Specialist #53

Under high pressure, water molecules move further apart with increasing pressure

At high pressure, the average distance between water molecules paradoxically increases.

Scientific Explanation

One of the most counterintuitive behaviors of water under high pressure is the density-distance paradox: although the density increases with rising pressure — meaning the water becomes more compact — the average distance between nearest-neighbor oxygen atoms simultaneously grows. At first glance, this appears contradictory: how can something become denser if the molecules are moving farther apart?

The resolution lies in distinguishing between nearest neighbors and overall packing. At low pressure, water forms a tetrahedral network with about four nearest neighbors at a fixed distance of roughly 2.8 angstroms. The tetrahedral geometry, however, leaves large voids between the network positions. Under high pressure, additional molecules are forced into these voids — so-called interstitial molecules. The total number of molecules per unit volume increases (higher density), but at the same time the distance between hydrogen-bonded nearest neighbors grows slightly because the inserted molecules stretch the bonds.

X-ray and neutron diffraction experiments at pressures up to several gigapascals confirm this effect. The first coordination shell of oxygen shifts to larger distances, while the second coordination shell moves closer and begins to overlap with the first. It is important to note that this paradox remains a subject of active research. Different experimental methods and simulation techniques yield slightly different details, and the precise pressure and temperature dependence continues to be investigated.

Density-Distance Paradox in Water Under Pressure Diagram showing the paradoxical behavior of water under high pressure: as pressure increases, the density increases but the average distance between nearest-neighbor oxygen atoms also increases because interstitial molecules fill the spaces in the hydrogen bond network. Density (g/cm³) 1.3 1.2 1.1 1.0 2.90 2.85 2.80 2.75 0 0.5 1.0 1.5 Pressure (GPa) Both increase — the paradox! Density O–O nearest distance (Å)
The density-distance paradox: both density and nearest O-O distance increase with pressure.

Everyday Relevance

Although this phenomenon occurs only at very high pressures far beyond everyday experience, it matters for understanding water in the Earth’s interior. In the crust and upper mantle, water exists under extreme conditions where it influences chemical reactions, mineral dissolution, and even tectonic processes. The unusual structural changes under pressure determine how water functions as a solvent and transport medium in these environments. For materials science, this paradox also serves as a cautionary example: density and intermolecular distance are not always as simply linked as intuition suggests.