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**Ice I _{h}** (hexagonal ice crystal) (pronounced:

The crystal structure is characterized by the oxygen atoms forming hexagonal symmetry with near tetrahedral bonding angles. Ice I_{h} is stable down to −268 °C (5 K; −450 °F), as evidenced by x-ray diffraction^{[2]} and extremely high resolution thermal expansion measurements.^{[3]} Ice I_{h} is also stable under applied pressures of up to about 210 megapascals (2,100 atm) where it transitions into ice III or ice II.^{[4]}

The density of ice I_{h} is 0.917 g/cm^{3} which is less than that of liquid water. This is attributed to the presence of hydrogen bonds which causes atoms to become closer in the liquid phase.^{[5]} Because of this, ice I_{h} 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 do so only 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. The density of ice I_{h} increases when cooled, down to about −211 °C (62 K; −348 °F); below that temperature, the ice expands again (negative thermal expansion).^{[2]}^{[3]}

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 I_{h} is 1.31.

The accepted crystal structure of ordinary ice was first proposed by Linus Pauling in 1935. The structure of ice I_{h} is the wurtzite lattice, roughly one of crinkled planes composed of tessellating hexagonal rings, with an oxygen atom on each vertex, and the edges of the rings formed by hydrogen bonds. The planes alternate in an ABAB pattern, with B planes being reflections of the A planes along the same axes as the planes themselves.^{[6]} The distance between oxygen atoms along each bond is about 275 pm and is the same between any two bonded oxygen atoms in the lattice. The angle between bonds in the crystal lattice is very close to the tetrahedral angle of 109.5°, which is also quite close to the angle between hydrogen atoms in the water molecule (in the gas phase), which is 105°. This tetrahedral bonding angle of the water molecule essentially accounts for the unusually low density of the crystal lattice – it is beneficial for the lattice to be arranged with tetrahedral angles even though there is an energy penalty in the increased volume of the crystal lattice. As a result, the large hexagonal rings leave almost enough room for another water molecule to exist inside. This gives naturally occurring ice its rare property of being less dense than its liquid form. The tetrahedral-angled hydrogen-bonded hexagonal rings are also the mechanism that causes liquid water to be densest at 4 °C. Close to 0 °C, tiny hexagonal ice I_{h}-like lattices form in liquid water, with greater frequency closer to 0 °C. This effect decreases the density of the water, causing it to be densest at 4 °C when the structures form infrequently.

The hydrogen atoms in the crystal lattice lie very nearly along the hydrogen bonds, and in such a way that each water molecule is preserved. This means that each oxygen atom in the lattice has two hydrogens adjacent to it, at about 101 pm along the 275 pm length of the bond. The crystal lattice allows a substantial amount of disorder in the positions of the hydrogen atoms frozen into the structure as it cools to absolute zero. As a result, the crystal structure contains some residual entropy inherent to the lattice and determined by the number of possible configurations of hydrogen positions that can be formed while still maintaining the requirement for each oxygen atom to have only two hydrogens in closest proximity, and each H-bond joining two oxygen atoms having only one hydrogen atom.^{[7]} This residual entropy `S`_{0} is equal to 3.4±0.1 J mol^{−1} K^{−1} .^{[8]}

There are various ways of approximating this number from first principles. The following is the one used by Linus Pauling.^{[9]}^{[10]}

Suppose there are a given number N of water molecules in an ice lattice. To compute its residual entropy, we need to count the number of configurations that the lattice can assume. The oxygen atoms are fixed at the lattice points, but the hydrogen atoms are located on the lattice edges. The problem is to pick one end of each lattice edge for the hydrogen to bond to, in a way that still makes sure each oxygen atom is bond to two hydrogen atoms.

The oxygen atoms form a bipartite lattice: they can be divided into two sets, with all the neighbors of an oxygen atom from one set lying in the other set. Focus attention on the oxygen atoms in one set: there are `N`/2 of them. Each has four hydrogen bonds, with two hydrogens close to it and two far away. This means there are allowed configurations of hydrogens for this oxygen atom (see Binomial coefficient). Thus, there are 6^{N/2} configurations that satisfy these `N`/2 atoms. But now, consider the remaining `N`/2 oxygen atoms: in general they won't be satisfied (i.e., they will not have precisely two hydrogen atoms near them). For each of those, there are 2^{4} = 16 possible placements of the hydrogen atoms along their hydrogen bonds, of which 6 are allowed. So, naively, we would expect the total number of configurations to be

Using Boltzmann's principle, we conclude that

where k is the Boltzmann constant and R is the molar gas constant. So, the molar residual entropy is .

Another estimate first orients each water molecule randomly in each of the 6 possible configurations, then check that each lattice edge contains exactly one hydrogen atom. Assuming that the lattice edges are independent, then the probability that a single edge contains exactly one hydrogen atom is 1/2, and since there are 2N edges in total, we obtain a total configuration count , as before.

This estimate is 'naive', as it assumes the six out of 16 hydrogen configurations for oxygen atoms in the second set can be independently chosen, which is false. More complex methods can be employed to better approximate the exact number of possible configurations, and achieve results closer to measured values. Nagle (1966) used a series summation to obtain .^{[11]}

As an illustrative example of refinement, consider the following way to refine the second estimate method. According to the second estimation, six water molecules in a hexagonal ring would allow configurations. However, by explicit enumeration, there are actually 730 configurations. Now in the lattice, each oxygen atom participates in 12 hexagonal rings, so there are 2N rings in total, giving a refined result of .^{[12]}

By contrast, the structure of ice II is hydrogen-ordered, which helps to explain the entropy change of 3.22 J/mol when the crystal structure changes to that of ice I. Also, ice XI, an orthorhombic, hydrogen-ordered form of ice I_{h}, is considered the most stable form at low temperatures.

- Ice, for other crystalline forms of ice

**^**Norman Anderson. "The Many Phases of Ice" (PDF). Iowa State University. Archived from the original (PDF) on 7 October 2009.`{{cite journal}}`

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(help)- ^
^{a}^{b}Rottger, K.; Endriss, A.; Ihringer, J.; Doyle, S.; Kuhs, W. F. (1994). "Lattice Constants and Thermal Expansion of H_{2}O and D_{2}O Ice I_{h}Between 10 and 265 K".*Acta Crystallogr*.**B50**(6): 644–648. doi:10.1107/S0108768194004933. - ^
^{a}^{b}David T. W. Buckingham, J. J. Neumeier, S. H. Masunaga, and Yi-Kuo Yu (2018). "Thermal Expansion of Single-Crystal H_{2}O and D_{2}O Ice Ih".*Physical Review Letters*.**121**(18): 185505. Bibcode:2018PhRvL.121r5505B. doi:10.1103/PhysRevLett.121.185505. PMID 30444387.`{{cite journal}}`

: CS1 maint: multiple names: authors list (link) **^**P. W. Bridgman (1912). "Water, in the Liquid and Five Solid Forms, under Pressure".*Proceedings of the American Academy of Arts and Sciences*.**47**(13): 441–558. doi:10.2307/20022754. JSTOR 20022754.**^**Atkins, Peter; de Paula, Julio (2010).*Physical chemistry*(9th ed.). New York: W. H. Freeman and Co. p. 144. ISBN 978-1429218122.**^**Bjerrum, N (11 April 1952). "Structure and Properties of Ice".*Science*.**115**(2989): 385–390. Bibcode:1952Sci...115..385B. doi:10.1126/science.115.2989.385. PMID 17741864.**^**Bernal, J. D.; Fowler, R. H. (1 January 1933). "A Theory of Water and Ionic Solution, with Particular Reference to Hydrogen and Hydroxyl Ions".*The Journal of Chemical Physics*.**1**(8): 515. Bibcode:1933JChPh...1..515B. doi:10.1063/1.1749327.**^**Berg, Bernd A.; Muguruma, Chizuru; Okamoto, Yuko (2007-03-21). "Residual entropy of ordinary ice from multicanonical simulations".*Physical Review B*.**75**(9). doi:10.1103/PhysRevB.75.092202. ISSN 1098-0121.**^**Pauling, Linus (1 December 1935). "The Structure and Entropy of Ice and of Other Crystals with Some Randomness of Atomic Arrangement".*Journal of the American Chemical Society*.**57**(12): 2680–2684. doi:10.1021/ja01315a102.**^**Petrenko, Victor F.; Whitworth, Robert W. (2002-01-17). "2. Ice Ih".*Physics of Ice*. Oxford University Press. doi:10.1093/acprof:oso/9780198518945.003.0002. ISBN 978-0-19-851894-5.**^**Nagle, J. F. (1966-08-01). "Lattice Statistics of Hydrogen Bonded Crystals. I. The Residual Entropy of Ice".*Journal of Mathematical Physics*.**7**(8): 1484–1491. doi:10.1063/1.1705058. ISSN 0022-2488.**^**Hollins, G. T. (December 1964). "Configurational statistics and the dielectric constant of ice".*Proceedings of the Physical Society*.**84**(6): 1001. doi:10.1088/0370-1328/84/6/318. ISSN 0370-1328.