(gas hissing)
It's not often that
I think of an experiment for Periodic Videos
where I really want to know the answer,
rather than just having a regular demonstration.
Today, I thought of one of those experiements.
There is quite a well known effect
-known as a fuel-coolant interaction-
where a molten metal
drops, say, into water and causes an explosion.
And we've sort of had some of those
in some of our videos
with caesium and sodium in water;
But there you have a metal which gets hot through a chemical reaction.
I had suddenly thought:
Why don't we try dropping mercury
into liquid nitrogen
because mercury is hot, it's room temperature
liquid nitrogen is at -196° C
The hypothesis is that because the metal conducts heat so well,
the heat will come out very quickly and there'll be a
*woosh*
as all the nitrogen vaporizes.
The experiment was done in three stages.
So first of all: Liquid nitrogen with solid copper.
Same temperature as the mercury but because it's solid
there's no change in surface area,
it can't break up into small droplets
so it should be safer.
So you put it in and much to our pleasure
it really looked quite nice.
(ethereal music)
So, that part at least was safe.
But just to be doubly sure
he then dropped the mercury the first time
into a really big vessel
so that even if there was splashing and so on
none would come out.
(ethereal music)
The thing that particularly pleased me was that
when the mercury froze, because the freezing point of
mercury is -40° C,
there were quite nice crystals that you could see formed
on the surface of the metal
and there was this great lump of mercury at the bottom
solid.
But Neil had quite a lot of difficulty getting it out
at the end and it began to melt and so on.
But that wasn't part of the experiment
Then came the real experiment:
A small vacuum flask
where you can see inside without condensation
on the surface from the vapor pressure in the air
and he dropped in the mercury.
What's interesting is that as it goes into the liquid nitrogen,
the nitrogen boils, and so the mercury falls
almost encased in this little bubble
of nitrogen gas.
Then, when it gets to the bottom it goes
*splat* on the bottom, spreads out,
and then the transfer of heat
becomes much more violent
and there is a huge upwelling.
Mercury going this way,
and bubbles of gas going up.
(gaseous boiling sounds)
As an experiment it was really nice
it looked really good.
I'm not completely convinced that it demonstrates
how well heat is transferred from a liquid,
but you can see that it's far more violent
than when you drop in a lump of copper
because the surface area of the mercury
is much, much larger
and the heat travels out through the surface
through the interface between the liquid nitrogen
and the metal
So, I think
in the end, we have demonstrated the point.
That even with a difference of just over 200° C
you can get quite a violent boiling
Imagine now if you put in really hot mercury
if you put in boiling mercury into liquid nitrogen,
then you might get really quite a bang.
You might even break the vessel.
(Brady: Are you going to let us do that one, professor?)
Well I'd be happy for you to do it
Neil would not be happy,
we would smash his vessel
and he'd have to clear up the mess as well.