Science Interviews


Mon, 2nd Mar 2015

The explosive history of hydrogen

Dr Peter Wothers, University of Cambridge

Listen Now    Download as mp3 from the show Eureka! Experiments that Changed the World

In the 1600s Robert Boyle showed that he could make a gas when he mixed iron filings with an acid, but no-one knew exactly what it was he had made. It took another 100 years for an ex-Cambridge scientist,Hydrogen Bubbles Henry Cavendish (who actually left the University without a degree!) to figure out that it can combine with oxygen to make water. Chemist Peter Wothers told Ginny Smith about the explosive history of hydrogen...

Peter - Hydrogen had been discovered even before Boyle first prepared it. There was a physician, Van Helmont, who invented the word gas. He noticed these bubbles that come out when you mix acids with different substances and one of them was with metals, just like Boyle later looked at. But Boyle noticed that this gas was flammable. But later, this was studied. This was in 1736 now at the Royal Society, John (Mord) brought some samples to demonstrate in front of his colleagues there this amazing flammable gas. Now, I brought a balloon with me. If I just pick this up, you can probably hear that squeaky. He didn’t have balloons in those days. They had to use bladders. Okay, so this was the bladder from an animal.

Ginny - I'm quite glad you didn’t bring a bladder full of hydrogen.

Peter - So, he had some bladders filled with this flammable gas and he amazed his colleagues by igniting this gas. And we’re going to do this now.

Ginny - Are you sure this is a good idea?

Peter - Apparently, it’s never been done here before so I hope the ceiling isn’t too dusty.

Ginny - Okay, should we have a countdown everyone? 3, 2, 1... That was brilliant! So, there was a sort of fireball. I could feel the heat coming off it. but it was over very quickly and you could probably hear that kind of popping noise.

Peter - Exactly. So, this was the reaction of course with the hydrogen. Well, we now know with the oxygen from the air. But it really wasn’t understood at this time in 1736 what was going on. in fact, there's a lot of confusion between other flammable gases as well. I mean, we’re all familiar that we use gas for cooking at home. But that's not hydrogen of course. This is methane predominantly, but this is also flammable. So, these gases were mixed up. I mean, miners knew about flammable gases. They came across methane and so on and terrible explosions. And they compared in 1736 at the Royal Society, samples that had been brought up out of mines with this hydrogen. They noticed the flame is slightly different colour, but they didn’t know what was going on. now, we move to Henry Cavendish. He prepared hydrogen in 1766 and he’s credited with the discovery because he was really carful about how we prepared and he measured the density of this, and he studied very, very carefully. And he also mixed up different proportions of hydrogen with the air. He noticed that some of them went bang rather loudly. In fact, he worked out the best sort of ratio to get the biggest bang when the hydrogen combines with different volumes of air. But still, he didn’t know what was actually happening. A few years later, somebody noticed that they were exploding different amounts of hydrogen in air because it’s such fun and then they started exploding hydrogen with oxygen after oxygen had been discovered around 1774. And then they were mixing these proportions together, hydrogen, oxygen getting bigger bangs and then somebody noticed when you do this in a close container that there's a little sort of dew forming around the sides of the vessel. And this was the key observation that then Cavendish picked up on this and he started to study this really carefully and try to measure, are we forming water from these two gases from hydrogen and oxygen. Now, I thought we should try this now and this one that I need a volunteer from the audience.

Freddie - Freddie Osborne.

Ginny - What do we need Freddie to do? We’re not going to blow him up like we did the balloon, are we?

Peter - Sort of. But with the balloon, that was just filled with hydrogen. So, there's a nice sort of whoosh, a nice bit of heat. but now, we’re going to mix up hydrogen and oxygen and this is rather different.

Ginny - Now, didn’t you say that was going to be more explosive?

Peter - Exactly.

Ginny - Freddie is looking a little worried here.

Peter - So, we better give Freddie some safety goggles first of all. You put those one. Okay, so now, Freddie, which hand do you write with?

Freddie - The right.

Peter - The right hand, okay. We’ll use your left hand just to be on the safe side there. Now Freddie, I'm going to put into your hand just some – it’s washing up liquid. You see if you can keep this in your hand. So, you want cup your hand.

Ginny - So, we’re just putting a few drops of nice green washing up liquid into Freddie’s hand.

Peter - What I'm doing over here was something that Cavendish certainly didn’t do. We are splitting up some water here and this is making just the right proportions of hydrogen and oxygen mixed together.

Ginny - There's a little kind of glass jar with a couple of crocodile clips and then a tube coming out of the top. And then the end of the tube is going into Freddie’s hand where he’s got a little pool of washing up liquid and that's making lots of nice bubbles of the gas that's coming out of the jar. What's actually going on in this jar?

Peter - So, this is where we’re using electricity to split up the water. And as I say, this is something that Cavendish didn’t do. He had to prepare his two gases separately and then mix them together. So, he was preparing hydrogen, preparing oxygen, mixing them together. But this is what we’ve done here. We’ve just used the electricity to split up the water.

Ginny - So, rather than taking the hydrogen and the oxygen, combining them to make water, we’re doing the opposite, just so that we get the right amounts of hydrogen and oxygen for a really good bang.

Peter - Exactly. This is just for convenience here. So, the gas is now collecting in the bubbles here. So, these look like just the normal foam that you would find on the top of your washing up liquid on the bowl when you're doing the washing up. But these are, of course, filled with explosive mixture of hydrogen and oxygen, not normal fairy liquid bubbles at all. Now, we seem to have got quite a few bubbles. I'm just going to put these ear defenders on you Freddie. Can you hear me okay?

Freddie - Yup!

Peter - Okay, right. and now, we’re going to explode the mixture.

Ginny - So, Freddie has got a nice handful full of bubbles. Peter is going to light a match.

Peter - Here we go.

Ginny - And just to reassure our listeners, Freddie still has his hand. It’s in one piece. And what did it feel like?

Freddie - Not much. I don't really know how to describe it, but it didn’t really feel.

Ginny - You can't really anything.

Freddie - No.

Peter - So, all of the energy there was going into the sound really. So, it sounded very loud but yes, it wasn’t painful at all.

Ginny - And a round of applause for our brave volunteer. How did this change the way people thought about chemistry? I mean, water is something that's kind of so important and all around us all the time. And this was the first time people really understood what it was made of.

Peter - Not quite actually. they were still reluctant. Even Cavendish was reluctant to really – he didn’t fully understand what was happening here. So, what he’s learnt is that hydrogen and oxygen can react together and water is formed. But the problem was that of course, there was this ingrained doctrine that the world was made up of just the four elements from the Greeks. This is air, earth, fire and of course, water. So water was thought to be one of the most primary substances and element. And yet, he just showed that these two gases combined to form water. He thought that actually the two gases here, they were modified forms of water. That the hydrogen he thought was – well something he called phlogiston He thought this was phlogiston or maybe water that has too much of this phlogiston . And he thought the oxygen was something that was lacking phlogiston. It was water that didn’t have enough phlogiston. And so, when these two things reacted together, water was formed. He really didn’t understand quite what was happening. And so, this was the real breakthrough that happened later when the French chemist Lavoisier, finally understood exactly what was happening in this process that these two substances, these elementary substances reacted together to form water. Water then was no longer an element but was actually a compound made up of the elements hydrogen and oxygen.

Ginny - And there's lots of modern science going into hydrogen as a form of power I guess because it can explode like that.

peter - Well, this is so important now because, because there's a lot of energy that's released, as we’ve seen with the balloons and as we’ve seen with the bubbles on the hand, a lot of energy can be given out from this reaction as hydrogen and oxygen combine. But the important thing here is that the by-product aside from all these energy is just water. So, unlike all the fossil fuels that we’re burning, that would release carbon dioxide that we’re now beginning to appreciate isn’t necessarily a good thing for our environment, of course, when hydrogen reacts with oxygen, the only product is water. So, this is a very clean energy source.


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