Ice Ih
Ice Ih (pronounced: ice one h, also known as ice-phase-one) is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic that is occasionally present in the upper atmosphere. Ice Ih exhibits many peculiar properties that are relevant to the existence of life and regulation of global climate.
The crystal structure is characterized by the oxygen atoms forming hexagonal symmetry with near tetrahedral bonding angles. Ice Ih is stable down to −201 °C (72 K; −330 °F) where it should transition into ice XI, however, it can still exist in metastable form down to near −271 °C (2 K; −456 °F). Ice Ih is also stable under applied pressures of up to about 210 megapascals (2,100 atm) where it transitions into ice III or Ice II.
The density of ice Ih is 0.917 g/cm3 which is less than that of liquid water. This is attributed to the presence of hydrogen bonds which causes atoms to become more distant in the solid phase . Ice floats on water, which is highly unusual when compared to other materials.The solid phase of materials is usually more closely and neatly packed and has a higher density than the liquid phase. When lakes freeze, they only do so at the surface while the bottom of the lake remains near 4 °C (277 K; 39 °F) because water is densest at this temperature. No matter how cold the surface becomes, there is always a layer at the bottom of the lake that is 4 °C (277 K; 39 °F). This anomalous behavior of water and ice is what allows fish to survive harsh winters. Upon further cooling of temperature the density of ice Ih continues to decrease down to about −214 °C (59 K; −353 °F) where it begins to expand.
The latent heat of melting is 5987 J/mol, and its latent heat of sublimation is 50911 J/mol. The high latent heat of sublimation is principally indicative of the strength of the hydrogen bonds in the crystal lattice. The latent heat of melting is much smaller, partly because liquid water near 0 °C also contains a significant number of hydrogen bonds. The refractive index of ice Ih is 1.31.
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