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25 g of potash alum
2 beakers (250 ml or bigger
170 ml of hot water
Filter Paper or Watch Glass
Make sure all apparatus is clean and dry.
Place approximately 25 g of potash alum in a
beaker (250 ml or bigger) and add approximately 170 ml of hot water.
Stir the mixture until all the crystals have
dissolved. The solution may have a slightly cloudy or milky appearance due to
impurities in the technical grade of alum used.
(NOTE: don’t use all the alum you have been given – keep a few crystals back in
reserve in case you need to “seed” the solution later.)
Filter the warm solution through a filter funnel
(in which filter paper has been inserted) into another clean beaker.
Cover the beaker of solution with the watch
glass or fresh filter paper and set aside in a cool sheltered place and allow
it to stand undisturbed overnight. The first beaker and funnel can be washed
and dried and the residue and filter paper used for filtration discarded.
The next day, observe the beaker of solution.
The bottom should have become covered with a layer of smallish crystals which
formed spontaneously as the solution cooled. \
Pour off the clear solution above the crystals
into a clean beaker and set it aside for later, leaving the crystals behind in
the other beaker. If for any reason, no crystals have formed after leaving the
solution to stand overnight, the solution can be “seeded” by adding a crystal
from the original alum.
Let it stand overnight again and observe the
formation of crystals. Alternatively, if nothing has happened, crystal growth
may also be induced by scratching the bottom of the glass beaker with a glass
From the bed of crystals, one good symmetrical
crystal or group of crystals needs to be selected to act as “seed” for your big
Using a plastic spoon, spatula, tongs or
tweezers, transfer the selected crystal to the beaker containing the decanted
solution, trying to place it in the centre of the beaker. A good technique to
promote uniform growth is to suspend the crystal with a nylon thread tied round
a stirring rod or pencil resting on the rim of the beaker. This step is not
essential, however, and good a crystal can usually be obtained just by leaving
the crystal on the bottom of the beaker.