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A 2026 Science paper reports experimental evidence for a long-proposed liquid–liquid critical point in supercooled water. Researchers rapidly heated two forms of amorphous ice with infrared laser pulses, then used X-ray scattering to probe the brief period before the water crystallized. The “two types” are proposed low-density and high-density liquid states—not different kinds of water you can find at room temperature.
What the researchers found
In “Experimental evidence of a liquid-liquid critical point in supercooled water,” Seonju You and colleagues report evidence supporting a liquid–liquid critical point, where the proposed low-density liquid (LDL) and high-density liquid (HDL) states meet. The paper appeared in Science on March 26, 2026. Read the paper’s indexed record.
The result addresses a difficult question: whether water can exist as two distinct liquid states under deeply supercooled conditions. It does not establish two stable liquids at everyday temperatures, nor does it mean water is made of two chemically different kinds of H₂O.
How the experiment worked
Liquid water in the relevant supercooled region tends to crystallize quickly, making it hard to observe before it turns to ice. The team used amorphous ice—ice without the regular crystal structure of ordinary ice—as a starting material. Ultrafast infrared laser pulses rapidly heated samples of high- and low-density amorphous ice. X-ray scattering then provided structural information about the short-lived liquid before ice crystals nucleated and grew. The paper’s abstract and publication record are available from Science.
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Chemistry World describes the measurement window as a few microseconds and reports the result as the team’s claim of evidence for the transition. That short interval matters: the experiment probes a metastable liquid that exists only briefly before crystallization, not a sample that can be kept as two separate liquids under ordinary conditions. Chemistry World’s report quotes Pacific Northwest National Laboratory physicist Greg Kimmel calling the results “the most persuasive evidence to date” for a liquid–liquid critical point. That is an expert assessment of the evidence, not a claim that every question about water’s behavior is settled.
What “two types of water” means
LDL and HDL refer to proposed liquid states with different densities and molecular arrangements. They are both water: the distinction is how neighboring molecules are organized under particular conditions, not a change in chemical composition.
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Low-density liquid
The low-density picture is associated with a more open, tetrahedral arrangement. In a simplified description, a water molecule is surrounded by four hydrogen-bonded neighbors.
High-density liquid
The high-density picture involves more distorted arrangements, often described as having three or five hydrogen-bonded neighbors. These arrangements can pack more closely than the tetrahedral structure.
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The distinction is a useful simplification of molecular structure and density, not a consumer-facing classification. The hypothesis proposes that the balance and fluctuations between such local arrangements matter under extreme conditions; it does not identify two varieties in a glass of tap water.
Why the finding may help explain water’s odd properties
Water behaves unusually in several familiar ways, and researchers have proposed that changes in its local molecular structure help account for those anomalies. In the critical-point model, competition between LDL-like and HDL-like arrangements—and fluctuations in their balance—offers one possible explanation. Anders Nilsson is quoted by Chemistry World as saying, “The LLCP is important because it is the source of the water anomalies.” This is the model’s explanatory interpretation, not proof that the 2026 experiment directly explains every unusual property of ordinary water.
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How this fits the earlier hypothesis
The idea of two liquid states in water predates the 2026 paper. A 2020 review discussed the two-state model, and earlier experimental work had reported observations of a liquid–liquid transition in bulk supercooled water under pressure. The newer paper adds experimental evidence using amorphous ice, rapid laser heating and X-ray scattering; it is not the first proposal that water might have two liquid states.
A 2020 account from Sapienza University of Rome describes modeling in which water is metastable relative to ice below about 180 kelvin (about −90°C), and the modeled liquid density begins to fluctuate between low- and high-density values. That temperature belongs to the 2020 modeling summary; it should not be mistaken for a temperature measured in the 2026 experiment. Sapienza’s account of the modeling study explains the earlier theoretical context.
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