Not true. You can have a fire in a Chlorine atmosphere, with Chlorine as the oxidizer.
The original statement is true on Earth, away from science demonstrations, but we’re talking about the situation on an alien planet.
My first thought was that sulfur could substitute for oxygen. The reaction I was imagining was ammonia plus sulfur to form hydrogen sulfide and molecular nitrogen in a 3 to 1 ratio. It turns out (according to some quick google results) that what actually happens is you get a bunch of different nitrogen sulfur compounds with different ratios of N and S to go along with the hydrogen sulfide. They do react, but not in the way I thought they would.
I’d guess that a fluorine atmosphere would also support flame, but fluorine is significantly less abundant on a cosmic level than chlorine, so it is less likely that a planet could develop a fluorine atmosphere naturally.
Fluorine wouldn’t just support combustion it would burn nearly anything on the planet. It will set fire to just about anything.
It makes oxygen look tame.
My first thought about oxidisers was perchlorates. But actually making a flame is likely a hard call.
We might get better action with higher pressure. The OP didn’t require STP on the planet.
To get some idea about Flourine:
There is no ignition source here. The reactions are just Flourine gas applied to materials.
Spontaneous human combustion might be a myth on Earth. If we were in a Flourine atmosphere it would be the really.
The passage of time supports random chance. So, wouldn’t an atmosphere consisting of Chlorine, Fluorine, or any other combustible gas have burned up fairly rapidly even without any presence of oxygen? All it should take is a bolt of lightning, a large rock falling and creating a big spark, or something of that nature.
No; you need both an oxidising agent and a source of fuel. Oxygen on our planet supports fire; why hasn’t it all burned away? The answer is, because of life. Life constantly creates new oxygen, and replenishes the atmosphere. Oxygen is too reactive to remain on a planet with no life. If all life were to die, then the oxygen would vanish - some of this would be the result of vast forest fires as the dead trees burned. The rest of the oxygen would be absorbed by rocks and minerals.
Exactly the same would happen on a planet with a chlorine or fluorine atmosphere. These highly reactive gases would not form an atmosphere on a lifeless world, because they are too reactive. But if we imagine a form of life that produced chlorine gas via photosynthesis rather than oxygen, then the atmosphere of such a world would contain free chlorine -if and only if the biosphere persisted. If all the plants on such a world died out, all the free chlorine would disappear pretty rapidly (certainly within a million years).
Probably need to be clear about terminology. “Oxidiser” doesn’t just mean oxygen. It is a term used for anything that accepts electrons in a redox reaction. Redox - a contraction of reduction and oxidisation. A reduction agent is a compound that donates electrons. We talk of the reduction or oxidation potential of compounds. It is a relative metric, Fluorine is so powerful an oxidiser that it can burn nearly anything. Including things that normally we would consider totally inert. Like bricks. If you pour liquid Fluorine onto ice, the ice will catch fire burn and maybe explode. It will set fire to asbestos.
For what we are talking about here we just need an oxidiser and a fuel. In order to make fire as opposed to an exothermic reaction, we need a serious energy output. Something that all of Fluorine, Chlorine and Oxygen are ready to provide when combined with ordinary fuels. If you look at a list of oxidising agents, you see a long list that is pretty much made up of compounds that have a useful amount of loosely bound Oxygen, Chlorine or Fluorine. Nitric acid being one of the outliers. It is a sufficiently good oxidiser, and is also stable at room temperature, that it made a good oxidiser for rocket fuels, before being replaced with less corrosive alternatives like nitrogen tetroxide. The history of evil rocket fuels - and especially the various oxidisers is a favourite geeky subject. (Cf, the book Ignition!, and Derek Lowe’s "things I won’t work with")
For the purposes of the OP, with an ammonia atmosphere, we need a source of probably a solid oxidiser that will react with the ammonia with enough energy to become incandescent. That will be a tall order at ordinary temperatures and pressures.
I appreciate the detailed post. I guess I’m trying to understand this in the context of planetary evolution. I’m probably overthinking it as planets, during the course of their evolution, balance out into a relatively finished product. How they balance out depends, I would think, on their overall makeup.
And, critically, whether they have life or not.
The equilibrium state of current Earth with life, and the equilibrium state of Earth with the life magicked away are two very different atmospheres & hence very different planets.
I worked on a hydrazine rocket engine program once. Hydrazine is N2H4. The engine was firing into a high altitude chamber and the hydrazine simply decomposed after passing over a platinum screen. It should have formed free hydrogen and nitrogen. But when we opened the chamber after a test run you could notice an ammonia odor. Ammonia is NH3. There seemed to be surprise among the research engineers that this happened.
Hydrazine is so close to ammonia that I wondered if it might be a candidate for a flammable substance on my hypothetical world. But analogous molecules like hydrogen peroxide and ozone (reactive molecules that have extra oxygen) are rare on Earth because they are unstable, so hydrazine (with an extra nitrogen) would probably be a short-lived compound (unless it was a product of some biological process).
IMO whatever is the non-fuel component of our proposed redox reaction would have to be a product of a biological process.
The “burning” would ensure the consumption of this redox agent, and over historical time it would be gone. Unless replaced by an active process.
IMO “active” is pretty well a synonym for “life”. You could imagine other sources like asteroid bombardment, primordial outgassing, etc. But all of those sources are finite over timescales of planetary lifecycles. At least around here, “life” is the word for the magic process that can run counter to entropy and push the ordinary flow of chemistry towards a low-energy state in the opposite direction. And do so more or less indefinitely in time and more or less universally across the planet’s surface.
Always enjoy that channel, and this video was fascinating as typical.
I didn’t see the fume hood, do they have just a huge fume hood they are filming it inside? I have certainly seen other videos where you can see they are using a hood.
Seems strange for such a terrifying element. Checking up on sulphur tetraflouride and iron tetraflouride, it appears they are both very stable non toxic compounds. Is it that the flourine reacts so swiftly and completely that there is no significant stray flourine or harmful reaction products?
It’s ridiculously reactive. We had to take special training to handle it. It will explode if it touches dirt, rust, etc. When confined a small section explodes first and then it propagates explosive cell by cell. Each new explosion takes place at higher and higher pressures. Plus. according to the Air Force literature “a single drop will kill a rat in 20 seconds”. We wore full enclosed suits. The room with the hydrazine tanks had to be flooded 2 feet deep with water so no splashes could hit anything.