The mysteries of water have amazed scientists and are credited with making life as we know it possible. Unlike most liquids, which shrink when cooled, water molecules align and freeze in a spaced out crystal lattice. You observe the result often: ice floats on top of liquid water. If water were like most liquids, ice would sink.
The secret of floating ice has long been known: a phenomenon we call hydrogen bonds. Each water molecule has one oxygen and two hydrogen atoms (the famous H2O). Oxygen likes to hog the negative electrons of the molecule, which in turn makes the hydrogen somewhat positively charged. Following the adage "opposites attract", the somewhat positive hydrogen tugs on the rather negative oxygen of neighboring water, and oxygen tugs back. This forces the V-shaped water molecules to line up in a crystal lattice with a lot of space -- space that makes the frozen water lighter than its liquid form.
Almost 50 years ago, scientists speculated that when water is under a high amount of pressure, the attraction between the hydrogen of one water molecule and the oxygen of its neighbor could be strong enough to pull the hydrogen away from its owner, into the space between the two molecules.
But scientists have not been able to directly observe hydrogen bonds overwhelming the normally stronger bonds that hold water molecules together. Reaching the high pressures required for the normally weaker hydrogen bonds to break water apart requires a very small sample. Like looking for a small earring or screw that has been dropped on the floor, the job is easier with a really bright light.
In the case of looking for hydrogen atoms in water under pressure, the "light" consists of a stream of neutrons. Similar to how the light bouncing off your face and onto a mirror brings an image of your self to your eye, scientists can create an image of the atoms in water when they shine a stream of neutrons on it. Until recently, though, there has not been a stream of neutrons bright enough to see the small, pressured water samples.
The implementation of the Spallation Neutron Source at Oak Ridge National Laboratory in 2006 changed that. A team lead by Malcolm Guthrie of the Carnegie Institute for Science has designed a tool that exploits the ORNL Spallation Neutron Source to see the mystery of the disassociating water. And what they saw will forever change the expectation scientists have for water under pressure.
It turns out the dissociation happens two different ways, one at much lower pressures that predicted. This finding will influence our understanding of how ice behaves in extreme situations, like at the core of of a planet. But far more promising, the tools developed to solve this mystery of water can be used to shine a light on hydrogen in other situations. The breakthrough will help scientists to develop ways to use the incredible powers of hydrogen in novel materials for hydrogen storage, in fuel cells, or in other alternative energy applications.