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Alkali metal explosion explained

26 January 2015Matthew
Gunther

Dropping an alkali metal into water and watching it explode is a classic
classroom demonstration © Phil Mason

The chemistry behind dropping
sodium into water and watching it explode may require a rethink, according to
scientists in the Czech Republic. They say the violent reaction may be triggered
not by the ignition of hydrogen gas, as previously thought, but by
large charge instabilities within the metal.

An alkali metal exploding in water has been a practical mainstay for teachers
wanting to show the volatile power of chemistry to their students, with many
explaining the process as electrons leaving the metal to form hydrogen, which
then ignites to produce a small blast. But Pavel
Jungwirth and his colleagues from the Academy of Sciences of the Czech
Republic were unsatisfied with this explanation.

‘There is this one important condition for explosive behaviour and that is
you need to ensure an efficient mixing of reactants – like gunpowder,’ says
Jungwirth. ‘When the reaction [between sodium and water] starts running you are
developing hydrogen and steam, and that’s actually a buffer layer that separates
the reactants – so the reaction should self-quench, it shouldn’t be
explosive.’

He goes on to argue that if you carry out the classic high school
demonstration ‘it sometimes explodes and sometimes doesn’t’ because the
conditions may not be right, which hints at the importance of reactants
remaining in direct contact to produce an explosion.

To investigate, Jungwirth and his colleagues dropped a sodium-potassium alloy
droplet into water and watched the ensuing explosion on a high-speed camera
capable of 10,000 frames per second, allowing it to be viewed 400 times slower
than real time.

They found that after 300µs in the water, metal dendrites protrude out from
the drop and pierce the vapour layer around it. These spikes give the alloy a
much greater surface area and allow for it to quickly react with the surrounding
water. The reason for this rapid expansion is all to do with charge, Jungwirth
says.

‘A high school teacher [will] tell you that electrons leave the metal and
they go into water,’ he explains. ‘But what they don’t tell you is if the
electrons leave the metal, then the metal is highly positively charged.’

Using molecular dynamics simulations, the team showed the alloy becomes
incredibly unstable as a result of this excess positive charge. The charges
repel each other and lead to a ‘coulomb explosion’, producing the spikes the
researchers witnessed during their imaging experiments. The team claim this
expansion is the trigger for the alkali metals’ violent behaviour in water.

‘It’s an interesting paper,’ comments Neil Bourne, director of the National
Centre for Matter under Extreme Conditions in the UK. ‘It opens up questions for
the field.’ But he adds that the research may have benefitted from more refined
high speed imaging methods: ‘There are lots of new and exciting techniques they
could have used, which would make their conclusions more robust.’ He also
questions whether the work ‘directly relates to any industrial [processes]’.

Jungwirth contends, however, that the research community was not necessarily
their main target when carrying out this work. ‘I would be most happy if high
school teachers get the message,’ he says. ‘I think the biggest thing would be
if next time this experiment is performed in high schools, it’s connected with
the proper explanation. We’ll do our best to spread the
message.’

References

P E Mason et
al, Nat. Chem., 2015, DOI: 10.1038/NCHEM.2161

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