Ask A Scientist Chemistry Archive

name         Lucas
status       educator
age          20s

Question -   How much potassium can I put in water to have a safe
demonstration, without a huge explosion?
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The short safe answer is NONE. The reaction of potassium with water is
extremely hazardous to do except under the most controlled conditions. It
should NEVER be used as a classroom or lab demonstration. Its low melting
point (~60C) makes it especially dangerous. It reduces most all oxygen
containing compounds, including silicates (glass), so potassium fires are
essentially impossible to extinguish. I cannot emphasize too strongly the
safety issues. It belongs to the category of substances like F2 and liquid
ozone that obey Tiger's Law:
"If you are going to twist the tiger's tail, you better be prepared to deal
with its teeth!!"
That is a sanitized quote of author Tom Clancy, but you get the idea.

Vince Calder
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Lucas,

In light of the danger posed by per-oxidation of potassium -- the 
formation of a peroxide film on the metal's surface -- that can result in 
a nasty explosion when the metal is cut, I would simply abandon the 
demonstration of placing potassium on water. It is not worth the risk. The 
real danger with potassium is in the cutting operation -- not in the 
reaction with water.

Personally, I would not use potassium in a demonstration and would use it 
in a reaction ONLY of the were no safe alternatives.

Sodium, dangerous as it is, will suffice if the water-reaction demo is 
done with care. Simple flame tests can illustrate the spectral differences 
between the alkali metals -- and, such tests can be safely run on the ions 
in solution.

Regards,
ProfHoff 917
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