We're not sure whether water is secretly two different liquids

The problem

Water breaks the rules other liquids follow: it expands when it freezes, is densest at 4 degrees Celsius, and gets stranger the colder it is pushed below freezing without crystallizing. In 1992 a computer simulation proposed a startling explanation: a low-density and a high-density form of liquid water that separate below a 'liquid–liquid critical point', predicted near 200 kelvin and around 1000 atmospheres. The problem is that this region is so prone to freezing that catching the two liquids experimentally is extraordinarily hard, so the hypothesis remains debated. Simulations and difficult experiments increasingly support it, but its existence and exact location in real water are not settled.

Why it matters

Water is the medium of all known life and of vast swaths of chemistry, geoscience, and climate, so knowing whether its anomalies stem from two competing liquid structures would refine everything from protein behavior to cloud and ice physics. It is a rare case where a single substance's basic phase diagram is still genuinely uncertain.

Progress so far

• 1992Poole, Sciortino, Essmann, and Stanley propose two liquid phases and a liquid–liquid critical point from simulations
• 2023experimental liquid–liquid phase separation observed via ultrafast heating of low-density amorphous ice
• 2024new work tightens the constraints on where the critical point sits, while its existence in real water remains actively debated

References

1. Nature Physics (2024) — constraints on the location of the liquid–liquid critical point in water
2. Nature Communications (2023) — experimental liquid–liquid phase separation via ultrafast heating of low-density amorphous ice
3. Wikipedia — background on water's anomalies