[Music and laughter]
This is amazing, you get this pulsating of the reaction.
My name is Steve Mould,
I've got my own YouTube channel where I make science videos.
Some of them are dangerous and if they are then I assess the risks.
It's an important part of my job.
But sometimes you find out the hard way that there are risks you didn't know about.
For example I was burning some aluminium powder for a project
and I wanted the fire to go out.
So I poured water onto it which you should never do.
So I made it my mission to figure out why that's problem,
why you can't put out a metal fire with a water fire extinguisher.
You can't put out a metal fire with a carbon dioxide fire extinguisher either
which is really counter intuitive.
So that's what this video is about.
We're going to use magnesium instead of aluminium.
It's a little easier to set light to.
This is magnesium turnings.
I'm going to spread it out a little bit.
[Wooshing sound]
Alright.
And let's see if we can put it out.
It will not go out with a carbon dioxide fire extinguisher.
If anything, it makes it worse.
The next thing is to try a water fire extinguisher.
This is really dangerous so we're going to go up onto the roof of the Ri.
We're going to use a squeezy bottle instead of an actual water fire extinguisher
just because I've got a bit more control over this.
Just going to let it get going.
That's crazy bright, isn't it? Alright.
[Laughter]
So with a water fire extinguisher it's even worse. It's explosive.
It's a bit like fireworks.
So why can't you put out a metal fire
with standard fire extinguishers like water and carbon dioxide?
To figure it out, let's do it again in a more controlled environment.
So I'm going to fill this beaker with gas carbon dioxide.
To do that I've got solid carbon dioxide in the bottom, dry ice,
if I add some water, it should start to fill with carbon dioxide,
and you can actually see that happening there.
And let's try and put out a normal fire with the carbon dioxide.
So I've got a piece of wood here.
We're going to set fire to it.
The wood is reacting with oxygen in the air.
That's the chemical reaction of burning.
It's the carbon in the wood.
And you also need heat as well. It's the fire triangle, right?
Fuel, heat and oxygen.
So what happens when I dip this into the carbon dioxide?
It goes out straight away.
That's because we're removing one of those things from the fire triangle.
We're removing oxygen. So the chemical reaction has to stop.
But let's try it again with magnesium.
It doesn't go out until it's all burnt.
So carbon dioxide can put out a wood fire for the reasons you already know.
But it can't put out a metal fire.
But actually when you think about it, it's weird that carbon dioxide,
it's weird that this can put out a wood fire.
Because if you think about it,
the explanation is that carbon dioxide starves the fire of oxygen.
Except that carbon dioxide has oxygen in it.
It has two oxygens for every carbon.
In that really hot environment of a fire, this carbon dioxide molecule is unstable.
The oxygen can be removed and the oxygen will preferentially stick to
or react with the atom that pulls on it more strongly.
And in the case of a metal fire it's the metal atoms that win
and you're left with carbon that is stripped of its oxygen.
Whereas with the wood fire you've got oxygen that's either going to
bond with the carbon in the wood
or it's going to stick with the carbon in the carbon dioxide.
There's no difference between the two, it's a tug of war between carbon and carbon.
Nothing is going to happen.
The case with water, it's the same thing happening.
The reason using water is even worse is that water can boil explosively,
taking bits of fuel with it and spreading it around.
And the other thing is, when you strip oxygen away from H2O, from water,
you're just left with hydrogen and hydrogen gas is also explosive.
So don't use carbon dioxide or water on a metal fire.
It's a class D fire so you have to use powder which basically is -
you drop a solid mass over the fire and that smothers the fire.
One final cool thing is that you can burn magnesium
inside a block of solid carbon dioxide, otherwise known as dry ice.
So we're going to give that a whirl.
We've created a little dip inside the block of carbon dioxide here.
We're going to fill that with some some magnesium.
So the heat from the fire is turning the solid carbon dioxide
into gaseous carbon dioxide. It's sublimation. It goes straight from a solid to a gas.
And then it's getting so hot
that the magnesium can steal oxygen away from the carbon dioxide
to form magnesium oxide. This is amazing.
You get this pulsating of the reaction. It glows and dies down and glows again.
That's because the oxygen is being used up, and then more of it is becoming available.
And I think now we're reaching the end
as that white light turns into more of an orange glow.
Just black body radiation because it's still hot in there.
Wow.
So there you go, more videos like that over on my channel.
I recommend you start with a video about chladni figures.
Link in the description.
And as always, don't forget to subscribe to the Ri channel as well.
Both channels are on Patreon so you can support us that way too.