Chapter 3 - Crystal Chemistry
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Chapter 3 - Crystal Chemistry 3a - Growing Crystals
Experiment 3.1
Copper sulphate crystals
• copper sulphate
• test tube
• conical flask
• crystallising dish
• pencil
Put 8 measures of copper sulphate into a clean conical flask and add 3cm
of water from a test tube. Gently warm the conical flask until all the copper
sulphate has dissolved. Pour the solution into a crystallising dish and leave
it somewhere warm for several days until all the water has evaporated away.
If possible prop up one side of the crystallising dish, say with a pencil, so the
solution is not spread too thinly over the bottom of the dish.
You will have formed some large blue copper sulphate crystals. Grown in this
way crystals do not have a regular shape, but look carefully at them (use a
magnifying glass if you have one) and decide which of the 3 shapes shown
opposite do copper sulphate crystals most resemble?
You can redissolve the crystals in water and grow them again if you wish.
If you grow a lot of small crystals and want to grow bigger ones, try growing
them where it is not so warm, the water takes longer to evaporate away and
they grow bigger.
Do not throw the crystals away when you have finished crystal growing.
Put the crystals somewhere warm until they are thoroughly dry and return
them to your copper sulphate container.
Experiment 3.2
Aluminium potassium sulphate crystals
• aluminium potassium
• sulphate test tube
• conical flask
• crystallising dish
• pencil
Repeat Experiment 3.1 using 8 measures of aluminium potassium sulphate
in place of copper sulphate, and 6cm of water instead of 3. Which of the 3
shapes shown opposite do aluminium potassium sulphate crystals most
resemble?
Experiment 3.3
Sodium sulphate crystals
• sodium sulphate
• test tube
• crystallising dish
• pencil
Put 4 measures of sodium sulphate in a clean dry test tube and add 3cm of
water. Heat the solution until it just boils and pour it into a crystallising dish,
leaving any residue in the test tube. Put the crystallising dish somewhere
warm for several days until all the water has evaporated away. If possible
prop up one side of the crystallising dish, say with a pencil, so the solution
is not spread too thinly over the bottom of the dish. Sodium sulphate
crystals will be left in the crystallising dish when all the water has
evaporated away. Initially the crystals are colourless but they quickly
become white as they lose some of their
water of crystallisation
. This
process is called
efflorescence
.
Which of the 3 shapes shown opposite do sodium sulphate crystals
most resemble?
In Experiment 1.5 you made some crystals of copper sulphate.
Crystals are solid substances in which all the particles are
arranged in a regular pattern. Crystals can be many different
shapes, some are simple shapes like cubes, rhombohedra or
long needles.
Crystals form in
solutions
when the solution contains as much
of the substance (the
solute
) as it can dissolve. The solution
is said to be
saturated
with the solute. Any excess amount of
solute that is present in the solution forms
crystals
. As most
substances increase in solubility as the temperature is
increased, one way to form crystals is to start with a hot
saturated solution and let it cool. As it cools the amount of
solute needed to keep the solution saturated decreases and
the excess is deposited out of solution as crystals. Crystals
formed in this way are usually small ones.
The other way to form crystals, and the way that must be used
if big crystals are wanted, is to start with a saturated solution
and to let the solvent slowly evaporate away. As it does so the
excess solute is deposited as crystals. A general rule for
crystal growing is that
the slower the crystals grow the
bigger they will be
.
Using the chemicals supplied with this Chemistry Lab you can
grow big crystals if you are patient and let them grow over
several days. To grow really big crystals you will need to buy
more chemicals.
Cubic
Rhombohedral
Long needle