Scientists Found a Second Critical Point Explaining Why Water Seems to Break All The Rules

Water is abundant but far from simple. A new discovery brings us closer to understanding its complex physics.

Water is the most mundane liquid on Earth, yet it almost breaks the rules of thermodynamics. Every other known liquid shrinks and becomes denser as it cools. Water does the opposite. The more you cool water after it freezes, the less dense it becomes, which is why ice floats so easily in ponds, lakes, and oceans.

Now, physicists think they know the exact reason why. Researchers at Stockholm University have provided direct experimental evidence of a long-theorized critical point inside supercooled water. Located at roughly -63 degrees Celsius and 1,000 times normal atmospheric pressure, this extreme threshold marks the spot where two distinct phases of liquid water collide and become one.

This is the second critical point of water known to physicists. The first sits at the opposite extreme. If you boil water to about 374 degrees Celsius and crush it under 218 times normal atmospheric pressure, the boundary between liquid and gas completely vanishes, creating a supercritical fluid.

Although the newly found critical point is reached at rather extreme conditions, it underlies the instabilities that generate quirks like water expanding the more you cool it down below 4 degrees Celsius.

Racing Against the Freezing Clock

To find this elusive phase, scientists had to subject water to extreme conditions of temperature and pressure. In this extreme, sub-zero environment, supercooled water normally freezes into solid ice almost instantaneously. Measuring the liquid before crystallization takes over requires instruments with extraordinary sensitivity.

“We have to do everything very quickly,” chemical physicist Anders Nilsson of Stockholm University told Science News.

The research team, led by Nilsson, traveled to the Pohang Accelerator Laboratory in South Korea. To bypass the freezing problem, they did not start with liquid water at all. Instead, they loaded tiny samples of amorphous ice — a special type of ice with a disorganized, jumbled molecular structure rather than a normal crystal lattice — into a vacuum chamber.

The researchers fired an intense, nanosecond blast from an infrared laser to rapidly heat and melt the amorphous ice. Microseconds later, before the newly formed liquid could refreeze, they probed the sample using an ultra-short X-ray laser to capture its density and physical structure.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

“What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges,” says Nilsson.

The Ripple Effect of a Watery ‘Black Hole’

The X-ray snapshots confirmed what physics had long suspected. At pressures below the critical point, the supercooled water violently snapped back and forth between two distinct liquids: a high-density phase and a low-density phase.

RelatedPosts

Water trapped in carbon nanotubes starts freezing at the temperature it should be boiling
Climate change will make droughts more intense, more frequent
Just by adding water to silicon nanoparticles, scientists almost instantly produced hydrogen
Saturn’s rings are raining down — in about 100 million years, they’ll be gone

But when the team dialed in the exact temperature and pressure of the critical point, that boundary vanished. The two liquids merged into one highly unstable state. Inside this zone, the water suffered a sort of thermodynamic identity crisis, exhibiting a chaotic range of both high and low densities, oscillating between states.

This extreme point acts like a structural anchor for the liquid’s behavior everywhere else. The massive instability it creates radiates outward, affecting water across a huge range of normal temperatures and pressures, even at ambient conditions inside your cup of coffee. This may explain why everyday water seemingly breaks so many conventions.

Another remarkable finding of the study is that the dynamics of the system slow down as it enters the critical point. The water became so bogged down by its own deep instability that its structural changes practically stalled.

“It looks almost that you cannot escape the critical point if you entered it, almost like a Black Hole”, says Robin Tyburski, researcher in Chemical Physics at Stockholm University.

A Critical Point for Physics

For the physicists who have spent their lives running computer simulations of water’s bizarre behavior, seeing concrete proof of this critical point marks the end of a grueling hunt.

“I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists and we also know there is no life without water. Is this a pure coincidence or is there some essential knowledge for us to gain in the future?“ says Fivos Perakis, an associate professor in Chemical Physics at Stockholm University.

“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 implications of these findings on waters importance in physical, chemical, biological, geological and climate related processes. A big challenge in the next few years,” Nilsson added.

The findings appeared in the journal Science.