It's always good to start video off with a bang and there could hardly be an easier way
of doing that than with this stuff.
What we have here is a contact explosive called nitrogen triiodide.
It's a highly unstable substance that detonates when disturbed.
Now you might have heard of nitroglycerine which is another kind of contact explosive which goes off when you hit it with a hammer.
but this stuff is in a league of its own. Even something like a mosquito landing on it will set it off.
Now the reason this compound is so unstable is all to down to how it's put together.
It's nitogren triiodide so we have a nitrogen atom - this is a trivalent atom. That means it likes to form three bonds with other atoms.
So when we bring it together with iodine, it forms bonds with three iodine molecules,
and they all go on this side of the nitrogen atom because of the way the nitrogen's electrons are arranged. So the iodines all
go on over here. And you can kind of see that there's a bit of a problem here which is that
there's not a lot of room for all these iodine atoms,
they're much bigger than the nitrogen atom and so they're all kind of squeezed together
and all repelling each other. And this creates something called bond strain or steric strain,
which makes the whole molecule less stable than you'd expect it to be given the components
Now chemists would say that this results in a low activation energy for
the decomposition of this molecule. Everyone else would probably say that
it is liable to fall apart.
One of the characteristic products of this reaction is the beautiful plume of purple iodine gas.
And you can see this again if I chuck this tiny piece of solid iodine onto this hot plate.
This iodine is formed along with nitrogen gas as a result of all those triiodide molecules falling to pieces and recombining into a different arrangement.
This recombination releases all of the energy, setting off more triiodide molecules, and more and more
in a chain reaction that races through the rest of the substance, decomposing all of the molecules
and releasing a huge amount of energy in a tiny fraction of a second.
Now we filmed the same reaction with a high speed camera to see just how fast it really is
Here at 37,000 frames a second
And again at a whopping 59,000 frames a second
Even at this highest frame rate, we can see that the reaction occurs across just a couple of frames
To go that fast, the shockwave that's propogating through the reaction must be travelling at a couple of miles per second.
That translates to around 7,000 miles an hour and that's actually about what we would expect to happen
because after all this is the propogation of a disturbance.
In other words, it's the movement of a sound wave
As this acoustically propagates a reaction, we should expect it to proceed at about the
speed of sound. Not the speed of sound in air but the much faster speed of sound in the solid NI3 crystals.
The speed of sound in solids is generally in that ballpark - a few thousand miles an hour.
Finally, when we played back the slow motion footage, there was one other thing that we saw.
If you look carefully you can see, maybe just for a frame or two, that the triiodide flashes as it detonates
And you can see this on both of the videos.
Now I'm not sure exactly what the cause of this is. It happens far too fast to see with a normal camera, or with the naked eye
I suspect it might be a result of all that heat being produced by the release of
such an enormous amount of energy in such a small space and short period of time.
And then that heat dissipates incredibly quickly as the gaseous products expand.
But maybe not though - if you have an alternative explanation or some
suggestions for how we might investigate this then drop in the comments below.
Otherwise that's all for now and I will leave you with a triiodide super montage