Is water secretly a dual liquid switching between two states?

Nature · Published 22 October 2026 Swipe more like this →

In short: Water behaves unlike almost any other substance on Earth, expanding as it freezes and defying standard liquid physics. For decades, scientists suspected that this strange behavior stems from a hidden reality: that water may actually exist as two competing liquids made from the exact same molecule.

According to the 'two-state hypothesis', water consists of a blend of two separate liquid types—low-density liquid (LDL) and high-density liquid (HDL)—instead of being one homogenous material. Hydrogen bond networks are perpetually created and destroyed as water molecules move around at a molecular scale. As temperature fluctuates, these molecules shift between more compact HDL structures and more open LDL arrangements. The balance between these two competing forms is believed to drive many of water's most anomalous properties.

Proving this has been historically difficult because researchers lacked a consistent way to describe or compare the microscopic structures of supercooled water—water that remains liquid below its normal freezing point due to a lack of nucleation sites like impurities or scratches. Because different structural descriptors (such as tetrahedral bond order and local density) were developed independently, they used different scales and dimensions, making it nearly impossible to determine which measurements accurately captured the most important features of these transitions.

To solve this, researchers at the University of Osaka utilized an AI model trained on computer simulations. The system evaluated 16 different structural descriptors to identify the most effective ways to distinguish between the two liquid states. By providing a unified framework to compare these models, the study offers some of the first pieces of evidence for the decades-old theory that water molecules are constantly switching places at a microscopic level.

This discovery matters because it provides a clearer framework for understanding one of nature's most mysterious substances. By identifying which structural features truly define the transition between HDL and LDL, scientists can better predict how water will behave under extreme pressures and temperatures, potentially unlocking new insights into its role in planetary science and chemistry.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Is water actually two different liquids hiding in plain sight?
• Is water actually two different liquids masquerading as one?
• Is water actually two different liquids existing in one?
• Is water actually made of two competing liquids?
• Why water behaves so strangely compared to other common substances
• Tide pools are miniature, high-stakes laboratories where life clings to the edge of existence.
• The invisible line at the bottom of a river that defines international borders.
• Why rivers are never just one stream but a complex, branching family tree
• Why rivers are changing: how warming and urbanization rewrite the flow of water
• Underwater avalanches of sediment are the primary architects of the deep ocean floor
• Why the ocean's surface temperature is a moving target for global climate scientists
• Why are underground lakes so rare compared to surface bodies of water?

More in Water · More in Nature · Latest · A–Z · All topics