A domestic research team has experimentally confirmed, for the first time in the world, a clue to solving water's oldest mystery. They observed for the first time that water can switch between two distinct liquid states and that the point where the boundary between them disappears exists around minus 60 degrees Celsius.
The Ministry of Science and ICT said on the 27th that a research team led by Professor Kim Kyung-hwan at Pohang University of Science and Technology POSTECH, together with a team led by Professor Anders Nilsson at Stockholm University in Sweden, has observed water's "liquid–liquid critical point" for the first time in the world. The findings were published the same day in the international journal Science.
Water is the most common and important substance, but it is also considered the hardest to understand. Density is a prime example. Most liquids become denser as the temperature drops and their molecules pack more tightly, but water becomes most compact at 4 degrees Celsius and then, as it gets colder, turns looser instead. Rivers and lakes freeze from the surface down rather than from the bottom up in winter because of this property. Colder water at the surface is lighter and stays on top, and as the top freezes first, liquid water remains below.
To explain these unique properties of water, scientists have proposed a hypothesis for about 30 years. Water is not a single uniform liquid but can switch between a high-density liquid state, where molecules are packed more tightly, and a low-density liquid state, where they are arranged more loosely. According to this hypothesis, as water cools it first becomes denser like other liquids, but below a certain temperature the proportion of the low-density state begins to grow. Based on this, it becomes possible to explain why the overall density peaks around 4 degrees Celsius and then decreases again at lower temperatures.
The crux of testing this hypothesis was whether a liquid–liquid critical point actually exists. At very low temperatures and high pressures, the two liquid states are distinctly separated, but at a certain temperature and pressure, the boundary between them disappears and they appear as a single liquid—this point is called the liquid–liquid critical point.
Simulations and theoretical studies have suggested the possibility of a liquid–liquid critical point near minus 60 degrees Celsius under high pressure. However, because water freezes extremely quickly in the cryogenic regime, it has been difficult to confirm this directly through experiments. There had been no decisive experimental evidence to support or refute its existence.
Professor Kim's team has pursued this critical point for the past 10 years. In 2017, they measured, for the first time in the world, water that did not freeze even below minus 45 degrees Celsius, and in 2020 they extended the observation range to minus 70 degrees Celsius, showing that water can indeed have two liquid states. In this study, they went further and directly captured where the two liquid states merge into one.
The team created an extremely small amount of unfrozen water at minus 70 degrees Celsius and tracked molecular motion in units of one-tenth of a trillionth of a second. As a result, they confirmed that the point at which water's two liquid states begin to merge into a single phase—the liquid–liquid critical point—exists around minus 60 degrees Celsius (with an error of ±8 degrees). They provided experimental grounds for a hypothesis that had been proposed only in theory for decades.
Professor Kim said, "Through 10 years of research, we have added weight to the hypothesis that room-temperature water is in a state where the boundary between high-density water and low-density water has disappeared, and that water's unique properties emerge as the ratio of the two states changes with temperature," adding, "A more accurate understanding of water's characteristics can ultimately improve accuracy across industry and research."
The team plans to further narrow down the location of the proposed critical point through an experiment at the Pohang 4th-generation accelerator in May.
References
Science (2026), DOI: https://doi.org/10.1126/science.aec0018