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If someone asks me to number the most notable "textbook experiments" I would definitely name "the comparison of the reactions of alkali metals with water" among the most favorites.

We just happened to do such a reaction with small pieces (approximately $\text{7.5 g}$) of Li, Na and K. As we were supposed to write down anything that we observe, I came across an interesting phenomenon.

It seems that the answer is so simple, I didn't get it through many resources.

Li piece is moving rapidly (better call it spinning) on the surface of water as it is reacting with it and a subtle explosion sound is heard.

Why does the motion of Li on the surface of the water in the beaker take place? Could the heat released by the reaction be causing current in the water (I don't believe so, as the possibly hot water isn't involved in any convection currents)? Info you might need:

Our chemistry teacher cut a non-oxidized piece of well-preserved lithium. The piece of the reactant wasn't of any distinguishable geometrical shape. After repeating the experiment for several times, I sensed a headache that as I believe is because of release of the impurities of the sample.

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  • $\begingroup$ Hydrogen evolves, but why does it evolve in such an uneven fashion as to cause motion, and not just jiggling back and forth.. $\endgroup$ – CowperKettle May 7 '16 at 3:59
  • $\begingroup$ Could there also be some steam being generated from the heat of the reaction (at the lithium surface)? Same effect as the $\ce{H2}$ gas. $\endgroup$ – guest Aug 14 '17 at 21:25
  • $\begingroup$ I believe surface tension of water might play a role, too. $\endgroup$ – Ivan Neretin Aug 14 '17 at 21:34
  • $\begingroup$ 7.5 grams seems a lot! $\endgroup$ – Eashaan Godbole Aug 15 '17 at 5:15
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What happens when you put lithium (or sodium or potassium) in water? What is the chemical reaction that is occurring?

The alkali metals react with water to produce alkali hydroxides and hydrogen gas:

$$\ce{2Li(s) + 2H2O(l) -> 2LiOH(aq) + H2(g)}$$

The two products of the reaction of water and lithium are lithium hydroxide (which dissolves) and hydrogen. Hydrogen is not very soluble in water, and it is much less dense than air. The motion of the lithium on the surface of the water is created by the escape of the hydrogen gas, which provides upward force.

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This is a very speculative answer to your comment on spinning. The spinning presumably occurs because the under surface of the lithium is not flat and also the metal itself is not a perfect cuboid so will not be levitated evenly by the gas produced but will be tilted w.r.t. the surface. There will then be slight force to the side causing it to tilt, if it then momentarily touches the surface then it will start to pivot about this point and so spin as it is levitated again. The angular momentum once started may cause it to continue to do so in the same direction even if it momentarily touches the surface again. As there is hardly any sideways force, as the gas is underneath the lithium pushing it up and flowing round the sides of it as it escapes, sideways motion seems less likely than spinning.

An additional factor is that if it translated across the solution the gas underneath would be less effective to levitate it as the lithium is always moving over water where no reaction has occurred. It would then touch the water and so stop moving until gas built up again. Thus rotation seems to be the lowest energy option.

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