The Origin Of Water’s Unusual Properties

Water behaves strangely in comparison to other compounds with the same molecular structure. It also possesses many unusual properties that defy what we currently know about chemistry and physics. If you look at how its density, specific heat, viscosity and compressibility respond to changes in pressure and temperature, what you will get is not what you’d expect on other liquids – it will be the complete opposite.

Most substances shrink when they are cooled resulting in an increase in the density. But if you freeze water, its density decreases and it expands. For example, if you look at a glass of ice water, you’d expect that water at zero degree Celsius being surrounded by ice would sink to the bottom of the glass, but instead the ice cubes float. Thanks to this unique property, or else the oceans, rivers, lakes would have frozen solid long, long time ago.

Water has the highest density at 4 degrees C. If you cool it further down below that, the expansion starts again. And, it even speeds when it gets colder. Its compressibility, heat capacity, viscosity and many other properties become increasingly strange as well. But now, researchers at Stockholm University – with the help of ultra-short x-ray pulses at x-ray lasers in Japan and South Korea – have established that at -44 degrees C, water ceases to exhibit its strange behavior.

“What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how it fluctuated between the two states,” explains Anders Nilsson, Professor of Chemical Physics at Stockholm University in a news release. “For decades there has been speculations and different theories to explain these remarkable properties and why they got stronger when water becomes colder. Now we have found such a maximum, which means that there should also be a critical point at higher pressures.”

Water can exist in two liquid states. Both states have different ways of bonding the water molecules together, and this is what makes it unique. Fluctuations occur between these two states, but they reach their peak at -44 degrees C. In other words, the water will stop behaving strangely when it’s cooled to -44 degrees C. So the answer to why water has many unusual properties boils down to its ability to shift from one liquid state into another and its ability to increase its fluctuation and strangeness upon cooling.

Interestingly, normal water, heavy water all exhibit different unusual properties, and even more so by the lighter one.

“The differences between the two isotopes, H2O and D2O, given here shows the importance of nuclear quantum effects,” says Kyung Hwan Kim, postdoc in Chemical Physics at Stockholm University. “The possibility to make new discoveries in a much studied topic such as water is totally fascinating and a great inspiration for my further studies,” says Alexander Späh, PhD student in Chemical Physics at Stockholm University.

“It was a dream come true to be able to measure water under such low temperature condition without freezing” says Harshad Pathak, postdoc in Chemical Physics at Stockholm University. “Many attempts over the world have been made to look for this maximum.”

“There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen,” Anders Nilsson explains. “Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime. The next stage is to find the location of the critical in terms of pressure and temperature. A big challenge in the next few years.”

The study, entitled “Maxima in the thermodynamic response and correlation functions of deeply supercooled water” has been published in the journal Science.

Source: Stockholm University