BIO In the literature: partially resolved

Does real water have a liquid-liquid critical point

In plain words

The proposal is that deeply cooled water can exist as two liquids of different density, separated by a phase transition that ends at a critical point. Computer models of water show this, and a 2026 experiment reported seeing it in real water, but an independent confirmation is still missing.

Precise statement

Determine whether H2O (and D2O) has a first-order transition between a high-density liquid (HDL) and a low-density liquid (LDL) ending at a liquid-liquid critical point (LLCP), versus the singularity-free scenario, a critical point at negative pressure, or the stability-limit conjecture. The LLCP is established in several classical and machine-learned first-principles water models, and ultrafast heating of amorphous ice with X-ray scattering has reported a discontinuous-to-continuous crossover with critical fluctuations. An answer is an independent confirmation, by a different technique or group, of critical scaling (diverging correlation length with 3D Ising exponents) in metastable bulk water with crystallization excluded.

What would settle it

Independent measurement of critical scaling or of a density discontinuity in bulk metastable water inside the region where ice forms rapidly, with crystallization excluded, consistent with the 2026 report.

Status in the literature

Unverified note

A 2026 ultrafast X-ray experiment (You et al., Science 2026, https://doi.org/10.1126/science.aec0018) reported a discontinuous-to-continuous liquid-liquid transition with critical divergence near $210 \pm 8\,\mathrm{K}$, following a 2020 report of the transition under pressure; independent confirmation and an equilibrium measurement in bulk water are still lacking.