It may sound like a Zen koan, but it's a serious scientific question: How many molecules of water does it take to make the smallest possible ice crystal?
Because crystals are defined by a repeated, three-dimensional arrangement of molecules, you can't necessarily take any small group of bonded-together
molecules and call them a crystal. That's especially true for water: When it freezes, the weak hydrogen bonds that loosely bind the water molecules
together pull the disordered clusters of molecules (left) into a more open—but also more rigid—cagelike arrangement (cross section of cluster at
right). This roomy lattice is also why ice is less dense than water (and therefore floats). So to calculate the minimum number of molecules needed to make
an ice lattice, a team of researchers shone infrared lasers on clusters of water molecules containing between 80 and 500 molecules. The team paid
particular attention to how much energy the clusters absorbed from the lasers between the wavelengths of 2.63 micrometers and 3.57 micrometers—the range in which
the oxygen-hydrogen bonds in water continually stretch and shrink. A particular peak of energy absorption occurred at a wavelength of about 3.125
micrometers—denoting the spectral characteristic of ice—and only appeared for clusters containing more than 275 water molecules, the
researchers report online today in Science. That number of molecules yields a tiny ice cluster between 1 nanometer and 3 nanometers across—the ultimate
in crushed ice.
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