17/07/2018

The Science & Mathematics blog presents the phenomena of ice.

Reproduction from the IMPA Science & Mathematics blog, published in O Globo, and coordinated by Claudio Landim.

Jefferson C. Simões , professor at the Institute of Geosciences of UFRGS

Marcia C. Barbosa , professor at the Physics Institute of UFRGS

When we admire the beauty of the fifty shades of blue of icebergs floating in the Patagonia region, we don't realize how bizarre ice can be. But why are there shades of blue when surfaces covered in snow are white? The crystals of frozen water (ice) without contaminants that we can observe on parts of the icebergs' surface absorb waves in the red range and emit in the blue range. Snowflakes, on the other hand, are an aggregate of ice crystals and a lot of air, a more disorganized structure, reflecting and emitting energy at all wavelengths, and thus we observe the sum that is white. In addition to the shades of blue of pure frozen water, there are icebergs with parts of other hues. Importantly, icebergs are not formed by the freezing of seawater. They were part of and broke off from glaciers, which are formed by the accumulation of snow crystals over thousands of years. As the years pass, air is expelled, ice crystals grow, and snow forms the ice of the glaciers. Sometimes, mineral impurities or biological material from the sea can freeze at the bottom of the iceberg, resulting in layers of ice with other colors (green, black).

But the iceberg isn't just about reflection. It exhibits a myriad of other bizarre phenomena. The simple fact that an iceberg floats on seawater is already unusual. For most materials, lowering the temperature in the liquid phase causes the system to transition to the solid phase, becoming more ordered and compact. After all, when we organize something, we can fit more into less space. This compaction makes the material denser, and denser materials sink in less dense materials. An iron bar sinks in liquid iron. But water contradicts this intuition. Frozen water arranges itself in a more open and less dense structure than liquid water. The origin of this anomaly in water lies in the fact that the water molecule, H2O, forms a V-shaped structure with oxygen at the center. Since oxygen has more protons, it attracts electrons more than hydrogen, making the region near the O negative and the region near the H positive. This charge distribution of a molecule interacts with its neighbors, forming hydrogen bonds. A typical ordered structure would be water in the center of a cube and neighboring bodies of water at the vertices of the same cube. When we heat the system, these bonds break and more molecules can enter the cube, increasing the density, allowing the less dense ice to float on denser water. But contrary to what is sometimes taught in high school physics classes, the submerged part of an iceberg is not nine times the size of what we can see above the sea surface. The iceberg has a lot of snow and, in addition, is formed of fresh water (which is less dense than the seawater in which the iceberg is floating). Conclusion: we see about one-seventh of the iceberg's size. That is, if it is about 50 m high, there are another 300 m hidden underwater.

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