this word is
crystallization or perhaps also
recrystallization because chemists
always want to get the chemicals they
make as pure as
possible and a good way of purifying
chemicals is to make crystals of them
when they're in solution you can have
all sorts of
impurities but when they form crystals
the cryst crystals contain a much purer
compound than in the solution and the
impurities are left in the solution
generally the crystal is a very precise
arrangement of molecules all the same
fitting together like pieces in some
sort of construction toy the impurity
has a different shape so it doesn't fit
in properly occasionally you're unlucky
and you get an impurity that's the wrong
shape and then you can't get rid of it
and each time you recrystallize the
thing so you make some solution form
some crystals filter them out redissolve
them form more crystals and so on and
each time you crystallize it you get a
purer and purer
compound sometimes in the old days
people crystallized thousands of times
to get something really
pure now the problem is that when you
have a solution
even if you want cool it down which is
the standard way of getting crystals to
form the crystals cannot form unless you
give something small some tiny bit for
them to First Crystal to form Round And
once the first one goes the whole lot
goes sometimes it can just be dirt there
is a famous organic chemist here called
Perkins who invented the D called Mo
but he was famous because he had this
huge beard and it was said he was really
successful as a chemist because all
sorts of bits fell from his beard into
the solution and helped the crystals
grow in the solution there are other
examples where people sometimes scratch
the side of their
Beaker this forms tiny fragments of
glass so that they also can act as a
nucleus for the crystals formed around
one of my friends David Jones found a
student scratching the wall of the lab
with a glass rod and when he was asked
why he said because in the instructions
it says scratch the wall with a glass
rod to get
crystallization but most chemistry
students are much brighter than that so
this is a
crystallization of a really concentrated
solution which is used for warming your
hands on cold days so in here we have a
solution I believe of sodium
acetate and it's been sitting in my desk
drawer for
years because although it's a very
concentrated
solution there's nothing to start at
crystallizing and inside here there is a
small metal disc you can see floating
around and I believe it may not work
that if we Flex this disc it will be
enough to start within
crystallizing so let's see what happens
we Flex the
disc there it's
going can you see the crystals spreading
out and it's getting very warm why is it
getting warm it's getting warm because
when crystals form when you bring the
ions together they generate
heat so if you feel it put your hand
down here it's nice and warm oh yeah so
it's gone almost solid and you can see
lines of crystals that have
grown along here it's now
crystallized so what do you do if you're
rich you just throw it away if not you
boil it for some hours in water the
solution gets very hot and everything
redissolves and then if you allow it to
cool slowly
and you've dissolved up all the crystals
it will go into a solution like it was
before and can sit in my desk drawer for
several more years