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Title: Location of water's liquid-liquid critical point
Plain: 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.
Precise: Assuming the LLCP exists, determine $(T_{c}, P_{c})$ for H2O. Published model results and extrapolations from experiment span roughly $T_{c} = 170 \text{ to } 230\,\mathrm{K}$ and $P_{c}$ from about $0 \text{ to } 2e9\,\mathrm{dyn}/\mathrm{cm}^{2}$ ($0 \text{ to } 2\,\mathrm{kbar}$), all approximate. An answer is a measured or reliably computed $(T_{c}, P_{c})$ with error bars, consistent with thermodynamic data at accessible conditions.
Settled by: Experimental location of the LLCP, or first-principles simulations including nuclear quantum effects whose predictions match measured response functions of supercooled water.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: A 2026 ultrafast X-ray experiment placed the critical temperature near $210 \pm 8\,\mathrm{K}$ (You et al., Science 2026); the critical pressure and an independent confirmation remain open.
Posed since: (empty)