The ice in your freezer is just one of twenty-two kinds
Nearly all ice on Earth is a single hexagonal form, the one that floats in drinks. Squeeze water hard enough or chill it in odd ways and it locks into other crystal arrangements, with twenty-two crystalline phases observed so far. Out in space, the most common ice may have no crystal structure at all.
Ordinary ice, labelled Ih, was given its accepted structure by Linus Pauling in 1935. Oxygen atoms sit at the corners of crinkled hexagonal rings joined by hydrogen bonds, meeting at nearly the tetrahedral angle of 109.5 degrees. That geometry leaves the rings so roomy that another water molecule could almost fit inside, which is why ice is less dense than liquid water and floats. The same rings explain why liquid water is densest at 4 °C.
Water also breaks a general rule. Most liquids freeze at higher temperatures under pressure, but thanks to its hydrogen bonds, water under some pressures above one atmosphere freezes below 0 °C. Ordinary ice holds together down to about −268 °C and up to roughly 210 megapascals, where it gives way to ice II or III. Gustav Tammann first described ice II in 1900 during high-pressure experiments, and found both types could be kept at normal pressure if chilled with liquid air.
Some ice has no long-range order at all. Cooling liquid water to about −137 °C within milliseconds leaves no time for crystals to form, producing a glassy amorphous solid. Observations show this is the dominant form in space, so it is thought to be the most common ice in the universe. At the other extreme, a theorised superionic ice above 50 gigapascals would adopt a cubic structure, and at around 1.55 terapascals ice might even turn metallic.
Hydrogen atoms add a subtle twist. Each oxygen keeps two hydrogens nearby, but which of its bonds they sit on can vary, so disorder stays frozen in even near absolute zero. Pauling estimated this leftover entropy by counting arrangements. Ice, water and vapour coexist at 0.01 °C under a pressure of 611.657 pascals, a point that once defined the kelvin until May 2019.
Source: Phases of ice
