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 Rob ert 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.
* *
*
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.
