We've been cleaning out a lab, so they can be refurbished, so we can teach our students better.
And, hidden at the back, we found a really long tube
which Pete License had made to do the barking dog experiment, our favorite experiment.
But, he'd found that it was a bit awkward to use in lectures, so it'd been forgotten
Barking dogs are usually done in quite a short tube.
A bit over a meter long.
And this one is seriously long.
Probably three or even four meters long
So, you probably all remember because you've seen it lots of times in our videos.
It's the reaction of carbon disulfide with N2O, nitrous oxide.
And we got our barking dog ready to go. Let's see if it barks
So, you fill the tube with nitrous oxide gas,
add some water.
Then you pour in carbon disulfide that's a liquid.
And then you shake the tube, and the water mixes everything up.
[Give a good] mix so you can ensure that we get all of that carbon disulfide going into the vapor phase
so it'll react really quite quickly with our oxidant, the N2O.
So the water, as you turn the tube, sloshes up and down
and acts rather like a piston to help mix the carbon disulfide
which is evaporating with the N2O, so you get a gas phase mixture.
And, of course, the more you shake it,
all being well, the better it's mixed.
So, with the first one we tried, what happened was that we were in a bit of a hurry.
And I don't think, perhaps, we mixed it well enough.
Because what happened was that, when the Tube was lit,
the first 30 centimeters also lit up and the flame was almost stationary.
Neil had time to walk away from the experiment.
And then, slowly, the flame started accelerating down the tube.
And then, surprisingly, in the middle, it almost stopped,
might have even gone up a bit.
And then, it went with the nice woush down to the bottom.
And it gave a very nice deep note
which those of you who study physics will realize the longer a tube you have, as in an organ tube, the deeper the note.
So that wasn't surprising.
But, what I thought was that, perhaps,
some of the effect that we'd seen was because of the bad mixing.
So, in some places, we had almost too much carbon disulfide.
It was rich in fuel.
And, in other places, there was a lot of nitrous oxide and not much carbon disulfide
which explained this sort of stop start of the whole thing.
The other thing which you might see in, particularly the slow-motion pictures,
is that the reaction gives out quite a lot of ultraviolet light
which makes Neil's white Lab coat fluoresce.
It suddenly looks purple, and quite impressively so.
I persuaded Neil to clean the tube,
and try an experiment the next day.
Now, the reason why it took some persuading
is because, when the sulfur is formed on the walls,
it's initially formed in a form of sulfur that's called plastic sulfur
which is made up of long chains of sulfur atoms
which are insoluble in most common solvents.
So, you can't just wash it out.
However, if you wait a few days,
then the plastic sulfur turns into so-called rhombic sulfur
which contains S8 rings
which is dissolved in all sorts of common solvents.
So, you just put in this liquid and you can wash it out in a few minutes.
So, normally, we just wait and then it's easy to clean.
But, this time, Neil was really scrubbing hard.
The next day, we tried again.
He mixed for a long time.
And, this time, the reaction went much better.
It still began with quite a slow start.
But, then it went there more smoothly, with some bouncing like we've seen before.
Possibly the bouncing again due to the shockwave hitting the bottom.
And we got some very nice fluorescence from Neil's lab coat.
And the sound was really a very deep woom.
But, if you look at the tube, the sulfur is still not deposited uniformly.
So, I think we got the mixing good enough to get a good demonstration.
We didn't get the mixing good enough to get a completely uniform reaction.
And I think this illustrates a very important point about chemical reactions
that mixing can be very important for the results.
And the bigger the vessel you have,
the more important it is to get your mixing uniform or you can get some very strange results.
The second thing was that we noticed something that we've seen before.
The flame shoots along the tube and then, suddenly, stops and bounces
and goes rather more slowly, but oscillating.
