I'm now going to do one of my favorite reactions.
It's not very spectacular,
but I think it is very satisfying.
It's the reaction between hydrogen peroxide and potassium permanganate.
Let's begin with a bit of water.
So I'll add some potassium permanganate,
which gives it a nice pinkish-purple colour.
You've probably heard me saying already that,
my grandfather used to put permanganate in his goldfish tank,
so the fish would swim around in purple water.
Didn't seem to do them any harm.
We now add a little bit of sulfuric acid.
The acid doesn't change the colour at all.
So you can see now we got some nice purple potassium permanganate,
and now we'll take some hydrogen peroxide.
Sam had a big bottle that she needs to be used up.
And we'll add it and see if we can see anything happening.
So watch carefully.
I think it may be flat.
Try another bottle.
We'll get some of my own hydrogen peroxide instead.
Brady: Gone flat?
Yes.
Well, hydrogen peroxide can decompose into oxygen and water.
And if you keep it too long, it goes off.
So I'll get some more.
So we got some more. There isn't really much of it but perhaps this will be enough.
I'm not very impressed. Do you?
Brady: It's not the best thing I've seen.
Hydrogen peroxide has the formula H2O2
And it has quite a strange structure.
This is a model that I use for the students.
I'm sorry that the atoms are all the same colour, but you imagine:
This one, and that one, the ones I'm holding with my hands, are hydrogens.
And the two black ones are oxygen.
And the important thing is you can see it has quite short hydrogen-oxygen bonds,
And a very long oxygen-oxygen bond, which is weak, and breaks very easily,
which is what makes hydrogen peroxide so reactive.
It also has this rather strange structure,
that this bond is at right angles to that one.
If I hold it up, this one is horizontal, that one is vertical.
And so it has a rather strange symmetrical structure.
I once hit myself very hard on the nose while demonstrating this in a lecture.
So you have a long oxygen-oxygen bond that makes hydrogen peroxide very reactive.
Pure hydrogen peroxide will set fire to almost all organic matter
So if you put it on a piece of cloth... Whoosh! It goes up in flames.
So normally people use hydrogen peroxide as a dilute solution.
And here, you can see we have one solution which is called 40 volume,
which means 1 litre of liquid gives 40 litres of oxygen gas, if it decomposes
And this sample here, which you can see, has more aggresive warnings, more dangerous,
This is 100 volume, and so the hydrogen peroxide that you buy from the chemists for bleaching your hair is much weaker.
It may only be 5 or 10 volumes, so this is 10 times stronger.
You wouldn't bleach your hair with this, might go up in smoke.
Trying again. This time I hope a bit better
I put warm water in here so the reaction may be faster, and I'll put in most of the acid as well.
Then a bit of permanganate.
(Brady laughs) And... Now we've got enough peroxide still left
You can see it's decolourising straight away.
(Chuckles)
That's pretty cool.
(Brady laughs)
Brady: What's going on there?
So what's happened now, I didn't wash the thing out properly, and we had some peroxide left.
And now it's reacting faster than we can add the permanganate
(Chuckles)
So this is actually much better than I intended,
because I was going to make it purple and make it go clear,
and here you can see I'm pouring in a purple solution, and it's disappearing!
Brady: That's brilliant!
And you can see, it's releasing oxygen from the hydrogen peroxide
So when it hits the surface, the manganese, which is bound to oxygen in permanganate,
reacts and loses its oxygen and forms manganese ions, manganese 2+ (Mn2+) , for those of you chemists,
which are completely colourless.
So it goes from this very intensely coloured salt to the colourless one.
So it's like magic, isn't it?
Brady: You'll have to remember this trick. Professor: Yes.
Brady: Lovely.
Brady: This wasn't what you wanted to show was it?
No, but it's even better
Then I now have to explain to the technicians why I've used the whole bottle of permanganate solution.