a block of ice in them. In many cities, it was not unusual to have a regular ice delivery service during the summer. For the first half of the 19th century, ice harvesting had become big business in America. Frederic Tudor, who became known as the “Ice King,” worked on developing better insulation products for the long distance shipment of ice, especially to the tropics. The advent of artificial refrigeration technology has since made delivery of ice obsolete.

In 400 BC Iran, Persian engineers had already mastered the technique of storing ice in the middle of summer in the desert. The ice was brought in during the winters from nearby mountains in bulk amounts, and stored in specially designed, naturally cooled refrigerators, called yakhchal (meaning ice storage). This was a large underground space (up to 5000 m³) that had thick walls (at least two meters at the base) made out of a special mortar called sārooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, and which was known to be resistant to heat transfer. This mixture was thought to be completely water impenetrable. The space often had access to a Qanat, and often contained a system of windcatchers which could easily bring temperatures inside the space down to frigid levels on summer days. The ice was then used to chill treats for royalty on such occasions.

Commercial production

Ice is now produced on an industrial scale, for uses including food storage and processing, chemical manufacturing, concrete mixing and curing, and consumer or packaged ice. Most commercial ice makers produce three basic types of fragmentary ice: flake, tubular and plate, using a variety of techniques. Large batch ice makers can produce up to 75 tons of ice per day.

Ice production is a large business; in 2002, there were 426 commercial ice-making companies in the United States, with a combined value of shipments of $595,487,000.

For small-scale ice production, many modern home refrigerators can also make ice with a built in icemaker, which will typically make ice cubes or crushed ice. Stand-alone icemaker units that make ice cubes are often called ice machines.

Uses

Sports

Ice sailing on the Żnin Small Lake

Ice also plays a role in winter recreation, in many sports such as ice skating, tour skating, ice hockey, ice fishing, ice climbing, curling, broomball and sled racing on bobsled, luge and skeleton. Many of the different sports played on ice get international attention every four years during the Winter Olympic Games.

A sort of sailboat on blades gives rise to ice yachting. The human quest for excitement has even led to ice racing, where drivers must speed on lake ice, while also controlling the skid of their vehicle (similar in some ways to dirt track racing). The sport has even been modified for ice rinks.

Other uses

Ice pier during 1983 cargo operations. McMurdo Station, Antarctica

• Engineers used pack ice's formidable strength when they constructed Antarctica's first floating ice pier in 1973. Such ice piers are used during cargo operations to load and offload ships. Fleet operations personnel make the floating pier during the winter. They build upon naturally-occurring frozen seawater in McMurdo Sound until the dock reaches a depth of about 22 feet (6.7 m). Ice piers have a lifespan of three to five years.
• The manufacture and use of ice cubes or crushed ice is common for drinks.
• Structures and ice sculptures are built out of large chunks of ice. The structures are mostly ornamental (as in the case with ice castles), and not practical for long-term habitation. Ice hotels exist on a seasonal basis in a few cold areas. Igloos are another example of a temporary structure, made primarily from snow.
• During World War II, Project Habbakuk was a British programme which investigated the use of pykrete (wood fibres mixed with ice) as a possible material for warships, especially aircraft carriers, due to the ease with which a large deck could be constructed, but the idea was given up when there were not enough funds for construction of a prototype.
• Ice can be used to start a fire by carving it into a lens which will focus sunlight onto kindling. When one waits long enough, a fire will start.
• Ice has even been used as the material for a variety of musical instruments, principally by percussionist Terje Isungset.
• Ice can be used to reduce swelling(by decreasing blood flow) and pain by pressing it against an area of the body.

Ice and transportation

U.S. Coast Guard icebreakers near McMurdo Station, February 2002

Ice can also be an obstacle; for harbors near the poles, being ice-free is an important advantage; ideally, all year long. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vardø (Norway). Harbors which aren't ice-free are opened up using icebreakers.

Ice forming on roads is a dangerous winter hazard. Black ice is very difficult to see, because it lacks the expected frosty surface. Whenever there is freezing rain or snow which occurs at a temperature near the melting point, it is common for ice to build up on the windows of vehicles. Driving safely requires the removal of the ice build-up. Ice scrapers are tools designed to break the ice free and clear the windows, though removing the ice can be a long and laborious process.

Far enough below the freezing point, a thin layer of ice crystals can form on the inside surface of windows. This usually happens when a vehicle has been left alone after being driven for a while, but can happen while driving, if the outside temperature is low enough. Moisture from the driver's breath is the source of water for the crystals. It is troublesome to remove this form of ice, so people often open their windows slightly when the vehicle is parked in order to let the moisture dissipate, and it is now common for cars to have rear-window defrosters to solve the problem. A similar problem can happen in homes, which is one reason why many colder regions require double-pane windows for insulation.

When the outdoor temperature stays below freezing for extended periods, very thick layers of ice can form on lakes and other bodies of water, although places with flowing water require much colder temperatures. The ice can become thick enough to drive onto with automobiles and trucks. Doing this safely requires a thickness of at least 30 centimetres (one foot).

For ships, ice presents two distinct hazards. Spray, and freezing rain, can produce an ice build-up on the superstructure of a vessel sufficient to make it unstable, and to require it to be hacked off or melted with steam hoses. And icebergs — large masses of ice floating in water (typically created when glaciers reach the sea) — can be dangerous if struck by a ship when underway. Icebergs have been responsible for the sinking of many ships, the most famous probably being the Titanic.

For aircraft, ice can cause a number of dangers. As an aircraft climbs, it passes through air layers of different temperature and humidity, some of which may be conducive to ice formation. If ice forms on the wings or control surfaces, this may adversely affect the flying qualities of the aircraft. During the first non-stop flight of the Atlantic, the British aviators Captain John Alcock and Lieutenant Arthur Whitten Brown encountered such icing conditions - Brown left the cockpit and climbed onto the wing several times to remove ice which was covering the engine air intakes of the Vickers Vimy aircraft they were flying.

A particular icing vulnerability associated with reciprocating internal combustion engines is the carburetor. As air is sucked through the carburettor into the engine, the local air pressure is lowered, which causes adiabatic cooling. So, in humid near-freezing conditions, the carburettor will be colder, and tend to ice up. This will block the supply of air to the engine, and cause it to fail. For this reason, aircraft reciprocating engines with carburettors are provided with carburettor air intake heaters. The increasing use of fuel injection—which does not require carburettors—has made "carb icing" less of an issue for reciprocating engines.

Jet engines do not experience carb icing, but recent evidence indicates that they can be slowed, stopped, or damaged by internal icing in certain types of atmospheric conditions much more easily than previously believed. In most cases, the engines can be quickly restarted and flights are not endangered, but research continues to determine the exact conditions which produce this type of icing, and find the best methods to prevent, or reverse it, in flight.

At different pressures

Most liquids freeze at a higher temperature under pressure, because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: water freezes at a temperature below 0 °C under a pressure higher than 1 atm. Consequently, water also remains frozen at a temperature above 0 °C under a pressure lower than 1 atm. The melting of ice under high pressures is thought to contribute to the movement of glaciers. Ice formed at high pressure has a different crystal structure and density to ordinary ice. Ice, water, and water vapour can coexist at the triple point, which is exactly 273.16 K (by definition) at a pressure of 611.73 Pa.

Phases

Phase	Characteristics
Amorphous ice	Amorphous ice is an ice lacking crystal structure. Amorphous ice exists in three forms: low-density (LDA) formed at atmospheric pressure, or below, high density (HDA) and very high density amorphous ice (VHDA), forming at higher pressures. LDA forms by extremely quick cooling of liquid water ("hyperquenched glassy water", HGW), by depositing water vapour on very cold substrates ("amorphous solid water", ASW) or by heating high density forms of ice at ambient pressure ("LDA").
Ice Ih	Normal hexagonal crystalline ice. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic.
Ice Ic	A Metastable cubic crystalline variant of ice. The oxygen atoms are arranged in a diamond structure. It is produced at temperatures between 130-150 K, and is stable for up to 200 K, when it transforms into ice Ih. It is occasionally present in the upper atmosphere.
Ice II	A rhombohedral crystalline form with highly ordered structure. Formed from ice Ih by compressing it at temperature of 190-210 K. When heated, it undergoes transformation to ice III.
Ice III	A tetragonal crystalline ice, formed by cooling water down to 250 K at 300 MPa. Least dense of the high-pressure phases. Denser than water.
Ice IV	A Metastable rhombohedral phase. Doesn't easily form without a nucleating agent.
Ice V	A monoclinic crystalline phase. Formed by cooling water to 253 K at 500 MPa. Most complicated structure of all the phases.
Ice VI	A tetragonal crystalline phase. Formed by cooling water to 270 K at 1.1 GPa. Exhibits Debye relaxation.
Ice VII	A cubic phase. The hydrogen atoms positions are disordered; the material shows Debye relaxation. The hydrogen bonds form two interpenetrating lattices.
Ice VIII	A more ordered version of ice VII, where the hydrogen atoms assume fixed positions. Formed from ice VII, by cooling it below 5 °C.
Ice IX	A tetragonal metastable phase. Formed gradually from ice III by cooling it from 208 K to 165 K, stable below 140 K and pressures between 200 and 400 MPa. It has density of 1.16 g/cm³, slightly higher than ordinary ice.
Ice X	Proton-ordered symmetric ice. Forms at about 70 GPa.
Ice XI	An orthorhombic low-temperature equilibrium form of hexagonal ice. It is ferroelectric.
Ice XII	A tetragonal metastable dense crystalline phase. It is observed in the phase space of ice V and ice VI. It can be prepared by heating high-density amorphous ice from 77 K to about 183 K at 810 MPa.
Ice XIII	A monoclinic crystalline phase. Formed by cooling water to below 130 K at 500 MPa. The proton-ordered form of ice V.
Ice XIV	An orthorhombic crystalline phase. Formed below 118 K at 1.2 GPa. The proton-ordered form of ice XII.
Ice XV	The predicted, but not yet proven, proton-ordered form of ice VI. Thought to be formed by cooling water to around 108-80 K at 1.1 GPa.

See also

Detail of an ice cube

Ice formations Amorphous solid water Anchor ice Black ice Diamond dust Firn Frazil ice Frost flowers Glacier Iceberg

Icicle Ice circle Ice crystals Ice nucleus Ice spike Névé Needle ice Sea ice Slurry ice

Climate Ice age Ice sports Figure skating Icebiking Ice climbing Ice hockey Tour skating

Man-made ice Ice cream Ice cube Ice hotel Pykrete

Further reading

• Gosnell, Mariana. (2005). Ice : the nature, the history, and the uses of an astonishing substance. New York: Knoph. ISBN 9780679426080.
• Marling, Karal. (2008). Ice : great moments in the history of hard, cold water. St. Paul, MN : Borealis Books. ISBN 9780873516280

External links

• The National Snow and Ice Data Center, based in the United States
• The phase diagram of water-steam-ice: WebCalculation
• The phase diagram of water, including the ice variants
• Webmineral listing for Ice
• MinDat.org listing and location data for Ice
• The physics of ice
• The phase diagrams of water with some high pressure diagrams
• A recent discovery about how ice melts
• 'Unfreezable' water, 'bound water' and water of hydration
• Electromechanical properties of ice
• Estimating the maximum thickness of an ice layer
• Sandia's Z machine creates ice in nanoseconds
• Amazing ice at Lac Leman
• The Surprisingly Cool History of Ice