We're going to do a very well known experiment called the elephant's toothpaste.
We're using concentrated hydrogen peroxide, as you can see.
We're going to put some of that in our 5 litre conical.
And then I'll explain the rest as we go along.
There are lots of videos on YouTube of this already
So, we'd thought we'd explain—or try and explain—why it's so spectacular.
And of course, Brady filmed it in slow-motion,
which allows you to see quite a lot of the detail as the reaction develops.
Final ingredient to our reaction is a saturated solution of potassium iodide.
So this is what's going to make this reaction go.
So, at this point I think, Neil, you need to move that way, because I'm going to
very rapidly be going that way as well once this has been added in.
So, are you ready?
3, 2, 1
The reaction is a reaction of hydrogen peroxide.
I've got rather a crude model here.
It is H2O2: Two hydrogens, two oxygens
The key part about this molecule is that the bond between the two oxygens
is rather long and rather weak.
You may remember that
when you break bonds, you have to put energy into a molecule.
And when you make bonds, you get energy back.
The overall energy that you have from a reaction, that's where the heat comes out
or you have to put heat in, depends on the balance between
the energy needed to break the bonds, and the energy needed to make them.
So, in this case,
the reaction is, overall, that two molecules H2O2
form two molecules of water, and one molecule of O2.
And, O2 (oxygen) has a double bond between the two oxygen atoms.
So, if you look at it overall,
all you have done is to break two single bonds and make one double bond.
Because in both of them, you have four O-H bonds.
Four here, and four here.
And usually,
the key to this reaction is that two single bonds need less energy to break them
than you get back when you've formed one double bond.
That is, the double bond is more than twice as strong as two single bonds.
So, you're breaking two single bonds between oxygen atoms,
making one double bond between two oxygen atoms and getting energy out.
And this is what makes the reaction work.
Now, like most reactions,
you need some energy to get it started.
Because hydrogen peroxide does not spontaneously fall apart at room temperature.
So, in this reaction
in the demonstration, you have hydrogen peroxide,
a bit of soap to cause foam
Little bit of soap into there
—Is it just like detergent, or? —Yeah.
Just washing-up liquid, liquid detergent, whatever you want to call it.
Sometimes, people put in a bit of coloring so it looks more dramatic, the foam.
And then you have to drop in a catalyst, which
reduces the amount of energy needed to break the first few bonds.
Now, the final ingredient to our reaction
is a saturated solution of potassium iodide.
There's a choice of different catalysts you can use
Sam used potassium iodide.
The potassium is not important, it's the iodide itself
which can lower the energy needed to break the oxygen-oxygen bond.
And because all molecules
at room temperature have a certain amount of energy,
with a catalyst, there's enough energy in the system to get the reaction started.
3, 2, 1
Now, once the reaction starts,
each molecule that reacts gives out some heat.
So, the temperature rises,
chemical reactions go faster, as the temperature rises,
so more molecules react,
more heat comes out,
and you get a runaway process.
So, the reaction goes faster, and faster, and faster.
So, what he's doing now is actually using the oxygen of the foam to keep that splint lit.
If you look at the reaction products, oxygen is a gas.
So, you've got a liquid that is producing a gas, which has a much bigger volume than the liquid.
And also, because the reaction mixture is heating up,
you're also producing some steam.That's the vapour of water.
Though, the volume of steam is probably much less than the volume of the gas.
The material needs to expand, because the gas is at room temperature,
so it expands. And by having a soapy solution, you can make a foam
so people can actually see the gas expanding.
The reaction would work just as well without the soap, but then you wouldn't see the oxygen coming out.
There is a second very important and totally physical aspect to this reaction,
and that is that the reaction is done in a conical flask
with sloping sides to a relatively small diameter neck.
And you will see the same thing in many of the demonstrations on YouTube.
So, what happens that as the foam rises,
because the tube is getting narrower, the speed at which is goes up has to accelerate.
And so, it comes up with really quite a high speed,
and shoots up into the air.
And, of course, as you get the column of the foam going up, there's nothing supporting it,
so when it runs out of momentum, gravity makes it collapse down again.
I was really excited to see that foam
overcoming a little camera at the side.
So now, we can actually demonstrate that this conical flask is important
because Sam wanted to do the reaction in
one of our big test tubes. Our barking dog tubes
which have straight sides.
She thought it would be really spectacular.
Neil thought it would be a waste of time.
In fact, they were both wrong.
It was not bad, but not nearly as dramatic as in a conical flask.
Wow!
All right, you were right! Okay, you're were right.
The takeaway message of this demonstration
is that the underlying chemistry provides the energy to get the reaction going.
But, you need a catalyst to set it off.
And then, the buildup of heat and the conical flask
gets it to go with a real "WOOSH!"
So, next time when you see the elephant's toothpaste, think of the science.
Not the clowning of the people doing it.