The surface of water is denser than the bulk

Scientific Explanation

Intuitively, one would expect the surface of a liquid to be less dense than its interior, since surface molecules lack neighbors on one side. In water, however, the opposite is true: measurements reveal that the near-surface layer has a higher density than the bulk water below.

This phenomenon relates to the reorganization of hydrogen bonds at the surface. In the bulk, water forms a loose three-dimensional network with many tetrahedrally coordinated bonds. At the surface, bonding partners are missing on the air side, and the remaining molecules rearrange into a more compact configuration. The hydrogen bonds in the surface layer become stronger and more directional, pulling molecules closer together. This compaction effect outweighs the expected loosening.

Neutron scattering and X-ray reflection measurements have confirmed that the near-surface layer (the top one to two nanometers) indeed has a higher density than the interior. This effect is closely linked to the unusually high surface tension of water.

Everyday Relevance

The increased density at the water surface contributes to its exceptionally high surface tension, which explains many everyday phenomena: insects walking on water, paperclips floating on the surface, and water droplets assuming a nearly spherical shape. All these effects would be less pronounced if the surface were less dense and therefore less cohesive.