We all know water, right? It's two hydrogen atoms and an oxygen atom bonded together. We need it to live, so we try and conserve it and keep it clean. We also bottle it, flavor it and debate whether sparkling or mineral water is better.
But that's all on the surface, really. It turns out that even our knowledge of that well-known water molecule can be tricky, and we're not just talking about when changes between a liquid state and either a gas or solid state. No, it would appear that water can go from liquid to another liquid under the right circumstances.
Slippery little devil.
That substances change to different states isn't new. As New Scientist explains, "... all substances have a high-temperature critical point where their gas and liquid phases converge, but a handful of materials display a mysterious second critical point at low temperatures."
This low temperature point is found in substances like liquid silicon and germanium. When cooled to the right temperatures, both of these substances will turn into different liquids of different densities. Their respective atomic compositions remain the same, but those atoms shift into different configurations, and that results in new properties.
Reports of something like this happening to water caught the attention of two Boston University researchers, Peter Poole and Gene Stanley, in 1992. Apparently, water's density would begin to fluctuate more at lower temperatures, an odd thing since a substance's density should fluctuate less as it gets colder.
Poole and Stanley's team tested this idea by simulating water cooling past its freezing point while still remaining a liquid, a process called supercooling. These computer simulations confirmed that the density fluctuations were occurring, with each a phase in its own right, according to New Scientist. This claim, however, was a controversial one, with the common explanation for this weird supercooled state being a disordered solid state that lacked the crystalline features of ice.
Proving this with actual water would be difficult, too. This critical point of oddity was minus 49 degrees Fahrenheit (minus 45 Celsius), and even supercooled water could spontaneously turn into ice at that point.
"The challenge is to cool water very, very, very fast," Stanley told New Scientist. "Studying it needs clever experimentalists."
One of those clever experimentalists is Anders Nilsson, a professor in Chemical Physics at Stockholm University in Sweden. Nilsson and a team of researchers published two different studies about water's potential critical point in 2017, both arguing that water can exist as two different liquids.
The first study, published in June 2017 in Proceedings of the National Academy of Science (US), confirmed the Poole and Stanley simulations of water shifting through high and low densities. To determine this, the researchers used X-rays at two different locations to follow the movements of and distances between H2O molecules as they shifted between states, including from a viscous liquid to an even more viscous liquid with a lower density. This study didn't determine the point in which a liquid-to-liquid transition took place, however.
The second study was published in Science in December of that year, and it pinpointed a potential temperature of this phase oddity. Since water has a habit of building ice crystals around any impurities, researchers dropped ultra-pure droplets of water into a vacuum chamber and cooled them down to minus 44 Celsius, the temperature they began to notice peak changes in the liquid's density. They again used X-rays to follow the shifts in water's behavior.
Critics of the latter study who spoke to New Scientist, while impressed by the technical feats Nilsson's team achieved, were skeptical of the results all the same, chalking it up to water's weird behavior below freezing points, or that another critical point is somewhere near that temperature.
Tougher to freeze
A study published in Science in March 2018, conducted by a different team of researchers, seems to back up the research performed by Nilsson's teams, albeit via a different method.
These researchers monitored the heat in a solution of water and a special chemical called hydrazinium trifluoroacetate. This chemical essentially acted as antifreeze and would prevent the water from crystallizing into ice. In this experiment, the researchers adjusted the temperature of water until they noticed a sharp change in the amount of heat the water absorbed, around minus 118 F (minus 83 C). Since it couldn't freeze, the water was swapping densities, low to high and back again.
A scientist not involved in the study, Federica Coppari at Lawrence Livermore National Laboratory in California, told Gizmodo that the experiment provides "a compelling argument for the existence of liquid-liquid transition in pure water" but that it's only "indirect evidence" and that more work is needed with other experiments.
Drops of life
At this point in the scientific discourse, the reason for understanding water's weird properties may not be entirely clear or applicable immediately, but there are good reasons for getting to the bottom of it.
For instance, water's wild fluctuations could be essential for our very existence. Its ability to shift between liquid phases could've spurred life to develop on Earth, Poole told New Scientist, and research is currently being conducted to understand how proteins in water react in a range of different temperatures and pressures.
Futurism explained another, more practical reason to understand water's weirdness, following the publication of Nilsson's June 2017 study. "[U]nderstanding of how water behaves at different temperatures and pressures can help researchers to develop better purification and desalination processes."
So whether it's unlocking the secrets of life or creating better drinking water, understanding water can make a big difference.