The Real Science of an Alkali Metal Explosion

You may remember the classic alkali metal explosion demonstration in one of your early chemistry classes. Many educators use this experiment to show the volatile power of chemistry. The thought was that the unstable reaction was caused by the ignition of hydrogen gas, but scientists in the Czech Republic have found new information behind this classic demonstration by using high-speed video.

The researchers began investigating the science behind this experiment by dropping a sodium-potassium alloy droplet into water. From there, they recorded the explosion with a high-speed camera that is capable of capturing 10,000 frames per second.

Of course, there’s a video.

This from Chemistry World:

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.

Read the full article here.

As the electrons leave the metal, it gains a highly positive charge. Because of this excess positive charge, the alloy becomes incredibly unstable. The charges then repel and produce to a coulomb explosion, which produce spikes – a chemistry secret that has been hiding for hundreds of years.

This from Popular Science:

Without the spikes, the team hypothesizes, the explosion might not happen at all. Instead, the initially-formed steam and hydrogen gas might sit in a layer around the sodium, keeping the water and sodium from reacting any further.

Read the full article here.

While the scientists hope this finding will impact the research community, the main target was educators who will now be able to provide a proper explanation for this classic chemistry experiment.

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