Firing up chemistry

05 December 2017

Question

Are there any chemicals that would create fire when mixed together?

Answer

Chris Smith asked chemist, Phillip Broadwith from the Royal Society of Chemistry, to fire up an answer to this question from Nick. 

Phillip - Well Nick, absolutely yes. They’re used in rocket propellants all the time and they’re called hypergolic mixtures. There’s one called Aerozine 50 which is a mixture of hydrazine and dimethylhydrazine which have unstable nitrogen-nitrogen bonds, and then dinitrogen tetroxide. But it doesn’t really matter what they are, the point is that when you mix them together you get a chemical reaction to start with, that generates a lot of heat and then you have all the things that you need for a fire. You have a fuel and you have an oxidiser which, initially, is the chemical oxidiser but then once you’ve got a fire going you can use oxygen from the air as the oxidiser, and then you’ve got a source of ignition. This is why they’re used in rocket engines for things like steering, you don’t need an extra electrical system that can go wrong to fire the rocket.

Or, if you’re wanting to power a missile, you’ve got something you can store for long periods of time at relatively easy temperatures as long as you can handle the corrosion. And then, when you need to fire your rocket, or missile or whatever you can just mix the two and off you go.

There are some more readily available mixtures. I would say before I say any of this, just be very careful, don’t really try this at home. Just watch the YouTube videos instead.

Chris - Don’t really try it at home or maybe just don’t try it at home?

Phillip - Don’t try it at home at all.

Chris - What are those readily available mixtures?

Phillip - Take glycerine - just household glycerine used in baking and potassium permanganate, which is a very highly oxidising thing, and you mix those two together, you can generate fire.

Chris - Kate?

Kate - Is this a “bang” fire or just a nice flame like a Christmas pudding?

Phillip - It depends on the mixture. Some of them will go on fire very quickly and very intensely like the rocket fuels. Glycerine and permanganate: there’s a kind of induction period where you have the chemical reaction going and they just get hot, and smoke, and then you get a flame. Then it will burn - quite an intense fire because you’ve got a lot of oxidiser there, but it’s just a fire.

Chris - Best thing I ever saw was a couple of chaps from UCL came along and did a chemistry demonstration and they poured liquid oxygen on a digestive biscuit and lit it, and the flame was a metre and a half high; it hit the ceiling. It was absolutely phenomenal!

Phillip - Well, you’ve got a high concentration of fuel. All of the carbohydrates from the biscuit and then you’ve got a huge amount of oxidiser - liquid oxygen - in there. It’s a massive concentration but you still, for that mixture, need an ignition source. You need…

Chris - He lit it with a very long stick.

Phillip - Yeah.

Chris - We were closer than he was, put it that way.

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