Obtain samples of the three metals in the title, a
gram or less of each will suffice
Ensure that safety procedures are followed: use
safety glasses, gloves and a lab apron, require
students to stand back at least 3 meters
Turn out the lights so that any flame will be clearly
visible
Add a very small amount of each metal to the water in
this order: Li, Na, K using a separate 500 mL beaker
for each metal; this will demonstrate the increase in
reactivity as elements farther toward the bottom of
the group are dcemonstrated
Show the basicity of the solution by adding an
indicator that is clear in neutral solution but
colored in basic solution (good indicators include:
o-Cresolphthalein, Phenolphthalein,
m-Nitrophenol, or Thymolphthalein)
Notes About What's Going On
The chemical equations for these reactions, in order
of reactivity and in order of the amount of heat
evolved are as follows:
2Li + 2H2O -> 2LiOH + H2
+ heat
2Na + 2H2O -> 2NaOH + H2
+ heat
2K + 2H2O -> 2KOH + H2 +
heat
These elements belong to group 1, the alkali metals,
and therefore exhibit a similar reaction upon
immersion in water
The reactivity of these elements as you go down the
group to higher atomic numbers
Contrast this with the reactivity order of the
halogens, where under some circumstances,
F>Cl>Br>I.
Contrast this also with the reactivity of the noble
gases, none of which typically undergoes chemical
reactions
Note: when Li/Na/K are done reacting they are all +1
cations and have noble gas electron configurations
This is why we study electron configurations
The products of these reactions are hydroxide ion
(OH-) and hydrogen gas (H2)
The hydroxide ion causes the water to be basic
The hydrogen gas can be lit on fire because the
reaction is exothermic
As the hydrogen burns, it becomes water:
2H2 + O2 ->
2H2O
Safety
As always, no one but a trained chemist should perform this demonstration.
Adding alkali metals to water is very dangerous
because the hydrogen gas that is released can burst
into flame and even explode violently
Handle the metals carefully and ensure that the bulk
of the metal stays covered with the paraffin oil
Only add a very small amount of each alkali metal to
the water, and use a large excess of water so that a
complete reaction can be assured
Cleanup
Simply neutralize the solutions with 1 M acid and
pour down the drain
Check for neutral pH using litmus paper or an
appropriate pH indicator solution