Water’s metal tally

An international team of scientists has succeeded in making droplets of water behave like a metal.

On a chemical level, metals are substances made from a lattice of positively charged atoms sitting in a sea of free electrons. The ease with which the electrons can move is what makes metals so good at conducting electricity.

Insulators, on the other hand, keep their electrons firmly locked in bonds, making them poor conductors. Pure water is a very strong insulator – the only reason tap and rainwater conduct electricity is because they have salts dissolved in them. It’s not the water itself that’s conducting, but the ions in it.

But a study published in Nature has demonstrated a way to make the water itself metallic – in very specific conditions.

At a Berlin synchrotron called BESSY II, the researchers used an alloy of sodium and potassium metals (Na-K) to donate electrons to the pure water. Na-K is liquid at room temperature, which makes it easier to manipulate, and sodium and potassium (both alkali metals) are reactive and quick to share electrons.

A droplet of na-k being coated with water vapour. Water becomes iridescent and colourful as it turns into a metal.
A droplet of Na-K being coated with water vapour. Note the colour of the water as it becomes metallicised. Credit: HZB

But because they’re so reactive, sodium and potassium are also usually explosive on contact with water. The researchers mitigated this by using very small amounts of both materials. Inside a vacuum chamber at BESSY II, they combined a small flow of water vapour with a droplet of Na-K coming from a nozzle. The water formed a very thin layer on top of the droplet, filling with electrons and becoming metallic.

“You can see the phase transition to metallic water with the naked eye,” says Dr Robert Seidel, co-author on the paper.

“The silvery sodium-potassium droplet covers itself with a golden glow, which is very impressive.”

The researchers then confirmed these observations with data from the synchrotron, using techniques called optical reflection spectroscopy and X-ray photoelectron spectroscopy to show that the electrons in the water were behaving like a metal.

“Our study not only shows that metallic water can indeed be produced on Earth, but also characterises the spectroscopic properties associated with its beautiful golden metallic lustre,” says Seidel.

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