Volume 94 Issue 35 | p. 11 | News of The Week
Issue Date: September 5, 2016

Liquid alkali metal alloy floats on water rather than explodes

Material makes colorful transformation into alkali metal hydroxide
Department: Science & Technology
News Channels: Analytical SCENE, Materials SCENE
Keywords: inorganic chemistry, materials, sodium, potassium, alkali metal, water, explosion, solvated electrons
A liquid drop of sodium-potassium alloy floating on water turns blue from solvated electrons.
Credit: Angew. Chem. Int. Ed. Engl.
Image of a liquid drop of sodium-potassium alloy colored blue from solvated electrons.
A liquid drop of sodium-potassium alloy floating on water turns blue from solvated electrons.
Credit: Angew. Chem. Int. Ed. Engl.

When a chunk of alkali metal gets tossed into water, it explodes. But when a team of scientists gently placed a liquid drop of a sodium-potassium alloy on top of a water surface, they observed a different but equally spectacular process (Angew. Chem. Int. Ed. Engl. 2016, DOI: 10.1002/anie.201605986).

Philip E. Mason and Pavel Jungwirth of the Czech Academy of Sciences and Tillmann Buttersack and Sigurd Bauerecker at Braunschweig University of Technology studied the drop’s transformation using high-speed imaging and optical spectroscopy.

At first, the alloy and water react to produce alkali metal hydroxides, hydrogen, and heat. The alloy’s buoyancy and the gas production limit contact between the metal and the water so that the reaction proceeds nonexplosively. An inert atmosphere prevents hydrogen ignition.

About 0.3 seconds into the reaction, the interacting surfaces turn blue from solvated electrons—the phenomenon is visible to the naked eye despite the electrons’ submillisecond lifetime in water. The drop continues to heat to the point that at about two seconds, the alkali metals begin to evaporate and the drop glows red. At about three seconds, the metal vapor clears and the alloy’s temperature falls as it completely transforms into transparent molten alkali metal hydroxides. Supported by a layer of steam, the drop floats for another second before falling into the water and bursting dramatically as the hydroxides and water mix.

A liquid drop of a sodium-potassium alloy deposited on water starts reacting to produce alkali metal hydroxide, hydrogen, and heat. Solvated electrons turn the drop blue and then black. Subsequently, the drop turns blue again and then red as the drop heats enough for the metals to evaporate. Finally, the metal vapor clears and the drop’s temperature falls as it transformes into transparent molten alkali metal hydroxides before dropping into the water and bursting as the two materials mix.
Credit: Angew. Chem. Int. Ed. Engl.
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Rev.Crash (Tue Sep 06 19:01:08 EDT 2016)
Did I just witness the Transparent Aluminum of StarTrek?
Jyllian (Wed Sep 07 19:55:48 EDT 2016)
Ha! Not quite. :)
MAINAK GANGULY (Wed Sep 07 03:21:25 EDT 2016)
how did the liquid drop of Na/K alloy was gently placed on water surface?
Lucas (Thu Sep 08 07:59:13 EDT 2016)
Probably using a pipette.
Jyllian (Mon Sep 12 12:26:56 EDT 2016)
The researchers used a syringe.
S. Dasgupta (Thu Sep 08 00:42:29 EDT 2016)
I notice the drop going clockwise. Initially it goes back and forth a little and then clockwise.
Interesting. Any particular reason?
Jyllian (Tue Sep 13 11:29:27 EDT 2016)
I asked Pavel Jungworth, and he replied: "I'd say the sense of rotation is accidental although we did not really focus on this aspect of our experiments."
Philip McCorkle (Wed Sep 14 13:56:56 EDT 2016)
They probably have a fume extractor, judging by the nice vortex action in the vapors. The rotation of the vortex is probably due to the Coriolis Effect. The rotation of the ball is due to momentum imparted by the vortex. Eh?
Huang S. Lin (Thu Sep 08 09:04:57 EDT 2016)
The alloy od K and Na is different form element of Na, it appears the electron orbit has interaction, therefore the alloy reacts with water is different form element. Even the physical characteristics are differences.
Mr Michael T Deans (Thu Sep 08 14:35:09 EDT 2016)
My research, read SCIENCE UNCOILED or email me for the latest summary, describes how catecholamines adrenaline, noradrenaline and dopamine exchange 3 Na for 2 K ions. Since morphine creates larger complexes, it also accounts for drug addiction.
vishal chaurasiya (Mon Sep 12 13:15:38 EDT 2016)
Norman Oppenheimer (Mon Sep 12 18:04:06 EDT 2016)
When sodium or potassium are dissolved in liquid ammonia they initially form stable, deep blue solutions (solvated electrons). As the concentration of the metal is increased eventually a point is reached where a phase separation occurs and a metallic, golden solution begins to float on the blue solution (delocalized electrons like in a metal). Is this occurring on the metal droplet in water? In the video the droplet goes from blue to black but also it is briefly golden in color. Is this indicating that water can support both solvated electrons and delocalized electrons for significant periods of time; e.g., seconds? A very interesting observation.

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