Accessibility links

Skip
to content

Skip
to local navigation

Accessibility
Help

BBC iD
Sign in

BBC navigation

News

Sport

Weather

Earth

Future

Shop

TV

Radio

More…

Search
term:

Science & Environment

Home

UK

Africa

Asia

Australia

Europe

Latin
America

Mid-East

US &
Canada

Business

Health

Sci/Environment

Tech

Entertainment

Video

26 January
2015 Last updated at 17:11
GMT

Share this page

Delicious

Digg

Facebook

reddit

StumbleUpon

Twitter

Email

Print

Metal explosions 'driven by charge'
By Jonathan Webb Science reporter, BBC News

The video captured 'spikes' of metal shooting
outwards into the water

Continue
reading the main story

Related Stories

A metal so light it floats on oil

Whatever happened to kids' chemistry sets?

Using super-fast video footage and
computer simulations, scientists have revised our understanding of explosions
well known from high-school chemistry.

Chemists in the Czech Republic and Germany captured images of the alkali
metals sodium and potassium exploding on contact with water.

They saw a flash of bluish purple and "spikes" of metal shooting
outwards.

This suggests that the reaction is kick-started by positive charges repelling
each other.

Writing in the
journal Nature Chemistry, the researchers say their results explain how an
explosion can be sparked, if the conditions are just right, despite there being
very little contact area between the water and the metal.

"If you want to have an explosive reaction... you need a lot of contact
between the reactants," said senior author Prof Pavel Jungwirth, from the
Academy of Sciences of the Czech Republic.

"And this is exactly what we don't have here."

When a small nugget of potassium or sodium is dropped into water, the
interface between metal and water is only small - and is rapidly obscured by a
layer of gas that the reaction produces.

"So the reaction, as it is explained in the text books, should be
self-quenched - it shouldn't lead to this runaway, explosive behaviour," Prof
Jungwirth told the BBC.

He and his colleagues set about solving this conundrum by studying the very
beginning of the reaction - less than a millisecond (0.001 seconds) - when it
either becomes explosive or simply fizzes along.

"We needed another physical mechanism that would ensure the mixing of the
metal and the water," explained Dr Phil Mason, who conducted most of the
experiments.

Many pilot experiments helped establish the right
conditions for an up-close, detailed study

But to capture the reaction in enough detail, the team had to secure the loan
of a high-speed videocamera - which costs about $100,000.

"When you say you want to film some explosions with these cameras... people
usually don't want to rent them to you," Prof Jungwirth said.

After many trials, the team settled on a method for reliably producing a
small explosion, which they could observe in fine detail.

They used an alloy of sodium and potassium, which is a liquid at room
temperature, to make sure that a nice clean drop of metal would make rapid,
clean contact with the water - and to stop the metal oxidising on the way, they
dropped it in a tube full of argon, rather than air.

"We went through quite a lot of kit before we worked out how to do it
safely," Dr Mason said.

Finally, in collaboration with a laboratory from the Braunschweig University
of Technology in Germany, the chemists captured the footage they needed.

The blue flash and 'spikes' of metal seen after about
0.3 milliseconds are indicative of a 'Coulomb explosion', driven by
electrostatic repulsion

Two tell-tale pieces of evidence from the video suggested a new
interpretation of how these reactions become explosive.

Spikes of metal could be seen shooting very rapidly out into the water,
within less than half a millisecond of the two surfaces coming into contact. And
there was a flash of a bluish purple colour at about the same time.

These two observations are consistent with a "Coulomb explosion", where the
metal becomes positively charged and those charges push each other outward,
causing the spikes to form.

Prof Jungwirth explained this phenomenon has been known since the 19th
century, but never spotted in this context.

"If you put enough charge on [a droplet], the repulsion between the charges
will be larger than the cohesion energy of the droplet... and it will
disintegrate," he said.

"And the way it disintegrates is that the droplet shoots out these spikes,
sometimes called Raleigh columns."

In this case, the metal droplet becomes positively charged when electrons
flood away from it in the very early stages of the reaction.

"The electrons are very light, so they almost immediately leave the metal and
move into water," Prof Jungwirth said.

This is consistent with the blue flash, he added: "That's actually the
absorption of light by the electrons."

The team also built computer simulations of the reaction that match their
observations, explaining how repulsion between the positive charges left in the
metal can produce the spikes, which in turn expand the surface area of the metal
and accelerate the reaction - creating the explosion.

"That's what also makes us feel more confident," Prof Jungwirth said. "On a
microscopic scale, when we do the calculations, the atoms really behave as we
see on the macroscopic scale, on the cameras."

Follow Jonathan on Twitter

More on This Story

Related Stories

A metal so light it floats on oil
11 APRIL 2014, MAGAZINE

Whatever happened to kids' chemistry sets?

01 AUGUST 2012, MAGAZINE

Share this page

Delicious

Digg

Facebook

reddit

StumbleUpon

Twitter

Email

Print

More Science &
Environment stories
RSS

Kepler
find reveals 'dawn of galaxy'

An ancient solar system similar to our own is discovered by scientists at
the University of Birmingham studying data from the Kepler telescope.

Planet hosts giant ring system

Best ever view of asteroid Ceres

Top Stories

Apple
profit 'biggest in history'

N Korea leader 'to visit Russia'

Japan condemns Goto hostage video

Asylum group 'legally held at sea'

Bank chief warns of pandemic threat

Features & Analysis

Cool
customer

The village that has just got its first fridge

The
twins of Auschwitz

How a Nazi doctor experimented on identical siblings

Counting
the cost

Germans see little point in more concessions to Greece

Near
miss

How pallbearers almost dropped Churchill's coffin

Most Popular

Shared

1: Apple earnings beat
expectations

2: Earthquake felt in
mid-Hampshire

3: Auschwitz survivors appeal to
world

4: Shetland celebrates Up Helly
Aa

5: Nepalese pretend to bust
furniture in #SmashChairChallenge protest

Read

1: Apple earnings beat
expectations

2: N Korea leader 'to visit
Russia'

3: The twins of Auschwitz

4: Germans count cost as Greece
turns left

5: US admits snow forecast
errors

6: Why Finnish babies sleep in
boxes

7: Japan probes 'new IS hostage
video'

8: Kepler find reveals 'dawn of
galaxy'

9: England drives global football
deals

10: What does Kim Jong-un really
want?

Video/Audio

2: ss Great Britain wreck site
found Watch

3: 'I had to survive to tell the
world' Watch

4: 3D printed heart saves girl's
life Watch

5: Cumberbatch sorry for 'idiotic'
remark Watch

6: Drone bird aims to take
flight Watch

7: How criminals clone your
card Watch

8: Footage captures hippo chasing
boat Watch

9: Survivors gather to remember
Auschwitz Watch

10: Snowy Statue of Liberty
time-lapse Watch

BBC Future

What
extreme cold does to humans
Goosebumps and broken eyelids Read
more...

Programmes

Click
Watch

The future of elevator technology - lifts that can climb up to 1km in the
air and even travel sideways

Services

Mobile

Connected
TV

News feeds

Alerts

E-mail
news

About BBC News

Editors' blog

BBC College of
Journalism

News
sources

Media
Action

Editorial
Guidelines

BBC links

Mobile
site

Terms of Use

About the
BBC

Advertise With
Us

Privacy

Accessibility
Help

Ad
Choices

Cookies

Contact the
BBC

Parental
Guidance

Copyright © 2015 BBC. The BBC is not responsible for the
content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style
sheets (CSS) enabled. While you will be able to view the content of this page in
your current browser, you will not be able to get the full visual experience.
Please consider upgrading your browser software or enabling style sheets (CSS)
if you are able to do so.