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19th February 2020
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Nice idea

Posted 11/06/2013 by sevans

Just when you thought you knew all that there was to know about something, along comes another idea that throws you completely and shows you that things are really much more complex than they seem. So it was this week with a new report about the chemistry of one of the fundamental molecules of life, water, with researchers in the US revealing a striking discovery about how ice behaves under pressure - work that changes ideas that date back almost 50 years.

Crystalline ice comprises a surprising diversity of structures – at least 16 are known, formed because of the versatility of the hydrogen bonds that bind the molecules together as water freezes to ice. In a paper in 1964, researchers predicted that at high enough pressures, the hydrogen bonds could strengthen to the point where they could break the water molecules apart.

To date, researchers have attempted to observe this phenomenon in one of two ways: either by using spectroscopic techniques to observe the transition indirectly (as reported by several teams in the 1990s); or by visualising the hydrogen atoms or protons directly by bouncing neutrons off the ice and then measuring the scattering. 

While the latter direct observation method is preferred, until now researchers been unable to reach high enough pressures with large enough samples of ice to make the hydrogen atoms visible. That has now changed, however, thanks to a new method of direct observation involving the intensely bright supply of neutrons provided by the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee.

Using this approach, researchers at The Carnegie Institution for Science have obtained the first glimpse of the hydrogen atoms themselves in ice at unprecedented pressures over 500,000 times atmospheric pressure, they report in Proceedings of the National Academy of Sciences. Strikingly, what their results reveal is that the dissociation of water follows two mechanisms, with some molecules beginning to dissociate at much lower pressures and by a different mechanism than was predicted in the 1964 paper.

More ground-breaking discoveries are expected to follow, with the researchers pointing out that ‘being able to see hydrogen with neutrons isn’t just important for studies of ice’. 

Cath O’Driscoll - Deputy editor

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