Water on the dancefloor

I don't usually get too scientific on you, but I thought this was interesting. Did you know that scientists are still making discoveries about the molecular structure of plain old liquid water?

"One can visualize this as a crowded dance restaurant, with some people sitting at large tables, taking up quite a bit of room—like the tetrahedral component in water—and other people on the dance floor, standing close together and moving slower or faster depending on the mood or 'temperature' of the restaurant—like the molecules in the disordered regions can be excited by heat, the dancers can be excited and move faster with the music," Nilsson said. "There's an exchange when people sitting decide to get up to dance and other dancers sit down to rest. When the dance floor really gets busy, tables can also be moved out of the way to allow for more dancers, and when things cool back off, more tables can be brought in."

This more detailed understanding of the molecular structure and dynamics of liquid water at ambient temperatures mirrors theoretical work on "supercooled" water: an unusual state in which water has not turned into ice even though it is far below the freezing point. In this state, theorists postulate, the liquid is made up of a continuously fluctuating mix of tetrahedral and more disordered structures, with the ratio of the two depending on temperature—just as Nilsson and his colleagues have found to be the case with water at the ambient temperatures important for life.

"Previously, hardly anyone thought that such fluctuations leading to distinct local structures existed at ambient temperatures," Nilsson said. "But that's precisely what we found."

This new work explains, in part, the liquid's strange properties. Water's density maximum at 4 degrees Celsius can be explained by the fact that the tetrahedral structures are of lower density, which does not vary significantly with temperature, while the more disordered regions—which are of higher density—become more disordered and so less dense with increasing temperature. Likewise, as water heats, the percentage of molecules in the more disordered state increases, allowing this excitable structure to absorb significant amounts of heat, which leads to water's high heat capacity. Water's tendency to form strong hydrogen bonds explains the high surface tension that insects take advantage of when walking across water.