now I've seen this reaction lots of
times but this is the first time I've
seen it in high speed where we pull the
solution in quite
slowly but then Watch What
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Happens the reaction of copper with
ammonia is quite a well-known reaction
and
often students school children come
across it quite early on in their
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career it's a reaction which gives both
a precipitate and then a color change
it involves copper sulfate which in
solution is a fairly pale
blue and you can imagine in the solution
you have copper ion
cu2+ surrounded by six water molecules
four that are close together and two
that are rather further apart this does
not absorb light very strongly so the
solution looks very pale when you add
ammonia which is alkaline in when it's
dissolved in water water you precipitate
a pale blue
solid which is a mixture of copper
hydroxide and copper sulfate socalled
basic salt and then as you add more
ammonia this redissolves to give a very
intense blue color the blue color is due
to Copper ions the same copper ions but
now with four ammonia molecules
interacting with it in a tetrahedral
shape
this is a baby toy that I got in Italy
but it's quite fun so there aren't
really jewels and cheese attached to it
no no no this is just a there's Pizza as
well so when the copper goes from this
shape that is quite pale colored to this
one that is
tetrahedral the color comes much more
intense and it is a very nice
demonstration for people just beginning
chemistry to see how copper can form
so-called coordination complexes where
stable molecule like ammonia can
actually bind to The Copper ions and
what I find quite surprising is if you
watch
carefully it really goes
blue dark dark blue before you see the
precipitate whereas chemically it should
be the other way around you should see
the precipitate and then the dark blue
color and my only explanation and this
may well be wrong is that it takes time
for the precipitate to form crystals
that are big enough for you to see when
crystallites are very small their size
is shorter than the wavelength of light
so they appear invisible though they can
still absorb light and then as they grow
bigger you see physical clamps of them
because they're much bigger than the
wavelength of light and this growing of
particles can be relatively slow
compared to the initial precipitation
which forms really tiny ones when you
look at at the end it looks rather
beautiful it looks very like the planet
Earth seen from space
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