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22 Jan 2015 Vol. 347
#6220

Comet
close-up reveals a world of surprises Diverse landscape
of 67P suggests an unexpectedly complicated origin.

Surveys
reveal state of Afghan population From fertility to
mortality, new data will help guide aid efforts.

Japan's
nuclear renaissance dogged by waste challenge Nuclear
reprocessing is years late; repository is in limbo.

Malaria
may accelerate aging in birds Infection erodes telomeres
and life spans—hinting that there are no benign parasites.

Probing
the proton A newly upgraded accelerator explores the
seething maelstrom at the heart of matter.

Using
science to fight animal research Animal rights activist
Justin Goodman is using science's own tools to combat animal research.

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Chemistry

Why this popular chemistry experiment is a blast

Latest News

Why this popular chemistry experiment is a
blast

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Emily

By

Emily
Conover

26
January 2015 11:00 am
Comments

Explosions are fun—ask any budding chemistry student. That’s why dropping a
chunk of pure sodium into water is a classic
classroom demonstration. The resulting violent reaction can produce
impressive flames and a loud bang. Although the basic chemistry of the popular
experiment has long been understood, the details were not. Now, scientists have
captured the action using high-speed video cameras and discovered an unexpected
trigger. Less than a millisecond after sodium and water meet, the sodium
contorts into a sea urchin–like shape, growing spikes that shoot out into the
water and initiate a runaway reaction.

Sodium is one of the alkali metals, a group of famously reactive elements.
Because sodium is so reactive, it is rare in its pure form, which is solid at
room temperature but soft enough to cut with a knife. The sodium we’re most
familiar with, in table salt, is stable because it’s bound to chloride. Pure
alkali metals are a different story:  When they get together with water,
they erupt furiously. A flurry of electrons abandons the metal, interacting with
the water to form hydrogen gas and other byproducts. The reaction generates
heat, which melts the sodium, and because hydrogen gas is flammable, it ignites.
Or so the story goes.

But scientists led by chemist Pavel Jungwirth of the Academy of Sciences of
the Czech Republic in Prague weren’t satisfied. “There was a missing piece in
this explanation,” Jungwirth says. To produce an explosion, the chemicals
involved need to be well mixed so that the reaction snowballs instead of
sputtering out. But because only the surface of the sodium chunk contacts water,
only atoms in its outer layer can react. Moreover, the production of hydrogen
gas creates a layer that separates the sodium and the water, which should
further slow the reaction, resulting in slow bubbling rather than a kaboom.

To study the details of the detonation, Jungwirth and colleagues first had to
tame the reaction. The results of the demonstration are unpredictable—sometimes
it flashes and sometimes it fizzles, depending on small variations in the size
and shape of the chunks of sodium used. So the scientists used a liquid mixture
of sodium and another alkali metal, potassium, which they could slowly drip into
water in drops of a uniform size and shape. They arranged high-speed video
cameras to capture the reaction at thousands of frames per second. Then they
donned protective gear and stood back.

The cameras captured a never-before-seen effect. Less than a millisecond
after the reaction begins, tens to hundreds of spiky
metal protrusions pierce the water, the researchers report online today in
Nature Chemistry. The spikes appear, the researchers deduced, because
when electrons flee the metal for water, an intense positive charge builds up.
The mutual repulsion of those positive charges rips the metal apart, and it
blasts outward in tiny needles. This increases the surface area of the metal in
contact with water, generating a vigorous reaction. Computer simulations
performed by the researchers confirmed this effect, although for much smaller
quantities of sodium due to the limits of computing power.

Because the basic chemistry of the reaction is 19th century science, it’s an
unconventional subject for a modern study. “I haven’t thought much about
explosions like this,” says chemist Benjamin Schwartz of the University of
California, Los Angeles. But, he says, “the idea makes a lot of sense once you
see it.”

“The surprising thing is that we didn’t think of it before,” says chemist
David Bartels of the University of Notre Dame in Indiana. “We just [took] this
for granted without thinking it through.”

The detailed explanation of this reaction could be useful for preventing such
explosions in industrial applications that use alkali metals, including certain
nuclear reactors that are cooled with liquid metal. But the applications are an
afterthought, Jungwirth says. The true motivation for the research? “We like to
play with explosives.”

(Video credit: Mason et al.)

Posted in Chemistry

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Science| DOI:
10.1126/science.aaa6389

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