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.