we've done lots of experiments with the
alkali metals don't pop it into the
water and see what we can do with the
reaction you can see the orange sodium
flame but I'd technician Neil felt that
we weren't really doing them
scientifically enough we were just
dropping alkali metals into water and
seeing what happened and we've also done
the reverse experiment water falling
onto sodium but what Neil felt is we
should do something that was much more
controlled he wanted to break ampoules
glass vessels containing the alkali
metals under water so we can watch the
reaction before there is any air present
and also he had the second idea that we
should compare the same number of atoms
of the three metals that he chose sodium
potassium and cesium but today I'm going
to show you the first experiment which
was with sodium Neil's device begins
with a plastic box which is filled with
water with flat sides so you can video
and you can see both under the water and
what's happening on the surface inside
is a device which he hasn't made the
name yet a sort of like a thumb screw
but for test tubes where by turning a
shaft which goes a long way away from
the trough he can force a metal pin into
the side of the test tube
so that it breaks and you can film the
whole thing at different speeds and from
different angles
when the tube breaks water rushes in and
the reaction starts and the only way
that the metal can become in contact
with air is if it floats up to the
surface in our cesium tube it was
evacuated without any air in some of the
other tubes there was air you will see
in one of our videos the tube wasn't
quite properly sealed so it was full of
air but the amount of air in the tube is
very small compared to the amount of
hydrogen that you expect to be generated
by full reaction the experiments were
done outside so that we had bright
sunlight and also so if there was an
explosion it wouldn't cause any harm in
the building what you see is that as the
pin starts breaking and the bubbles are
coming out I believe are hydrogen and
the hydrogen just bubbles up to the
surface when it gets to the surface
there is some cloudiness which i think
is probably just an optical illusion
from the Sun there may be some mist of
water inside it because from some angles
they look transparent on the other hand
Neil and Brady think it might be
something more interesting and
unprepared to be corrected if we do more
experiments once the reaction gets going
you get more and more bubbles but to
begin with there isn't much evidence of
any fire or anything burning the
hydrogen just dissipates and then
gradually the reaction goes more rapidly
at the end there is a huge rush which in
our first video was about sodium
effectively wipes everything out on the
screen
partly because I think heat which is
generated by these reactions is
registered on video screens as just wet
light what I think happens at the end is
that sodium which has a density that is
quite similar to water is carried up by
the hydrogen bubble
and it's all reacts on the surface and
with the air and explodes there are a
number of questions that will strike you
immediately the first thing is it all
looks very rapid but from the point of
view of a chemist it's really pretty
slow it took a good fraction of a second
really fast chemical reactions take
place in a nanosecond a thousand
millionth of a second and here we're
talking point one of a second something
that's quite slow so this reaction is
fast but it is slow enough for
mechanical things like breaking of glass
and so on to take place what of course
we haven't done its we haven't analyzed
the bubbles and shown that they're
hydrogen but part of the reason for this
is that the reaction of alkali metals
and water is a very well-researched area
and I've got here a PhD thesis reactions
of the liquid alkali metals with water
which was published in 1976 this is not
cutting-edge research of course we now
have high-speed cameras so that we can
show you videos nice and easily but when
these guys were researching they had
high-speed cine cameras unfortunately
we've lost the film but we do have some
nice photos so what we take with this is
that a lot of this is already known the
reason why this research was being done
was because in the 1970s people were
building nuclear reactors with hundreds
of tons of liquid sodium in them which
also had water cooling and people wanted
to know what would happen if the sodium
leaked into the water system so I have
an eight-year-old colleague now retired
who supervised this research he looks at
all our videos with some amusement and
says when will the young generation ever
learn his point of view is that
you could save a lot of research time by
reading the old papers in fact there is
a cliche that you can save six months in
the lamp with two hours in the library
so you've seen sodium well you clearly
waiting like need to see potassium and
cesium but those will be in our next
videos