thumb|235px|A fountain in Boise, Idaho, February 2007 thumb|235px|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 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.
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thumb|235px|A fountain in Boise, Idaho, February 2007 thumb|235px|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 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 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
thumb|left|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.
