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Tuesday – method + materials
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.
(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.
- Wednesday – method +
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. Carefully
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 stirring rod.
From the bed of crystals, one
good symmetrical crystal or group of crystals needs to be selected to act as
“seed” for your big crystal. Using a plastic spoon, spatula, tongs or tweezers,
transfer the selected crystal to the beaker containing the decanted solution,
trying to place it centrally in 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.)
- Thursday – method +
Take a picture of the crystal,
preferably close to a ruler so that you can measure its size, and mark the level
of the liquid in the beaker. Record the date in your diary.
Cover the solution with a
loose-fitting paper hat that permits water to evaporate slowly whilst keeping
Allow the solution to stand in
a draft free location, not in direct sunlight or near a heater. The aim is to
keep the temperature as constant as possible.
Once a week record in your
diary the level of the solution in the beaker and if possible, without
disturbing your crystal, take a picture of it close to a ruler so that you can
estimate how much it has grown. Try and avoid disturbing crystals during the
growth phase as this may induce additional crystals to grow.
If small isolated crystals
appear, you may be able to carefully remove them with tweezers. Be careful not
to disturb your big crystal. If small crystals grow on the main crystal, remove
it, dry it with tissues, and carefully remove the adhering buds. Do not touch
the crystal with your fingers. The crystal is likely to be quite fragile, fairly
brittle, and easily damaged, so should not be dropped or bumped.
When it is decided to terminate
the experiment (after about 10 weeks or so), remove your crystal very carefully
from the beaker, dry it with tissues and with a ruler measure its length, width
and height. Compare the measurement, if you can, with the size of the original
crystal. Take a photo of the completed crystal (next to a ruler to confirm
To complete the artistic
component, students will digitally photograph the completed crystal, and using
the effects of light and shade, colour and other techniques, produce an artwork
featuring the crystal and demonstrating the principles of diffraction/
reflection/ dispersion using light and or objects/images (see examples). The
ingenuity of students will be encouraged to produce an innovative picture which
highlights the crystal.
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