Liquid-liquid transition and anomalies of supercooled water

5 problems

• Are water's compressibility maxima at 1 bar a Widom line?

  Contested

  Measurements on tiny falling droplets of supercooled water found peaks in compressibility near 229 K at ordinary pressure. Whether these peaks are a sign of a critical point at higher pressure or have another cause is not settled.

• Does real water have a liquid-liquid critical point?

  Partially resolved

  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.

• Location of water's liquid-liquid critical point

  Open

  If water's second critical point exists, its temperature and pressure must be pinned down. Model estimates disagree by tens of kelvin and by thousands of atmospheres.

• Which liquids have liquid-liquid transitions, and why?

  Open

  Besides water, a few substances such as phosphorus and sulfur show a jump between two liquid forms, while for others the claim is disputed. There is no rule that says which liquids should have such a transition.

• Is the 136 K calorimetric step the glass transition of water?

  Contested

  Glassy water made by very fast cooling shows a small heat-capacity step near 136 K, long taken as its glass transition, and a second step near 116 K was reported in high-density amorphous ice. Whether these steps mark glasses turning into ultraviscous liquids or something else is disputed.