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.