Boom! The Naked Scientists

To end our show this week, science demo superstar Dave Ansell showed Adam Murphy what happened when he put a lid on a fizzy drinks bottle containing liquid nitrogen...

Adam - We are almost at the end of our show, but we did call this show, Boom! The Naked Scientists, didn't we? I think it would be a little poor to leave you without actually giving you a proper boom. Now remember Katie at the start mentioned about covering your ears? Well we're getting into that part of the show now. So I'm going to go over to Dave who on his table has an ominous metal canister. So Dave, what is in the ominous metal canister?

Dave - So in here we have something which is a little bit chilly. Well by physics standards it's not particularly cold, it's about minus 196 degrees Celsius. This is what happens if you take normal nitrogen, which we're surrounded by in the atmosphere, and cool it down to minus 196.

Adam - And I'm guessing that this smoking plastic cup full of bubbling liquid is something I don't want to drink?

Dave - People have done this and died horribly.

Adam - so no then!

Dave - So this is nitrogen, it liquefies at minus 196 which means if it gets anything above minus 196 degrees Celsius, it boils and we can see kind of what effect that will happen on the liquid boiling by first cooling down this balloon.

Adam - Let's feel sorry for a balloon, this is a first.

Dave - This is a balloon I blew up with just my own breath earlier. You'll notice something happening to it.

Adam - You're putting the balloon in the thing. And that is far too big a balloon to go into that flask normally. So when you pull that out of the flask ... you have a tiny apple shaped little balloon.

Dave - I don't know if you can see the little bit of liquid, which is sloshing around in the bottom? Yeah. So that tiny amount of liquid is boiling to blow up the whole balloon again.

Adam - So is that, is that the air that's turned to liquid?

Dave - So we've liquified air, which takes up much less space than gaseous air. Um, so the balloon shrank and now it's boiled again and it's expanded about a thousand times, blowing up the balloon, which is why swallowing liquid nitrogen is a really bad idea.

Adam - Expanding would be bad. I don't want to be this balloon.

Dave - Yeah. Because basically you're 200 degrees Celsius above nitrogen's boiling point. Um, it's going to apply an immense pressure as it tries to turn into a gas and basically you're not going to be able to contain it.

Adam - So, pausing for effect, what can we do with putting liquid nitrogen in somewhere we're probably not supposed to unless you have scientific supervision!

Dave - And I've written many, many risk assessments over the years. Um, so this is actually one of the two major ways you can kill yourself with liquid nitrogen. The other one is by getting in a lift with it or in some kind of confined space, it excludes all the oxygen and you just suffocate. But the one we're looking at today is putting it into a sealed container. We have a half litre fizzy drinks bottle here, I am just going to put on some ear defenders as they will come in handy later.
Adam - Thank you. I was about to ask where are my ear defenders!?

Dave - And I'm now going to pour in some liquid nitrogen, at the moment this is completely safe. Apart from my fingers might get a little bit chilly.

Adam - So not completely, relatively.

Dave - So the point at which this becomes dangerous is if I put the lid on the bottle, because lemonade bottles are incredible pieces of modern engineering. They cost just a couple of pence and they're incredibly strong. They fail maybe at 10 atmospheres, so 10 times the pressure we feel now, which is about a hundred tons per square meter, it's so a lot, a lot of pressure. So putting the lid on is what makes this dangerous. To do that, I've got to come over here...
Adam - Safety first. We have a wheelie bin.

Dave - We have a wheelie bin to contain anything flying out. There's a wonderful video on The Naked Scientists website of this blowing up with no wheelie bin, and it's petrifying. So I'm about to put the lid on and shut the lid very quickly. Okay. Three, two, one...

Adam - Anyone else a little nervous? Going to slowly creep ... across the stage...

Dave - So what's happening very slowly is that -

[BOOM!]

Dave - The pressure built up until it went bang!