(a) Crystal growth
Sodium thiosulphate crystals grow rapidly from a super-saturated aqueous solution. The
formula for the crystals is Na2S2O3.10H2O. On heating, these crystals dissolve in some of
their water of crystallization. Put 3 cm of sodium thiosulphate crystals in a test-tube. Add
2 drops of water. Heat gently until all the crystals have dissolved. They appear to "melt".
Leave to cool. Crystals may not form unless you drop a tiny seed crystal of sodium
thiosulphate into the solution. Then crystal growth commences and spreads rapidly through
the whole solution. Watch the growth from one centre. Hold the tube in the hand while
crystallization occurs.
(b) Crystals of naphthalene grow from the melt
Put a little naphthalene on a glass slide. Hold over a flame until the crystals melt. Put a
cover slip over the liquid and allow to cool. Watch the crystals grow using a hand lens.
Sometimes crystals will grow from several points simultaneously to make boundaries where
they meet. Draw the shape of the boundary between the forming crystals and the melt.
View the crystals through Polaroid filters.
(c) Grow large crystals
(i) Use a 0.5 - 0.8 cm long seed crystal to start growing large crystals. Make seed crystals
by slow evaporation of 30 mL of saturated solution in a glass dish. Dry a selected seed
crystal. Tie a piece of clean cotton around it without touching the seed crystal with your
hands because impurities easily affect the size and shape of the crystal. Hang the seed
crystal 5 cm from the base of the container with a bent wire. Fill a jar (with a screw-on lid)
with a solution of the salt less than saturation strength before you put the seed crystal in
position. If little crystals grow on the surface of the seed crystal, then screw on the lid of
the jar to make the little crystals dissolve. Prevent crystals growing on the sides of a
crystallizing dish by rubbing Vaseline round the upper inside rim. Evaporation may be
increased by sitting the crystal growing jar on a tin with a 5 watt bulb mounted inside it. An
air flow over the solution surface given by a fan will also hasten crystal growth. You can
preserve large crystals by painting with a clear varnish.
(ii) You can also support the seed crystal at the end of a glass tube. Heat a 3 mm bore piece
of glass tubing in a flame until the end softens sufficiently to squeeze with pliers to make a
smaller hole. When the glass tubing cools, drop seed crystals on its end until one catches in
the smaller hole. Keep this crystal in place by dropping other crystals on it. Support the
glass tubing vertically so that the seed crystal at the end is immersed in the solution of the
salt. The seed crystal should then grow. Turn the crystal regularly so that growth on all
faces is equal.
sharon kalunda answered the question on April 12, 2019 at 07:59
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