X-ray laser experiment unlocks water's hidden critical state at -81°F

Researchers at Stockholm University have discovered evidence of a hidden liquid-liquid critical point in supercooled water at -81°F (−63°C) and about 1,000 atmospheres of pressure, using ultra-fast X-ray laser pulses. This critical point explains long-standing mysteries about water’s unusual behavior, including why ice floats and why liquid water expands as it cools below 39°F (4°C). The experiments captured water’s structure before it crystallized into ice, revealing that water can exist in two distinct liquid phases differing in molecular organization and bonding. At the critical point, these two phases merge, causing fluctuations that influence water’s unique physical properties even at ambient temperatures. The study clarifies why water’s density, heat capacity, compressibility, and viscosity respond atypically to temperature and pressure changes compared to most substances. Unlike typical materials that become denser as they cool, water reaches maximum density at 39°F and then expands upon further cooling. The researchers found that molecular dynamics slow dramatically near