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| Numerous metal spikes protrude from a millimeter-sized droplet of a Na/K alloy milliseconds (ms) after it makes contact with water. These spikes, which do not form on water droplets (right column), participate in alkali metal explosions. |
Explosion of Sodium And
Potassium
In
Water
Why Sodium And Potassium Really
Explode In Water
Inorganic Chemistry: High-speed photography and
modeling reveal that classic reaction gets its oomph from sudden repulsion
between alkali ions
By Mitch Jacoby
For decades, science enthusiasts have delighted at the famously
energetic way sodium and potassium explode on contact with water.
Researchers in Europe now show that the long-accepted explanation for
the way that process unfolds is incomplete (Nat. Chem. 2015, DOI: 10.1038/nchem.2161).
Chemists have long thought that tossing a chunk of
alkali metal into water, a time-honored tradition still practiced by some
lecturers and many chemistry thrill seekers, causes an explosion because the
metal dissolves, generating an extreme amount of heat and transferring
electrons to the water.
The dissolution step also generates steam and forms hydroxide ions and
hydrogen, which can be ignited, making the process even more energetic.
Some researchers have puzzled over how the process can occur so quickly,
though.
They recognized that the steam and hydrogen generated early on in the
reaction should form a buffer layer over the metal surface and impede water
from continuing to react.
To sort out the mystery, chemists Philip E. Mason,
Pavel Jungwirth, and coworkers at the Academy of Sciences of the Czech
Republic, in Prague, along with colleagues at Braunschweig University of
Technology in Germany, studied the process with ultrafast photography and
computational techniques.
A number of factors, including sample surface
cleanliness and temperature, can prevent chunks of alkali metals from exploding
on contact with water.
The team eliminated those variables and others by using a
sodium-potassium alloy that remains liquid at room temperature and a droplet
delivery system featuring a calibrated syringe.
The team observed that within a fraction of a millisecond of making
contact with water, the Na/K droplets form numerous spikes that protrude into
the water.
Molecular dynamics analysis indicated that nearly instantaneous transfer
of electrons from the spikes to the water rapidly generates positively charged
alkali ions, which vigorously repel and cause a so-called Coulomb explosion.
It is the speedy manner in which that process propagates and generates
reactive metal surfaces that triggers the overall explosion.
The researchers have figured out many of the key
aspects that enable this highly exothermic reaction to become explosive, rather
than self-quench, says Stephen E. Bradforth, a chemistry professor at the
University of Southern California.
He adds that the “beautiful” high-speed photography showing extremely
rapid development of long metal spikes and the role of Coulomb explosions at
the interface is “quite provocative.”
Michigan State University emeritus chemistry
professor James L. Dye notes that explosions of overcharged droplets in the gas
phase have been known since the work of Lord Rayleigh in 1882.
He says that anyone who has done or seen this classic demonstration will
appreciate the “graphic detail” of the reaction mechanism and the visuals
provided by this study.
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