Ice
Ice is the name given to any one of the 15 known crystalline solid phases of water. In non-scientific contexts, it usually describes ice Ih, 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 Ih occurs when liquid water is cooled below 0 C (273.15 K, 32 F) at standard atmospheric pressure.  It can also deposit from a vapor 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 is, in turn derived from Proto-Germanic *isaz.

# Characteristics
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 8% less dense than liquid water. Water 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 most dense, 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. 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 it. 
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 widely 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.  
This phenomenon does not seem to hold true at all temperatures. The extreme conditions found, especially, in Antarctica have been observed to make ice and snow not slippery. Explorers report that at very low temperatures snow loses its "glide", and pulling a sledge across it becomes like pulling a sledge through sand.

# Types of ice
Everyday ice and snow has a hexagonal crystal structure (ice Ih). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than Ih is the cubic structure (Ic).
With both cooling and pressure more types exist, the formation conditions for each being represented on the phase diagram of ice. These are II, III, V, VI, VII, VIII, IX, and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen disordered, these are IV and XII. Ice XII was discovered in 1996. In 2006,  XIII and XIV were discovered. Ices XI, XIII, and XIV are hydrogen-ordered forms of ices Ih, V, and XII respectively.
As well as crystalline forms, solid water can exist in amorphous states as amorphous solid water (ASW), low density amorphous ice (LDA), high density amorphous ice (HDA), very high density amorphous ice (VHDA) and hyperquenched glassy water (HGW).
Rime is a type of ice formed on cold objects when drops of water crystalize on them. This can be observed in foggy weather, when the temperature drops during night. Soft rime contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice. Hard rime is comparatively denser.
Aufeis is layered ice that forms in arctic and subarctic stream valleys.  Ice frozen in the stream bed blocks normal groundwater discharge and causes the local water table to rise, resulting in water discharge on top of the frozen layer.  This water then freezes, causing the water table to rise further and repeat the cycle. The result is a stratified ice deposit, often several meters thick.
Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.
Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice. Pancake ice is a formation of ice generally created in areas with less calm conditions.
Some other substances (particularly solid forms of those usually found as fluids) are also called "ice": dry ice, for instance, is a popular term for solid carbon dioxide.
In outer space hexagonal crystalline ice, the predominant form on Earth, is extremely rare.  Amorphous ice is more common; however, hexagonal crystalline ice can be formed via volcanic action.

# Uses of ice

## Ice harvesting
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 year-round, 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 sarooj, 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 that could easily bring temperatures inside the space down to frigid levels in summer days. The ice was then used to chill treats for royalty during hot summer days.

## Sports on ice
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 boating. 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.

## Ice and transportation
Ice can also be an obstacle; for harbors near the poles, being ice-free is an important advantage, ideally all-year round. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vard (Norway). Harbors that are not 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 that 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 labor-intensive 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 combat 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 centimeters (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 under way. Icebergs have been responsible for the sinking of many ships, a notable example 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 carburettor. As air is sucked through the carburettor into the engine the local air pressure is lowered, which causes adiabatic cooling. So, in humid close-to-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. Aircraft reciprocating engines with carburettors are provided with carburettor air intake heaters for this reason. The increasing use of fuel injectionwhich does not require carburettorshas 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 that produce this type of icing, and find the best methods to prevent or reverse it in flight.

## Other uses of ice
- 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 (6.7056 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.
- Pagophagia, a type of pica eating disorder, is the compulsive consumption of ice.
- 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 program 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 that will focus sunlight onto kindling. When one waits long enough, a fire will start.
- In global warming, ice plays an important part because it reflects 90% of the sun's rays. Furthermore, ice cores help provide historical climate information.
- In January and February 1658, the straits between the islands of Denmark, Great Belt and Little Belt froze over, allowing a Swedish army to March across the Belts and defeat the Danish army. The resulting Treaty of Roskilde ceded large areas of Denmark to Sweden.

# Ice 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 why glaciers move.
Ice formed at high pressure has a different crystal structure and density than ordinary ice.  Ice, water, and water vapor can coexist at the triple point, which is 273.16 K at a pressure of 611.73 Pa.

# Phases of ice