Water is crucial for life on Earth both directly and indirectly. It is a universal solvent and is one of the most studied liquids ever. And yet it appears to be hiding a secret. Water might not be just one liquid, it could be two.
Now, this might sound weird but it has been witnessed with other substances, such as silicon and germanium. These can be found in two density states depending on temperature and pressure. One is a low-density state and one is a high-density state. The point at which this occurs is called the second critical point, to distinguish from the critical point, which separates liquid states from gaseous states. The latter one for water is at a pressure of 218 atmospheres and a temperature of 374°C (705°F).
The existence of a second critical point for water is not a newly developed hypothesis. As reported in New Scientist in greater detail, scientists have been looking for this phenomenon for a few decades. In 1992, researchers at Boston University studied how the density of water changed as temperatures were lowered. They discovered fluctuations in the value. This was the opposite of what was expected. At lower temperatures, there should be fewer fluctuations, not more.
Computer simulations showed that the effects would continue as the temperature of liquid water dropped below its freezing point. It was a hint that something very peculiar was going on, but this was clearly going to be difficult to prove. It is possible to have pure liquid water below the freezing point. It’s called supercooled water and it has interesting properties. But it does freeze eventually. And the suggested region for water’s second critical point, at around -45°C (-49°F), seemed unreachable. Well, it did until recently.
Two independent groups have attempted to study the region. One group used an antifreeze molecule similar in structure to water, the other group used ultrapure droplets of water in a vacuum chamber. Although the setups were very different, both teams observed density variation at that fateful temperature.
While two different techniques showing similar results are encouraging, there might be multiple reasons to explain the density fluctuations and they might not be related to a second critical point. A lot more work will be necessary to prove it uncontroversially, one way or the other.
Two potential types of liquid water don’t only matter at unusual temperatures and pressures. They might help explain curious properties of water at room temperature and, according to some researchers, even matter when it comes to essential-to-life molecules such as proteins.