Ice
Ice is the solid form of water. The phase transition occurs when liquid water is cooled below 0 °C (273 K, 32 °F) at standard atmospheric pressure. Ice can be formed at higher temperatures in pressurized environments, and water will remain a liquid or gas until -30 °C at lower pressures. Ice formed at high pressure has a different crystal structure and density than ordinary ice.
Ice, water and water vapour can coexist at the triple point, which for this system is 273.16 K at a pressure of 611.73 Pa.
An unusual feature of ice frozen at a pressure of one atmosphere is that the solid is less dense than liquid water (10 % less). Ice has a density of 0.917 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 the 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, an important factor in Earth's climate.
As a crystalline solid, ice is considered a mineral.
Types of ice
Everyday ice and snow is Ice Ih, or hexagonal ice. 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 cubic structure ice (Ic). But cooling Ih causes a different arrangement to form in which the protons move, XI.
With both cooling and pressure more types exist, each being created depending 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. 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 Kurt Vonnegut's novel Cat's Cradle. Real Ice IX does not have the properties of Vonnegut's fictional ice-nine.
Rime is a type of ice formed by fog freezing on cold objects. It 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.
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 are also called "ice": dry ice, for instance, is a popular term for solid carbon dioxide.
Ice is a common metaphor for things that are cold, hard, and clear in some way; for instance, "ice" is a colloquial term for diamond, and also for some forms of the drug amphetamine.
Human relationship with ice
Ice has long been valued as a means of cooling. Icehouses were used to store ice during the winter so as to preserve perishables during the summer, and early refrigerators were known as iceboxes because they had a block of ice in them. The manufacture and use of ice cubes or crushed ice is common for drinks.
Ice also plays a role in winter recreation, namely the sports of ice skating, ice hockey, ice fishing, and ice climbing.
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 Vardø. Harbors that are not ice-free are opened up using icebreakers.
See also
