Thermite vs ice = explosion

[Travis]

On the latest episode of Mythbusters (http://dsc.discovery.com/videos/mythbusters-thermite-vs-ice-angle-3.html) they confirm that igniting thermite in a bucket on top of a block of ice results in a strange, concussive explosion. However the reason behind this is left unresolved by the episode.

Does anyone have any guesses as to what is causing this phenomena?

[paximperium]

No, don't spoil it for me. I recorded it on my DVR.

[Soapy Sam]

God.

[JoeyDonuts]

Claim that WTC core columns were actually made of ice in 3...2...1...

[jasonpatterson]

Thermite produces huge amounts of energy, so what I suspect is happening is that the molten iron burns a smallish hole through the surface of the ice, it flows into this hole and vaporizes a larger cavity underneath. The ice wouldn't all vaporize at that time (if nothing else ice chunks are easily visible after the explosion.) Enough should remain on the top and sides to make a passable chamber for a small rocket to form (the giant plume of death spraying out start at 0:06) due to steam production and further thermite reacting. The explosion would be due to steam production that is too great to be vented through the hole through the top of the ice, so it exploded.

I always knew there was a reason rockets aren't made from plastic buckets of ice. :D

[soylent]

I think it might be small amounts of hydrogen formed by aluminium reducing water. When the concentraction is high enough it goes boom.

[Cuddles]

I assume it's what Jamespatterson said. If you produce lots of gas in an enclosed space, you get an explosion once the pressure is too great to contain.

Why were they looking at this anyway? The interaction between thermite and ice hardly seems like a common topic of discussion that they'd be interested in investigating.

[JJM]

{snip} Why were they looking at this anyway? The interaction between thermite and ice hardly seems like a common topic of discussion that they'd be interested in investigating.If you watch "The Thing from Another World" (1951) you will see that they attempt to release the UFO from ice using thermite. The explosion is too violent, and destroys the ship. So, the connection is obviously related to UFO investigations. It confirms that an alien space ship encased in ice could be destroyed by thermite.

[Evilgiraffe]

I think it might be small amounts of hydrogen formed by aluminium reducing water. When the concentraction is high enough it goes boom.

If anything is reducing water, it'd be the iron. The aluminium is already fully oxidised, by virtue of having taken part in the thermite reaction in the first place, and can't reduce anything else.

[MRC_Hans]

Wow, that was one big boom! It must almost certainly be a chemical reaction, so I also guess at some reduction of water, possibly combined with the effect of all the thermite being ignited in a much shorter time than the normal burn (oxygen released?).

If the origin of the idea is somebody attempting to burn a nice hole in ice with thermite, then that somebody was sure in for a nasty surprise! :eek:

Hans

[casebro]

A couple of observations:

Volkswagen magnesium engine casing in a fire hardly burn- until you throw some ice onto them. Then they become magnesium flares.

A sealed can of warm beer thrown into a camp fire with get a hole in it, and fizzzzz. A sealed can of cold beer will explode.

So, heat and water do strange things to metals in the aluminum/magnesium/beer family. ;)

[BenBurch]

I think it might be small amounts of hydrogen formed by aluminium reducing water. When the concentraction is high enough it goes boom.

You would be correct, sir. Not just the Aluminum, but the IRON will disassociate the water and react with the oxygen.

[kookbreaker]

Why were they looking at this anyway? The interaction between thermite and ice hardly seems like a common topic of discussion that they'd be interested in investigating.

It was based off an internet video they found. They've been doing a lot of things like that since so many are faked.

[lomiller]

It was based off an internet video they found. They've been doing a lot of things like that since so many are faked.

They are also fond of anything that may explode...

[Madalch]

I worked briefly in an aluminum plant, and mention was constantly made of the dangers of steam explosions- when water gets into something as hot as a large pool of molten cryolite (or in the case of thermite, molten iron) it simply explodes into steam.

If the iron reacted with water to give hydrogen, which then burned, that wouldn't be any more exothermic overall than the molten iron burning, which it doesn't really do.

Of course, I've done the thermite reaction dozens of times, and often with the reaction vessel (half a pop can) stuck in a snowbank. Never a problem.

[BenBurch]

Hot iron is how some commercial hydrogen is made. Does not need to be molten.

[Madalch]

Hot iron is how some commercial hydrogen is made. Does not need to be molten.
Hot iron with water? I find that hard to believe, when sulphuric acid is so cheap.

[One Skunk Todd]

For all the apparent violence of the explosion it doesn't appear to have much effect on the foreground wreckage. Loose wires and debris don't appear to move, the camera barely shifts and the car vibrates a little. Very showy but not a lot of oomph.

[BenBurch]

Hot iron with water? I find that hard to believe, when sulphuric acid is so cheap.

It was the first commercial process, and was used to fill war balloons in the 1700s-1800s. Used again today in some solar-hydrogen energy systems and sold commercially.

Edit; Not just water, live, dried steam is used in the process.

[Madalch]

Edit; Not just water, live, dried steam is used in the process.
Dried steam? Superheated steam, I could see, but dried?

[BenBurch]

Dried steam? Superheated steam, I could see, but dried?

Dried steam is run through a set a baffles called a steam dryer that has the goal of removing all droplets that almost universally are in steam right from the boiler. Steam right from the boiler is usually called saturated steam. Superheated steam is also necessarily dried steam.

[BenBurch]

See; http://www.trainweb.org/j.dimech/6167/articles/etsd.html

[casebro]

Lye water in a pop bottle, slide in some rolled up tin foil, and snap a condom over the top- instant hydrogen balloon. Attach a wick. Light it and let her go. Watch for the utility company trucks after the old lady next door calls them to report flashes on the overhead wire.

Careful, it's an exothermic reaction, the bottle will melt the rubber.

Anyway, I don't think mere aluminum and water will make hydrogen, it takes lye in there. Other wise we couldn't cook in aluminum pots, or have aluminum around boats.

[BenBurch]

The lye keeps the passivation layer from forming.

[Madalch]

Anyway, I don't think mere aluminum and water will make hydrogen, it takes lye in there. Other wise we couldn't cook in aluminum pots, or have aluminum around boats.

Actually, the lye is only there to make the reaction product (aluminum hydroxide) soluble. If you put a piece of perfectly clean aluminum in water, it instantly becomes coated with a layer of aluminum hydroxide, which prevents the water from reaching and reacting with the rest of the aluminum. But aluminum hydroxide is amphoteric, and will dissolve nicely in strong base or strong acid, so aluminum will react readily with these solutions.

If you put water into molten aluminum, you will get a fire in addition to the steam explosion, since aluminum hydroxide can't coat the surface of a liquid.

ETA: What BenBurch said.

[ponderingturtle]

Lye water in a pop bottle, slide in some rolled up tin foil, and snap a condom over the top- instant hydrogen balloon. Attach a wick. Light it and let her go. Watch for the utility company trucks after the old lady next door calls them to report flashes on the overhead wire.

Careful, it's an exothermic reaction, the bottle will melt the rubber.

Anyway, I don't think mere aluminum and water will make hydrogen, it takes lye in there. Other wise we couldn't cook in aluminum pots, or have aluminum around boats.

Well aluminum general has a coating of aluminum oxide preventing it from reacting more, but liquid aluminum would not have this and be more reactive.

[Travis]

You know, from appearances, it looks a lot more like a steam explosion than hydrogen combustion. A violent phase change can produce this can't it?

[Madalch]

You know, from appearances, it looks a lot more like a steam explosion than hydrogen combustion. A violent phase change can produce this can't it?
I didn't watch the video, but it sounds reasonable to me.

[technoextreme]

You know, from appearances, it looks a lot more like a steam explosion than hydrogen combustion. A violent phase change can produce this can't it?
Wouldn't be the first time they have tested an explosion where the combination of extreme heat and extreme cold results in a disaster. The other scenario is the frozen turkey in the deep fryer.

[I Ratant]

I assume it's what Jamespatterson said. If you produce lots of gas in an enclosed space, you get an explosion once the pressure is too great to contain.

Why were they looking at this anyway? The interaction between thermite and ice hardly seems like a common topic of discussion that they'd be interested in investigating.
.
Mythbusters gets a TON of the silliness ideas from game-playing children, and occasionally will try to make sense of a made-up "myth".
It's hardly a situation anyone would ever encounter.
I failed to pay much attention to the episode, but it has generated hundreds of messages on their forum.

[PixyMisa]

Why were they looking at this anyway?
It goes boom!

[R.Mackey]

You know, from appearances, it looks a lot more like a steam explosion than hydrogen combustion. A violent phase change can produce this can't it?

It can, yeah... but not usually so suddenly without containment.

My own pure speculation is that it's a hydrogen effect, not just ice -> water -> steam. The report was pretty impressive, and it threw chunks large and small for hundreds of feet. I'm guessing the blast qualified as a low-order explosive, though it's hard to tell without a pressure measurement.

What could happen is if enough water was cracked, and possibly oxygen bound up in the iron as well, it would generate a hydrogen plume mixing unevenly with ambient air. Ignition source is right there, obviously. When this plume approached the right ratio, BOOM! Nice deflagration with a clear flame, something that fits the audio and video evidence.

But maybe not. I suppose it could also be the ice puddled, containing enough liquid water that a large chunk of it superheated and flashed to steam all at once. Seems unreliable, but who knows?

Jamie's observation was that the ice/water/steam might somehow trigger "aerosolizing" the thermite. I don't see that being reasonable, but it is possible that some similar effect is going on -- steam somehow helps melt or strip oxide coating from the metals and speeds up the already ongoing thermite reaction. I'd call this a long shot.

What would be interesting is to retry this (1) in an inert atmosphere, which would eliminate my leading hypothesis if nothing happens, and (2) in the presence of dry steam instead of ice, which if it does BOOM eliminates the second. Also might be worth trying with a regular bucket of water to see what happens. Better calorimetry, too, to see if atmospheric oxygen is contributing, will help a lot.

[BenBurch]

Too much fuel, suddenly the oxidizer appears... I had a rocket motor blow up on me that way when I was a teenager. (Yes, I was insane.)

[macdoc]

I think whoever made the comment about it creating it''s own rocket chamber makes the most sense...

clearest here

plume like a rocket motor when the thermite burrows into the ice
then boom as the pressure exceeds the containment

http://dsc.discovery.com/videos/mythbusters-thermite-vs-ice-angle-3.html

[Travis]

It can, yeah... but not usually so suddenly without containment.

My own pure speculation is that it's a hydrogen effect, not just ice -> water -> steam. The report was pretty impressive, and it threw chunks large and small for hundreds of feet. I'm guessing the blast qualified as a low-order explosive, though it's hard to tell without a pressure measurement.

What could happen is if enough water was cracked, and possibly oxygen bound up in the iron as well, it would generate a hydrogen plume mixing unevenly with ambient air. Ignition source is right there, obviously. When this plume approached the right ratio, BOOM! Nice deflagration with a clear flame, something that fits the audio and video evidence.

But maybe not. I suppose it could also be the ice puddled, containing enough liquid water that a large chunk of it superheated and flashed to steam all at once. Seems unreliable, but who knows?

Jamie's observation was that the ice/water/steam might somehow trigger "aerosolizing" the thermite. I don't see that being reasonable, but it is possible that some similar effect is going on -- steam somehow helps melt or strip oxide coating from the metals and speeds up the already ongoing thermite reaction. I'd call this a long shot.

What would be interesting is to retry this (1) in an inert atmosphere, which would eliminate my leading hypothesis if nothing happens, and (2) in the presence of dry steam instead of ice, which if it does BOOM eliminates the second. Also might be worth trying with a regular bucket of water to see what happens. Better calorimetry, too, to see if atmospheric oxygen is contributing, will help a lot.

They're always doing retests of old myths. Perhaps I should go over and suggest this on their forum.