Water shows a hidden liquid behavior that helps explain why ice floats

Researchers at Stockholm University used ultra-fast X-ray lasers to observe deeply supercooled water and reported evidence of a critical point where two distinct liquid phases merge; the results were published in Science.

Researchers report a new observation of water that may help explain why ice floats. A team at Stockholm University used ultra-fast X-ray laser pulses to probe water when it was deeply supercooled. By capturing the liquid structure on very short timescales before it crystallized, the researchers say they observed behavior consistent with a critical point linking two distinct liquid forms. Water is densest near 4°C, and below that temperature it expands, which is why solid ice is less dense and floats. Key findings: - The team used ultra-fast X-ray laser pulses to measure water structure at timescales short enough to see the liquid before it froze. - The researchers report evidence for a liquid-liquid critical point where two different molecular bonding arrangements become indistinguishable. - Molecular dynamics were reported to slow markedly near the critical point, producing strong microscopic fluctuations between the two liquid states. - The work was published in Science and is presented as a potential explanation for several of water's anomalous properties, including its density maximum near 4°C. Summary: The study provides experimental support for a decades-old hypothesis that water has a critical point in the deeply supercooled regime. Researchers say these fluctuations at the critical point could underlie some of water's unusual thermal and structural behaviors. The team has stated that the next stage is to study the implications of the finding for physical, chemical, biological, geological and climate-related processes.