Li sizzles, Na balls & K-pot explodes

Comparing the reactions of lithium, sodium and potassium with water.

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A lump of potassium explodes

The alkali metals all react speedily with water and with oxygen.

For this reason they are stored in oil to limit the oxygen and water in air getting to them in the chemical store.

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Lumps of lithium, sodium and potassium metals in hexane prior to dropping them into water

Students at GCSE are often asked to compare the trend in reactivity of the metals on going down the group, by comparing the observations made as each one reacts with water.

To this end experiments are sometimes carried out, demonstrating the reactions of lithium, sodium and potassium with water.

All three metals float on water and react quickly with it producing hydrogen gas.

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Potassium always catches fire producing a lilac coloured flame and explodes if too big a chunk is used.

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“Li sizzles, Na balls and K-pot explodes lilac”

Equations for the reactions are:

2 Li   +   2H2O  –>  2LiOH   +   H2
2 Na   +   2H2O  –>  2NaOH   +   H2
2 K   +   2H2O  –>   2KOH   +   H2

Students are also often asked to predict how rubidium and caesium metals would react with water.

The answer is that they would both react producing similar products according to the equations above, but even more vigorously than potassium, with caesium being the most reactive.

Links to movies on You Tube from which the above gif animation was made are here:

1. Short version 39 seconds

2. Longer version 2 minutes and 3 seconds

 

Why has no one has ever dropped a lump of Francium into water and made a movie of it?

 

 

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