Ice

          Solid phase of water formed when water vapor or liquid water freezes. The phase transition occurs when liquid water is cooled below 32° F (0° C) at standard atmospheric pressure. Ice formed at higher pressures has different crystal structures, densities, and other distinct physical properties than ordinary ice. When water freezes, the resulting hexagonal crystalline arrangement is caused by the sharing of the bonding hydrogen atoms between adjacent water molecules in an orderly three-dimensional pattern, much like that of a honeycomb.

          Author’s Note: Because ice crystals are composed of hexagonal columns that have a number of open regions and pockets, normal ice is less dense than the more densely packed molecules of water. Those differences in density and structure have profound implications for the Earth because ice floats, water expands a little over eight percent when it freezes, ice acts as an insulating blanket on the land surface, ice reflects sunlight, etc. At this point would be a good time to start thinking about the crystalline structure of snowflakes and why they exhibit hexagonal symmetry.

          Real World Problems: Until relatively recently scientists thought they knew why ice was slippery and why people on skates are able to glide about in a carefree manner, that is, if they are sufficiently skilled to stay upright. But now scientists are in the middle of a rousing debate about just that subject. So, exactly what makes ice slippery? It seems that the old explanation no longer cuts the mustard, so to speak. The previously accepted explanation seems to be flat out incorrect: that the pressure exerted along the blade of an ice skate lowers the melting temperature of the top layer of ice. Consequently, the ice melts and the blade glides on a thin layer of water that refreezes as soon as the blade passes. According to chemist Robert M. Rosenberg, pressure-melting effect is far too small to facilitate melting and thus slipperiness. That “explanation” also fails to explain why someone wearing flat-soled shoes, which have much greater surface areas than skate blades, exert even less pressure on the ice but also can also easily slip on ice, as can be attested to by many millions of people who walk outside after an ice storm in winter and fall on their posteriors or some other, less padded, body part.

          Two alternative explanations have been proposed to take the pressure argument’s place. The first, more widely accepted at present, fingers frictional heating as the culprit. According to that view, the gliding of a skate blade or a shoe sole over ice, heats the ice and melts it, creating a thin, slippery layer of water on the ice surface. But, then, why is ice slippery even if you stand nearly perfectly still? The other argument, which emerged only a decade ago, is based on the idea that perhaps the surface of ice is in itself slippery. That idea holds that water molecules at the ice surface vibrate more rapidly (greater amplitude) because no molecules are above to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing. The debate is far from settled so keep your eye out for additional heated discussion.

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Ice I through Ice XIV          Twelve different crystalline ice structures (polymorphs) that form under various temperature-pressure conditions plus two amorphous states. The stable phase of ice at normal sea-level pressures is known as ice I. The two closely related variants are hexagonal ice (Ih), whose crystal symmetry is reflected in the shape of snowflakes and occurs when tiny water droplets are frozen, and cubic ice (Ic), which is formed when water vapor is deposited at very low temperatures (-130° C). Amorphous ice can be obtained when water vapor is deposited at even lower temperatures or by compressing ice Ih at liquid nitrogen temperature. In addition to the elemental phases are clathrates, crystalline compounds composed of a large H₂O cage in which Xe, Ar, or CH₄, are entrapped.

          Author’s Note: Without getting into an extended discussion of each of the different forms of ice, it is important to note that ice Ih turns into a different type of crystal at about 30,000 pounds of pressure per square inch, which of course is a condition not found on the Earth’s surface. That crystalline form, Ice II, has been theorized to exist inside icier bodies in the outer solar system, like the Jupiter moons Ganymede and Callisto.

          Fun Stuff: In addition to those very real ice variants, a fictional “ice-nine” was created by the American novelist Kurt Vonnegut and featured in his book Cat’s Cradle. All you non-science types who have read the book and were worried about the potential adverse effects of ice-nine should take a deep breath and relax because the real ice IX does not exhibit any of the properties of Vonnegut’s fictional form. The idea may have been conceived originally by 1932 Nobel Chemistry Prize Laureate Irving Langmuir to entertain H. G. Wells when he toured the General Electric facility where Langmuir worked. In any event, Wells wasn’t amused and the story was forgotten by all save Vonnegut, at one time a GE employee, who turned it to his own literary devices.