X-ray diffraction reveals an unusually detailed structure
X-ray diffraction patterns of water show more structural detail than expected for a simple liquid.
Scientific Explanation
When X-rays are directed at a liquid and the diffraction pattern is analyzed, the result is the radial distribution function g(r) — a probability map showing how neighboring molecules are distributed at various distances around a reference molecule. In simple liquids such as argon or nitrogen, g(r) shows a single broad main peak followed by weak oscillations that quickly decay to the mean — the liquid has little structural order beyond its nearest neighbors.
Water presents a very different picture. The first peak at 2.8 angstroms (the typical distance between nearest neighbors in a hydrogen bond) is exceptionally sharp. Beyond it, a clear second peak appears near 4.5 angstroms and even a third around 6.5 angstroms. These multiple peaks reveal that order in liquid water extends over several molecular shells — a behavior more typical of crystals or very viscous liquids.
This far-reaching order arises from the network of directional hydrogen bonds. Each molecule arranges its neighbors in a nearly tetrahedral geometry, and this local order propagates through the bonds into the next coordination shells. The sharpness of the peaks diminishes with temperature (hotter water becomes more diffuse), but remains much more pronounced than in most other liquids even at 80 degrees Celsius.
Everyday Relevance
The detailed structure of liquid water is one reason it is such an unusually effective solvent. The structural order influences how water surrounds ions and molecules (solvation shell) and how chemical reactions proceed in aqueous solution. In biochemistry, the water structure around proteins and DNA governs their folding and function. Without water’s distinctive local order, the biochemistry we know would not be possible.