Dry Ice Experiments and Bombs

24 August 2009


Laura:: Recently, The Naked Scientists were visited by Dr Hal and his assistant, Dave Campbell from the University of Brighton. They stopped by to show us some of the amazing experiments they've been demonstrating on their Ministry of Chemistry Tour. First off, Dr Hal grabbed a handful of solid carbon dioxide and dropped it into a bucket of water.

Hal:: Here Dave, we've got a gas, it doesn't know it's a gas. It thinks as a solid. Carbon dioxide, solid CO2 at minus 78. It's a solid. It doesn't turn into a liquid. It turns straight into a gas. Look at that. Shooting right up there and what that's showing us is that gases occupy much more space than solids, which gave rise to them. We can see here that the solid is turning straight to a gas. It's going straight from solid to gas - you see the balls of gas coming from the (bits of) solids and you can also see that one mole of gas occupies far more space than one mole of solid.

Laura:: We saw huge plumes of carbon dioxide gas pumping out from the water, all from just a handful of solid carbon dioxide, subliming to produce much more gas than its volume as a solid. This happens because the molecules in the gas are free to move around, rather than being locked tightly together. But carbon dioxide gases are colourless and transparent, so the white smoky plume we could see was actually something else.

Hal:: Now, the white smoke you can see coming out, that's actually the water vapour. The CO2's propelling out the water vapour because it's still so cold, having to come out of the hot water, so we are making real clouds and where are they going, Dave?

Dave:: They're going down.

Hal:: Yep. There are clouds are going right down, proving that CO2 is denser than air. Now, the other interesting property of CO2 is we use it as a fire extinguisher. Okay, Dave, can you just light that splint for me?

Dave:: Sure.

Hal:: Dave's just lit a nice wooden splint and I'm putting into the CO2 smoke and the flame goes straight out.

Laura:: In order for something to burn, it requires a fuel. In this case, it was some wood and requires an oxidizer. In this case, oxygen from the air. By submerging the flame in the carbon dioxide gas, it was deprived of oxygen, stopping it from burning. More carbon dioxide coming up. But first, they look to the properties of liquid nitrogen. Unlike carbon dioxide, if you cool nitrogen gas to minus 196 degrees Celsius, it turns to a liquid.

Hal:: We've got some liquid nitrogen here and we're going to do the old liquid nitrogen and light bulb. Nitrogen is an inert gas, which means, although this looks suicidally stupid, I am indeed putting a light bulb into liquid nitrogen and I am going to turn it on. And what's going to happen is, I hope that light bulb will glow from the beautiful liquid nitrogen and make lots of bubbles and we can even get a raise in the number of bubble produced, as we turn the power up.

Laura:: We would never recommend you put a light bulb or any electrical equipment into a liquid. But because nitrogen doesn't conduct electricity, Dr. Hal was safe. Turning the power up gave the light bulb more energy which caused the nitrogen around it to bubble more vigorously, creating more bubbles. Next, Dr. Hal did something that seemed even more stupid.

Hal:: So now, we've got a light bulb with no glass because we broke the glass. I just got the filament which is made with tungsten and light bulbs normally have an inert gas atmosphere like argon.

Dave:: That's it?

Hal:: Because if you heat up tungsten, so it's red hot in air, of course it oxidizes and the light bulb burns out. And we've got liquid nitrogen which is of course air without the oxygen. And I'm going to put our light bulb without the glass into it and turn it on, and I think we can all see and hopefully, the listeners can hear that the bulb is glowing in our liquid nitrogen. But more importantly, it's not burning out.

Dave:: That is truly amazing.

Hal:: Yes.

Dave:: That is amazing. I mean, I can't - it's...

Hal:: And we have bright sparks.

Dave:: We certainly are.

Hal:: But if it goes wrong, it's not my fault.

Dave:: All right, yeah.

Hal:: We can leave electricity pumps out.

Dave: I think we should, yeah.

Hal:: Okay, so what we'll do is just pull this out and illustrate our theory that when I pull it out, it's going to glow for a little while and then it's going to go out all by itself.

Laura:: So, in the liquid nitrogen which doesn't react with all the chemicals the tungsten filament can become glowing hot with no ill effects, just like it would do in the argon gas inside a bulb. Once the filament is exposed to the air however, it reacts with oxygen and quickly burns out. Dr. Hal's explosive finale this week takes us back to solid carbon dioxide and the consequences of warming it up in a sealed container.

Hal:: What we got here, lemonade bottles.

Dave:: Yep.

Hal:: Polyethylene terephthalate, designed to withstand the internal pressure of carbonated drinks. This can withstand about six atmospheres. There's lots technology in them. They got little grooves in the threads of the lid doesn't pop when you open it up.

Dave:: Yep.

Hal:: And I was thinking to myself, what would happen if you put some carbon dioxide in which sublimes as we all know that is turns from a solid to a gas.

Dave:: Turned in to a gas, yeah.

Hal:: With some boiling water.

Dave:: Yes.

Hal:: And put a lid on and ran away. What should you think it's going to happen? I think the CO2 is gonna sublime and we've already worked out that one mole of a gas occupies much more than a mole of solid. So, I think it's going to have a big old bang. Now, these bottles withstand about six atmospheres. CO2 can produce, under certain circumstances, 50 atmospheres. So, it'll be a test of CO2 one: bottle nil. So, what I'm doing is I'm putting CO2 chips into the bottle. And the problem we're gonna have is, as soon as you put the boiling water in to sublime the CO2, the gas is going to try and get out. So, we have to use our ingenious long-stemmed funnel and a kettle of boiling water and what's going to happen is the water is going to splash around. Dave and I will get third degrees scalds, but as long as the Naked Scientists listeners get what they want, that's okay well.

Dave:: I think things are going to get exciting.

Hal:: So, let's remind everyone what's going on. We've got a lemonade bottle with about fifth full of solid CO2 that's put in 2 litres boiling water and then when we put the lid on and see what happens. Let nature take its course. What do you think, Dave?

Dave:: Okay.

Hal:: Okay. Three, two, one and you go, Dave?

Dave:: We're going. I'm pouring.

Hal:: Okay. Big white fumes coming out, hissing away. Keep going, Dave.

Dave:: I'm pouring. I'm pouring.

Hal:: Keep going, Dave. It's breaking up, Dave.

Dave:: I can't see.

Hal:: I can't afford this to go, Dave. Keep it going.

Dave:: I'm pouring. I'm pouring.

Hal:: Pouring, pouring, pouring. Come on.

Dave:: It's difficult to see. It's difficult to see.

Hal:: Keep going, Dave. Nearly there.

Dave:: Are you sure? Are you sure?

Hal:: Yeah.

Dave:: Keep pouring?

Hal:: Yeah, I think the water is full. It's getting bubbles, Dave. Keep it going.

Dave:: I'm pouring. I'm still pouring.

Hal:: Can you stop there?

Dave:: Okay.

Hal:: And we'll take the funnel out.

Dave:: Okay.

Hal:: Dave, ready on the - take the tube. Okay. Fingers in your ears, audience at home. Ready, Dave?

There we go. Big bang, CO2 for you. Dave. Are you all right?

Dave:: Yeah, I'm fine.

Hal:: Once again, CO2.

Dave:: Because she comes out with the goods every time though.

Hal:: She does, yeah.

Dave:: Yeah.

Hal:: I hope that's shepherd's pie in my trousers.

Laura:: And while that enormous bang echo through corridors of Cambridge University's Pathology Department, Dave and Dr. Hal were already preparing next week's explosive experiments.

We'll be blowing up hydrogen balloons, exploding ostrich eggs, and hearing the ear-splitting woof of the barking dog. That's coming up next week.


Something you will have probably seen in every cheesy film involving anything slightly scientific is bubbling pots producing huge quantiites of smoke. These all use dry ice, that is CO2 cooled below its sublimation point of -78°C (194K). This causes it to crystallise directly to form a solid known as dry ice.

If you put this very cold solid in water it rapidly heats up above -78°C and sublimes straight back to being a gas. This warms up to room temperature very quickly and expands by a factor of 750.

If the dry ice is put into warm water, the water evaporates on the surfaces of the bubbles forming water vapour. When this meets the cold carbon dioxide gas it condenses forming billions of tiny water droplets which make up a small cloud.

Because this cloud is in carbon-dioxide, and as gas denser than air, it sinks. So it can form a cloud sitting on the floor, an effect often used theatrically on stage.

It is possible to use the expansion of dry ice as it boils and a strong pressure vessel to produce what is known as a dry ice bomb. This works on the same principle as the
Liquid nitrogen bomb we did a few months ago.

These are very very dangerous, they are extremely violent and go off very quickly. Do not even think of doing this at home.

Dr Hal demonstrated this by putting a lot of dry ice in a lemonade bottle. He then added lots of boiling water to supply it with plenty of heat, put the bottle behind a blast shield and screwed on the lid.

The dry ice keeps subliming producing carbon-dioxide gas and the pressure builds up. Despite a lot of the gas dissolving in the water eventually the pressure builds up enough to make the bottle fail. Lemonade bottles are very strong and often fail at over 10 atmospheres, so the resulting explosion is very violent.


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