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today I want to show you a pair of
reactions that I think demonstrate
really quite nicely the difference
between detonation and deflagration the
two sorts of processes that can occur in
explosive reactions when you actually
watch them there isn't all that much
difference at normal speed detonation
sounds louder you put your hands over
your ears but they're both pretty
spectacular
so the first reaction which is in the
detonation is a reaction between red
phosphorus the less reactive allotrope
of phosphorus and potassium chlorate
which is used for percussion caps in toy
guns and also for the old-fashioned guns
when you fired these muskets and so on
they often use the percussion cap to get
it going
this reaction you take a mixture of the
two and put it on the surface on the
little plate and then hit it with a
hammer when the hammer comes down it has
quite a lot of kinetic energy and some
of that kinetic energy is converted into
heat by the shockwave and that is enough
to cause the detonation and as you watch
with a hammer coming down as it hits you
can see this explosion shooting out
sideways when it's left afterwards the
residue you can see a ring of red
phosphorus where it has been blown apart
it's called the detonation because it
goes off very suddenly one minute it's
sitting there and bang it's gone and
this is used to set off other explosives
and most explosives need to have a
detonator in order to make them go off
the detonator will be a reaction which
is very often
fired by mechanical force for example
the pin when you're firing a bullet the
detonation is needed to set off the
explosion quickly when you fire a bullet
you want the gun to go off as quickly as
possible or your enemy may be upon you
before the bullet has come out but you
don't want the main explosive in the
barrel of your gun to go off really
quickly because it would just blow the
gun apart and you would injure yourself
and your enemy would arrive uninjured
the second demonstration is a much more
smooth and slow reaction which is the
reaction of cotton wool with liquid
oxygen you've seen us demonstrating this
before but when you've seen it it has
gone with a great push okay because
we've been filming it at normal speed
when you film it at high speed it's much
slower and more poetic you see the match
slowly approaching the cotton wool and
then the cotton wool begins to warm up
inside partly because the concentration
of oxygen is probably higher inside
because it's trapped in the fibers and
also because the heat can't get out so
fast so you can see the reaction
building up and suddenly the flame comes
out and then the reaction builds up with
more and more intensity until it
completely washes out that the picture
and you have this huge white screen in
front of you this slow buildup is just
what you need say in a gun barrel to
accelerate the projectile as it's coming
out without blowing up the gun itself
but I think in this video what is nice
is that you can see that after this
there one or two pieces of the cotton
will
that the left almost completely
untouched presumably on the outside and
there's one crazy piece that takes off
rather like the lunar module coming off
the surface of the moon and disappears
out at the top and then some white
pieces gradually come down again so in
terms of watching it the deflagration is
really much more interesting but
chemists use the power of explosives by
choosing whether they have a
deflagration explosive or a detonating
explosive so that they can get the
effect they want from the particular use
of these explosives and quite often the
real effect is using the two together a
detonator to get the thing to start just
when you want and then a deflagration
explosive to produce the steady push
that you want to propel some sort of
project time
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