so what I thought we'd do is investigate
today an explosive mixture of methane
and air and to do that we're going to
use this very crude device it's just
simply can now me and Neil have doctored
it slightly by drilling a couple of
holes okay one in the bottom and one in
the top now what I'm going to do is I'm
going to fill the can from my gas tap so
I'm going to fill it up with gas so you
can hear it fill in methane is actually
found in our gas taps so here in our lab
we've got a gas tap here and methane
comes out of it along with some other
stuff as well to actually give it the
smell because normally me phone doesn't
really smell of anything so this is
methane carbon with four hydrogen's
around it ch4 methane is the simplest
compound of carbon and hydrogen and it
is enormous ly widespread molecule in
the world so what we're going to do is
actually use this to bubble methane gas
into a soapy water solution okay so
that's all is just a tub of soapy water
and it goes top goes on it is the basis
of so called natural gas the gas that's
found and deep in the earth and which
you can drill down and recover it's used
for heating and energy all over the
world so let's show you the combustion
of methane so simple combustion we're
going to use a Bunsen burner I just need
to light it so you can see the me
thing's burning quite nicely in the
atmospheric air and you're getting this
rather nice incandescent flame it's also
formed when plant material decomposes so
if you have a pond little lake and
plants material falls in and decomposes
on the bottom if you stick a stick in
bubbles of gas come up so all that's
happening and obviously I've got the
benefit be able to smell it as well as
here it is that the methane gas bubbles
are forming
so they're soap bubbles but instead of
it being air trapped inside each bubble
every single bubble there has
faith inside it if you collect the this
gas you can actually light it with a
match and it burns and I've done that
when I was younger and to prove its me
Fame and not just air that we're putting
into it I don't have match on a stick
like Pete does so that I don't burn
myself I'm going to coat myself in some
soapy water and then scoop up a handful
of this methane is also formed when
plant material decomposes inside the
digestive system of an animal it's
particularly dramatic and cows but it
happens inside you or me that this
material reacts is digested by bacteria
in your digestive system part of the
carpet and the food that you're eating
is converted to methane and you can
belch it out or expel it as wind from
the other end okay now then methane gas
and alkanes in general are very
energetic like you can see with a flame
and in fact it's similar compounds those
with a bigger perhaps a higher molecular
weight which are used to drive things
like cars buses and planes and they
basically use combustion or an explosive
combustion inside the combustion engine
of a car so you mix oxygen with the fuel
in this case we're going to use methane
in the appropriate amount such that they
mix to be stoichiometric Li correct such
that they explode into life and they
react instantaneously and cause a big
pop methane when it goes into the
atmosphere
behaves like carbon dioxide because it
can absorb radiation infrared radiation
and cause global warming and it causes
the absorbs the radiation via the
vibrations of the carbon hydrogen bonds
and the vibrations when they stretch and
also when they vibrate so called
scissors vibrations now what I'm going
to do is I'm going to fill the can from
my gas tank
so I'm going to fill it up with gas
methane in fact absorbs radiation much
more strongly than carbon dioxide in the
atmosphere but its life time the
molecule life in the atmosphere is
shorter because it eventually reacts
with oxygen and turns to carbon dioxide
but the methane that emitted by
intensive farming of cattle can be quite
a contribution to global warming okay
then when it's completed I'm going to
set fire to the top so now I have a
reservoir of gas inside my can which is
burning in the atmosphere okay now then
as we're using up the methane inside
something really cold happening
atmospheric air including oxygen is
being sucked in through the hole in the
bottom of the cap so as the methane is
being used more air is going in so if we
think about the can the amount of
methane is going down and the amount of
air is going up eventually they'll reach
an explosive regime and something
exciting might happen in the camp so
we're going to walk away from him a
little bit and wait so what we're
looking for now is the flame on the top
it's getting smaller and smaller and
smaller as the burning methane gets less
and less and less and then hopefully
that flame will pop back inside the
vessel and pop let's see it could take a
while because these cans they do take a
while to burn out so when the flame
disappears cover your ears it's not
going to be that bad
so inside the can we developed an
explosive regime or explosive
concentration of methane and air and it
instantaneously reacted and blew the top
off as it made lots and lots of hot co2
and hot water vapor