LIFE WAS A LOT SIMPLER when Aristotle had only four elements: air, earth, water, and fire. Later, he added heavenly aether, and things have gotten more complicated since then.
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It’s fire that interests me today. This was prompted by BBC CrowdScience’s “Why Don’t Some Things Burn?,” together with my usual Internet sleuthing.
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BBC Podcast. BBC’s Alex Lathbridge visited the Fire Research Centre at the University of Edinburgh and the archives of the Royal Institution of London to learn about “the fundamentals of fire… and why things react differently to heat.”
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“Fire has defined our species,” Alex says, “and it’s a fundamental part of our modern lives, from heating our homes to sending us into space.”
The Fire Triangle. Rory Hadden, Fire Research Centre, University of Edinburgh, explains that fire requires fuel, oxygen, and heat. “The fuel and oxygen react chemically, during that reaction energy is given off, and we see that as light coming off the flame.”
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What About Just Oxygen Mixing with Fuel? Oxidation occurs whenever oxygen is in the presence of a fuel (a wooden table, for instance, or a piece of metal). The rate of this oxidation is really slow, “and that’s where the heat comes in,” says Hadden.
“Most chemical reactions,” Hadden says, “are going to need some kind of kickstart to get the reaction going…. The great thing about a fire is it releases energy so that it has its own kind of kickstart for the next bit of reaction, so you get that sustained burning process.”
Can Fire be Flameless? Hadden says, “We call that a smouldering fire. A smouldering fire is one where the oxygen from the air around us directly attacks a solid, so you don’t actually have a flame produced. The most common example of that is a barbeque… it’s definitely burning but there’s not necessarily any flames there. It might be glowing orange a little bit….”
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So What About Flame? Hadden describes a candle’s flame: “Here, the fuel for the candle is the wax… and as the liquid is drawn up through the wick, it’s heated so then the liquid turns into a gas.”
Hadden says that the flame we see is the chemical reaction occurring in the gas phase: The fuel is a gas which produces the stable teardrop flame.
Why That Shape? Hadden says, “The energy that is released heats up the products of combustion and they rise up.” He then adds a curious bit: “If you were to go into space, for example, and light a fire,” in the absence of gravity the flame shape would be essentially spherical, not teardrop. (Of course, you’d have to supply the oxygen too.)
What Won’t Burn? Obviously, things that refuse to react with oxygen fail to combust. Concrete is one example. Others simply don’t oxydize rapidly enough. For this, Hadden uses a blowtorch first with a steel plate and then on iron filings.
“Give me a sign when you’re bored of this,” he says as he heats the steel plate. “Eventually it would get really hot, but nothing happens.”
By contrast, with the iron filings his blowtorch “creates a shower of glowing sparks… the iron filings have a large surface area that the oxygen can react with.”
The oxidation rate increases to the point that, in a sense, the filings “rust,” but very quickly indeed.
Magnesium. Not discussed in the BBC CrowdScience podcast is the oddity of the metal magnesium. I recall Pierre Levegh’s Mercedes-Benz and its magnesium bodywork igniting in the crash, adding to the horror of the 1955 Le Mans tragedy.
Wikipedia notes, “Magnesium metal and its alloys can be explosive hazards; they are highly flammable in their pure form when molten or in powder or ribbon form. Burning or molten magnesium reacts violently with water.”
Note, Mg + 2H2O → Mg(OH)2 + H2, this last one highly flammable hydrogen gas.
Who would have guessed a nice placid campfire could be related to such flammability? ds
© Dennis Simanaitis, SimanaitisSays.com, 2022