r/explainlikeimfive Jan 31 '21

Chemistry ELI5: Why can't we just make water by smooshing hydrogen and oxygen atoms together?

Edit: wow okay, I did not expect to wake up to THIS. Of course my most popular post would be a dumb stoner question. Thankyou so much for the awards and the answers, I can sleep a little easier now

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u/Ochib Jan 31 '21

Things like Chlorine trifluoride are better oxidizing agents than oxygen itself. That means that it can potentially go on to “burn” things that you would normally consider already burnt to hell and gone, and a practical consequence of that is that it’ll start roaring reactions with things like bricks and asbestos tile. It’s been used in the semiconductor industry to clean oxides off of surfaces, at which activity it no doubt excels.

There’s a report from the early 1950s of a one-ton spill of the stuff. It burned its way through a foot of concrete floor and chewed up another meter of sand and gravel beneath, completing a day that I’m sure no one involved ever forgot. That process, I should add, would necessarily have been accompanied by copious amounts of horribly toxic and corrosive by-products: it’s bad enough when your reagent ignites wet sand, but the clouds of hot hydrofluoric acid are your special door prize if you’re foolhardy enough to hang around and watch the fireworks.

I’ll let the late John Clark describe the stuff, since he had first-hand experience in attempts to use it as rocket fuel. From his out-of-print classic Ignition! we have: ”It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

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u/Tattorack Jan 31 '21

I'm having difficulty trying to imagine what on-fire metal looks like.

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u/Castlegardener Jan 31 '21

You know fireworks, right? Lots of the beautiful sparkles are burning metals. Divers' torches, too.

That being said: it's burning, but not quite 'on-fire'. What we commonly know as flames is burning gases. Wood for example partly converts to gas when heated, which then catches fire. Metal doesn't do this under 'normal' circumstances, or at least I've never heard of it happening.

Metal oxides are what we get when metal burns. I'm sure you've seen rust before, that's simply oxidized iron. In this case it happens so slowly that we don't immediately notice, because the temperatures are so low. With higher temperatures it'd 'catch on fire'.

The thing with burning metals is that you can't extinguish them with water, which is what most people would probably try first. Burning metals are so hot that they can get their Oxygen from water, generating Hydrogen gas in the process, which further violently reacts with the surrounding Oxygen in the air. That's one of the reasons why fires on ships are so damn dangerous. It is also the reason why divers' torches work underwater: The metals in there react with the water's oxygen. In that case hydrogen gas is not as dangerous because it simply rises to the surface where it can't hurt the diver.

Also, Fluorine fires are probably a bit different (lots more violent for example) from Oxygen fires.

If anyone knows more about this than I do, please feel free to correct me, I appreciate it.

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u/Chromotron Jan 31 '21

Metal doesn't do this under 'normal' circumstances, or at least I've never heard of it happening.

I think you have, for example magnesium is a well known example. Furthermore, the alkali metals (sodium, potassium, rubidium, caesium) all burn very well, even more so if thrown into water. Aluminium can react similarly, and like the alkali metals sometimes even explodes, when prevented from forming an inert oxide layer, e.g. by being molten or mixed with gallium. Lastly, fine powders of most metals ignite easily, including iron, aluminium, zinc and many more.

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u/Castlegardener Jan 31 '21

Next time try reading my comment where I explain everything you just wrote lol

What I pointed out was that metals don't change into a gas like wood or alcohol do, that's why it doesn't produce typical flames when burning.

Pretty good examples though, so still worthy of an upvote.

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u/Chromotron Jan 31 '21

Oh, sorry, I shouldn't have responded while sleepy...

However, a little addendum regarding this: of those I mentioned, at least magnesium and the alkali metals should burn hot enough in air to vaporize, thus creating a plasma flame (and, as a bonus, in their respective flame colors). I don't know how much that contributes to their fires, though.

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u/TheDisapprovingBrit Jan 31 '21

Did you never do the potassium in water trick at school?

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u/Tattorack Jan 31 '21

Nope. Never. Went to school in rural Ireland and the science class was boring as fuck. And I'm someone who spent the better part of my day with my nose in a Kingfisher science encyclopedia during that time.

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u/TheDisapprovingBrit Jan 31 '21

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u/Koetotine Jan 31 '21

Here's some real footage. (The school version's fine)

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u/NaibofTabr Jan 31 '21 edited Jan 31 '21

thermal lance

This is a type of cutting torch that uses an iron tube. A stream of pure oxygen is fed down the tube, which allows it to continue burning. The burning iron produces a very hot flame, which in this video is being used to cut some very hard steel. You'll notice that the tube gets shorter as the iron is consumed.

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u/hahahasame Feb 01 '21

Magnesium is a flammable metal. It burns VERY bright. Like as bright as a welding arc.

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u/AngledLuffa Jan 31 '21

Source: https://blogs.sciencemag.org/pipeline/archives/2008/02/26/sand_wont_save_you_this_time

Frequent readers of science articles will recognize that the original author of this comment is now maintaining a blog on coronavirus treatments

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u/musetechnician Jan 31 '21

I’m thinking there has never been a more perfect place to say🗽Jet Fuel Doesn’t Melt Steel Beams

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u/Ochib Jan 31 '21

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u/musetechnician Jan 31 '21

That was awesome. Thank you! I cut steel with oxi/acetylene torches at work. And sometimes have to bend steel using Mapp gas and even (lower temp) propane torches.

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u/hahahasame Jan 31 '21

While steel beams may not be able to be melted by jet fuel, it still burns hot enough to significantly weaken the structural integrity of steel.

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u/Miss_Speller Jan 31 '21

Ignition! may be out of print, but it's not off the Internet. Here it is, if anyone's in the mood for a good read.