Water is one of the most unusual liquids in the Universe. Its anomalous properties (expansion when cooled below +4 °C, high heat capacity, unusual compressibility, and the fact that ice floats) have puzzled scientists for more than a century. Now, an international team of physicists has, for the first time, obtained direct experimental evidence for the existence of a special “hidden” critical state of water that appears to be responsible for these peculiarities.

The results of the study were published in the prestigious journal Science.

What Was Discovered
Scientists managed to “capture” water in a deeply supercooled state — at a temperature of about −63 °C and a pressure of around 1,000 atmospheres. Using ultrafast X-ray pulses, they observed the structure of the liquid before it had time to turn into ice.

Professor Anders Nilsson, one of the study’s leaders, explained that they were able to literally observe how the transition between two liquid phases disappears and a new critical state emerges.

According to the data obtained, under extreme conditions water can exist in two distinct liquid forms, differing in the structure of their hydrogen bonds. At the so-called hidden critical point, these two phases merge, and the system becomes extremely unstable and sensitive to the slightest changes.

An “Echo” of the Critical Point in Ordinary Water
Even at familiar temperatures and atmospheric pressure, water, according to the researchers, carries an “echo” of this hidden state. Constant microscopic fluctuations and transitions between two structural forms explain its unusually high heat capacity, compressibility, surface tension, and many other unique properties.

Why It Matters
The debate about the nature of water’s anomalies has been ongoing since the time of Wilhelm Conrad Röntgen, who discovered X-rays. The new experimental data may finally resolve this long-standing scientific dispute.

The significance of the discovery goes far beyond fundamental physics:

It helps improve understanding of climate processes and the water cycle in nature.
It influences the modeling of geological phenomena.
It has direct relevance to biology: the unique properties of water made the emergence and development of life on Earth possible.

In Brief
Scientists have, for the first time, experimentally confirmed the existence of a hidden critical point of water at about −63 °C and high pressure. In this state, water exists in two different liquid phases that merge at the critical point. The “echo” of this hidden state explains the anomalous properties of water under normal conditions. The discovery, published in Science, may be key to understanding why water has the unique qualities that made life on our planet possible.