As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered a mineral.[2]
An unusual property of ice frozen at atmospheric pressure is that the solid is approximately 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. The density of ice is 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[3] of ice as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules that is less compact in the solid. Density of ice increases slightly with decreasing temperature and has a value of 0.9340 g/cm³ at −180 °C (93 K).[4]
The result of these properties is that ice floats on liquid water, which is an important factor in Earth's climate. It has been argued that natural bodies of water would freeze from the bottom up without this property,[5] resulting in the annual loss of wild life and vegetation.
When ice melts, it absorbs as much heat energy (heat of fusion) as it would take to heat an equivalent mass of water by 80 °C, while its temperature remains constant at 0 °C.
It has been theoretically shown that ice may be superheated beyond its equilibrium melting point. Simulations of ultrafast laser pulses acting on ice suggest it may be heated to room temperature for an extremely short period (250 ps) without melting. [6]
Light reflecting from ice often appears 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 brown, gray or green.[7]
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.[8]
Formation
Rime is a type of ice formed on cold objects when drops of water crystallize 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.
Candle Ice is a form of Rotten Ice that develops in columns perpendicular to the surface of a lake.
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