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Pure water is an almost perfect insulator.
Yes, water found in nature conducts electricity, but that is due to the impurities it contains, which dissolve into free ions that allow electrical current to flow. Pure water only becomes “metallic” (electronically conductive) at extremely high pressures, beyond our current ability to produce it in a laboratory.
But, as researchers demonstrated for the first time in 2021, it is not only high pressures that can induce this metallicity in pure water.
By putting pure water in contact with an alkali metal that shares electrons (in this case an alloy of sodium and potassium), freely moving charged particles can be added, turning the water into a metal.
The resulting conductivity only lasts a few seconds, but it is an important step in understanding this phase of water by studying it directly.
“With the naked eye you can see the phase transition to metallic water!” physicist Robert Seidel of the Helmholtz-Zentrum Berlin für Materialien und Energie in Germany explained in 2021 when the research was published.
“The silver drop of sodium and potassium is covered with a golden glow that is very impressive.”
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Under high enough pressures, virtually any material could, in theory, become conductive.
The idea is that if you squeeze the atoms hard enough, the orbitals of the outer electrons would begin to overlap, allowing them to move. In the case of water, this pressure is about 48 megabars, just under 48 million times the atmospheric pressure of the Earth at sea level.
Although pressures higher than this have been generated in the laboratory, such experiments would not be suitable for studying metallic water. So a team of researchers led by organic chemist Pavel Jungwirth of the Czech Academy of Sciences in Czechia turned to alkali metals.
These substances release their external electrons very easily, meaning they could induce the electron exchange properties of highly pressurized pure water without high pressures.
There’s just one problem: alkali metals are very reactive with liquid water, sometimes even to the point of exploding (there is a really cool video below).
Drop the metal into water and you’ll get a kaboom.
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The research team found a very ingenious way to solve this problem. What would happen if, instead of adding the metal to the water, you added water to the metal?
In a vacuum chamber, the team began by extruding from a nozzle a small droplet of sodium-potassium alloy, which is liquid at room temperature, and carefully added a thin film of pure water using vapor deposition.
Upon contact, electrons and metal cations (positively charged ions) flowed into the water from the alloy.
This not only gave the water a golden glow, but made it conductive, just as we should see in pure metallic water at high pressure.
This was confirmed by optical reflection spectroscopy and synchrotron X-ray photoelectron spectroscopy.
The two properties (the golden glow and the conductive band) occupied two different frequency ranges, allowing them to be clearly identified.
In addition to giving us a better understanding of this phase transition here on Earth, the research could also allow for detailed study of the extreme pressure conditions inside large planets.
On the Solar System’s icy planets, Neptune and Uranus, for example, liquid metallic hydrogen is believed to swirl in eddies. And it is believed that only on Jupiter are the pressures high enough to metallize pure water.
The prospect of being able to replicate the conditions inside the planetary colossus of our Solar System is truly exciting.
“Our study not only shows that it is possible to produce metallic water on Earth, but also characterizes the spectroscopic properties associated with its beautiful golden metallic sheen,” Seidel said.
The research was published in Nature.
A previous version of this article was published in July 2021.