we've been using a high-speed camera to
film hydrogen balloons exploding ha ha
ha ha I like you Neil
it's given me the answer or nearly give
me the answer to a question that's been
puzzling me for years
ever since our first element video we've
been using the match on the stick to
blow up balloons of hydrogen and the
thing that's always puzzled me is that
when they blow up the flame has a
reddish color and it's well known that
hydrogen when it burns for example in a
Bunsen burner or something like that
gives an almost colourless flame so why
should the balloon give a red flame or
reddish flame when it blows up and I
came up with an idea on one of our
videos that it may be the rubber of the
balloon that's burning recently I got an
email from one of our viewers called
Paul who said he thought I was wrong and
he thought that the hydrogen in the
balloon and the oxygen in the air are
not properly mixed so when the hydrogen
starts burning there's much more
hydrogen than oxygen and the heat from
the first part of the flame heats up the
rest of the gas and the hydrogen gas
gives out red light we were quite
interested to see if this was really
true and the obvious way to do is to
film the balloon going off in high speed
to see what happens to the rubber when
you light the balloon when we looked at
the video it was extraordinary the
rubber disappears and flies off into the
air before the gas really starts burning
at all and you can see the flames slowly
going through this massive gas sort of
balloon shaped mass of gas
so the orange color cannot be anything
to do with the rubber from the balloon
because that's flown away in one of our
videos
it goes up towards the ceiling so I was
wrong
and it's always good for a scientist to
be proved wrong but now you've got to do
an experiment to see what it might be
and Paul suggested an experiment he said
try burning a balloon filled with a
proper mixture of two parts of hydrogen
and one part of oxygen water is h2o so
what you need is twice as much hydrogen
as oxygen this is what chemists call a
stoichiometric mixture that's a mixture
that's just the right amount to give you
the product the problem is of course
that it explodes with a very loud bang
and we would quite frightened you know
it might might be bad for all sorts of
reasons since the gas is already mixed
the reaction will go faster so you get
much bigger shock wave so we've tried
the experiment and let's watch what
happens
if you look carefully you can see that
as the flame touches the balloon the
reaction goes really fast the balloon
lights up like an electric lightbulb
before it's lost its shape at all and
then it slowly spreads the bright light
which is much brighter than with the
hydrogen by itself because the reactions
going faster so the temperature goes up
much higher and what's interesting is
that the flame separates into two parts
from the point where you add the match
this is actually quite a well-known
effect when there gas explosions in
people's houses when they light a match
when there's gas in their house very
often the person who likes the match is
okay because the explosion spreads away
from them and it does huge damage and
they left in the middle feeling a bit
stupid because the house has gone it
really looks as if Paul is right that is
that the reaction goes completely
differently I really feel that I have
learned something about this reaction I
was talking to quite a senior physicist
yesterday over lunch and he was amazed
that the balloon would burst and the
rubber disappeared before the hydrogen
caught fire and so was I because it all
happened so fast that normally one
doesn't see this and your gut feeling is
that hydrogen explodes easily so of
course it'll blow up long before the
balloon does