Structure clarified properties of frozen water, from snowflakes to sea ice
In the late 1920s, the structure of water in its crystalline form—ice—was an area of much debate. “Data on the subject in the literature are very conflicting,” wrote William H. Barnes at the time, noting that textbooks and other resources featured varying structures.
A Canadian scientist who used a fellowship to visit London’s Royal Institution to work with crystallography pioneer William H. Bragg, Barnes wound up publishing the definitive structure of ice formed under natural environmental conditions. He showed conclusively that the oxygens are arranged in a planar hexagon, with the hydrogens located between the oxygens (Proc. R. Soc. Lond. A 1929, DOI: 10.1098/rspa.1929.0195). The hexagons assemble into parallel sheets, giving ice a structure somewhat akin to graphite.
Barnes’s “technique and skill were just amazing,” says Bruce M. Foxman, a chemistry professor at Brandeis University. “It’s just really some of the most beautiful experimental work.”
Barnes didn’t work at just one temperature—he characterized ice from 0 to –183 °C, using a liquid-air cryostat. Setting up the experiment would have been challenging, Foxman says, let alone analyzing the diffraction pattern. “Let’s not forget that there weren’t any calculators or computers; this was all looking at intensities and matching strong for strong, weak for weak,” Foxman emphasizes. The work also predated understanding of hydrogen bonding.
We now know that the structure determined by Barnes drives many of the properties of frozen water, from snowflakes to sea ice.
To form a snowflake, you start with a “little round ball of ice,” says Kenneth G. Libbrecht, a professor of physics at California Institute of Technology. Water molecules add preferentially to the edges of ice’s hexagonal sheets. As the sheets get larger, the corners stick out further, so they encounter more water molecules and grow ever faster. “Pretty soon you have a hexagon with branches and side branches,” and the stunning beauty of a snowflake, Libbrecht says.
Sea ice grows similarly, with the planes poking down into the water, says Pat Langhorne, a professor of physics at New Zealand’s University of Otago. The structure of ice also explains its unique lower density as a solid than a liquid. Ice’s ability to float on liquid water in turn gives it a key role in moderating Earth’s climate by reflecting sunlight.
“This material is not ordinary in any way,” Langhorne says. “The properties that it has are critical to life.”—Jyllian Kemsley