Chapter 3 - Crystal Chemistry 3a - Growing Crystals
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Chapter 3 - Crystal Chemistry 3a - Growing Crystals
If you are growing a copper sulphate crystal then you need to
add 60g of copper sulphate and 1 measure of sodium hydrogen
sulphate for every 100g (100ml) of water. (60g of copper sulphate
is 2 test tubes-full.) Tie a piece of sewing thread around your seed
crystal. Tie the other end of the thread around a pencil, or lolly
stick so that the seed crystal hangs in the middle of the container.
Carefully fill the container with the cold saturated solution and hang
the seed crystal in place. Put the container where it will not be
disturbed. Somewhere where the temperature does not change
much from day to night is best, otherwise the crystal may grow
during the cold night and dissolve again during the warm day!
A good tip is to grow the crystal only at night when the temperature
is falling. Each morning take the crystal out of the solution and lay
it on a piece of paper towel. Each night put it back into the solution.
A large crystal will grow over a period of several weeks. Take it out
of the solution from time to time to look at it and to remove any
small crystals that are growing on it and on the thread. If small
crystals grow on the sides and bottom of the container pour out the
solution, wash and dry the container and refill it, then continue the
experiment.
In this way over a period of weeks and months very large crystals
can be grown. It is easy to grow crystals but difficult to grow
perfect big crystals. Sometimes you may see competitions for
crystal growing. If you are interested in crystal growing you can
buy a book on crystals and crystal growing and get the chemicals
from a chemical supplier. Or you could start by buying a Crystal
Growing Kit.
Experiment 3.8
Growing large crystals
• aluminium potassium
sulphate
• saucepan
• glass container
• sewing thread
• pencil
Note: The are insufficient chemicals supplied in your lab to carry
out this experiment
. Before you can carry out this experiment you
need to buy more aluminium potassium sulphate or copper sulphate
from a chemical supplier.
To grow really large crystals you need to suspend a small one
(known as the
seed crystal
) in a
saturated
solution of the chemical
and let the water slowly evaporate away. As it does so the chemical
will grow as one large crystal on the seed crystal. The best
chemicals for growing large crystals are aluminium potassium
sulphate and copper sulphate. The method is described here for
aluminium potassium sulphate; you can substitute copper sulphate
if this is the crystal that you are growing.
First you need a seed crystal of aluminium potassium sulphate which
you have grown as in Experiment 3.2 (If you are working with copper
sulphate then the seed-crystal will be grown by Experiment 3.1).
Get a suitable container (e.g. an egg cup, a drinking glass or a jam
jar) the size of which will depend on the amount of the aluminium
potassium sulphate that you have available. Measure the volume
of the container that you are using.
You now need a saturated solution of aluminium potassium sulphate
that will fill your container. Prepare this by measuring into a small
saucepan 32g of aluminium potassium sulphate and 1 measure of
sodium hydrogen sulphate (to keep the solution acidic and prevent
decomposition of the aluminium potassium sulphate) for every 100g
(100ml) of water. (It is best to weigh the aluminium potassium
sulphate but, if this is not possible, 32g is 1 test tube-full plus
another 6cm in the test tube.) Gently heat and stir the solution until all
the aluminium potassium sulphate has dissolved. When cool enough
pour the solution into the container and leave it for 24 hours. Some
aluminium potassium sulphate crystals should be formed, leaving
a saturated solution. Filter, or carefully pour, off this solution into
another temporary container and wash and dry the container in
which you are going to grow the crystal. One of the crystals
deposited during this initial cooling may be a suitable seed crystal.
Pencil
Container
Thread
Seed
crystal
Saturated
solution
