A 2026 study reports experimental evidence for a long-proposed liquid–liquid critical point in supercooled water. The two states—low-density liquid and high-density liquid—are not two kinds of water you can pour from a bottle. They are proposed forms of liquid water under extreme, deeply supercooled conditions, where water’s molecular arrangements and density behave differently.
What did the scientists find?
In a paper published in Science on March 26, 2026, Seonju You and colleagues reported experimental evidence for a liquid–liquid critical point (LLCP) in supercooled water. The paper, “Experimental evidence of a liquid-liquid critical point in supercooled water,” appeared in volume 391, issue 6792, pages 1387–1391 (PubMed record; DOI: 10.1126/science.aec0018).
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The result supports a decades-old proposal that water can have two distinct liquid states at very low temperatures: low-density liquid (LDL) and high-density liquid (HDL). It does not mean that researchers have discovered chemically different H₂O molecules or two stable liquids under everyday conditions.
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How did the experiment work?
Water crystallizes rapidly in the deeply supercooled region where the proposed transition is expected, making it difficult to study as a liquid. The team used amorphous ice—ice without the orderly crystal structure of ordinary ice—as a starting material. They rapidly heated high- and low-density forms of amorphous ice with ultrafast infrared laser pulses, then used X-ray scattering to examine the short-lived liquid before ice crystals formed.
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Chemistry World’s March 27, 2026 report describes the measurement window as a few microseconds before crystals nucleated and grew. The laser created a fleeting opportunity to study the liquid; the scattering provided evidence about its structure. The experiment therefore probes water in a metastable, extreme regime, not two liquids held stably side by side at room temperature.
What are low-density and high-density liquid water?
The labels refer to proposed liquid states with different densities and molecular arrangements. In the picture outlined by a Sapienza University of Rome account of 2020 modeling, LDL has a more tetrahedral arrangement: a water molecule is surrounded by four hydrogen-bonded neighbors. HDL is more distorted, with local arrangements often involving three or five hydrogen bonds.
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At very low temperatures, the competition between these structures may produce liquid states with different densities. The two labels simplify a complex picture of molecular arrangements; they do not describe different chemical substances.
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Why would two liquid states help explain water’s odd properties?
Water has unusual properties compared with many liquids. The proposed explanation is that changes in the balance of its local structures—and fluctuations between them—may contribute to those anomalies. If the two-state model has a liquid–liquid critical point, fluctuations near that point could help explain why water behaves unusually.
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That connection is an interpretation of the model, not a claim that the 2026 experiment resolved every question about water’s anomalies. Physicist Anders Nilsson told Chemistry World, “The LLCP is important because it is the source of the water anomalies.” Greg Kimmel, a physicist at Pacific Northwest National Laboratory, called the new results “the most persuasive evidence to date of a liquid–liquid critical point [LLCP] in water.” Those are expert assessments of the evidence and its significance, rather than proof that all questions are settled.
What is new about the 2026 result?
The idea that water may have two liquid states is not new. The two-state model has a long history, and earlier work reported experimental observations of a liquid–liquid transition in bulk supercooled water under pressure. A 2020 modeling study also described evidence for a critical point.
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The contribution reported in 2026 is further experimental evidence for a liquid–liquid critical point, using rapid heating of high- and low-density amorphous ice and X-ray scattering. The Sapienza account of the 2020 model says that below about 180 kelvin (about −90°C), water is metastable with respect to ice and the modeled liquid density begins to fluctuate between low- and high-density values. That temperature belongs to the 2020 modeling summary; it is not a temperature measured in the 2026 experiment.
Does this mean ordinary water is two different liquids?
No. The claim concerns liquid states under deeply supercooled conditions, not two everyday varieties of water. Under ordinary conditions, the proposed relevance is that shifts and fluctuations in local molecular structures may help account for water’s familiar anomalies. The reported experiment does not show separate stable low- and high-density liquids at room temperature.
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