Researchers at Stockholm University used x-ray lasers to find evidence of a hidden liquid-liquid critical point in supercooled water at -81°F (63°C). This discovery explains why ice floats and how liquid water expands as it cools below 39°F (4°C).
Water’s behavior is unusual, unlike any other substance. It doesn’t shrink when cooled like most materials do; instead, its density increases and then decreases. Ice floats, and liquid water reaches its highest density at about 39°F.
The team used ultra-fast X-ray laser pulses to observe water’s structure before it crystallized into ice. They found that water can exist in two distinct liquid phases at low temperatures and high pressures. These phases merge when the conditions reach a certain point, creating a new critical state.
This discovery settles a long-standing scientific question and opens new avenues for research. The team believes that fluctuations between these two liquid states may be responsible for water’s unusual properties, such as its molecular dynamics slowing down near the critical point.
Source: https://interestingengineering.com/science/xray-laser-water-critical-point