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Votive candles.Photo: <a href="">Fr. Lawrence Lew, O.P.</a>, CC <a href="">some rights reserved</a>

Natural Selections: Flames

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What is a flame? Why is it shaped like that? How does it keep going? Martha Foley and Dr. Curt Stager answer some burning questions about rapid oxidation.

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I want to talk about flames or maybe about fire. I’m looking at candle flames and they have an edge. I mean a flame has a shape, but it’s not really a thing. What’s happening actually?

Yea it’s weird. It looks like a substance, but is it matter, is it energy? Why does it stay where it is?

If there’s no flame, is there fire? I got thinking about it and I got really kind of off in space with fire.

It seems like a kind of philosophical question, but it’s physics. Let’s just limit it to one flame, like on a candle or a fireplace. It’s got a certain shape to it (which actually it wouldn’t have if it wasn’t on Earth). So you think of a candle flame and it’s kind of broad on the bottom and round and sort of tapered above, sort of like a camp fire. If you went out into space and ignited something, it doesn’t look like that. It makes a sphere instead, not a tapered shape. And it doesn’t burn as well either.

But why is that?

It turns out that being in the air on the ground on Earth gives it that unique shape, because we're in a sort of sea of air. And the stuff feeding the fire is oxygen in the air. So when you burn something, you’re basically combining it with oxygen.

So when you light a candle, it doesn’t have that shape at the beginning. The flame is kind of ill defined. It’s on the wick, and then as the fire builds it gets this beautiful shape.

It sort of amplifies itself. The fuel is the wax, basically a form of petroleum. It’s burnable; it’s just a bunch of carbons and hydrogens. And the way you make the fire is you take those substances and combine them with oxygen in the air. Once you get it going, it feeds itself and releases heat and light. It consumes the wax and turns it into gases-- carbon dioxide, and water vapor. That’s the chemistry of it.

It looks different when you start it; you’re melting a little of that wax and getting it into a gaseous state by evaporating it a little in the wick. That lets the oxygen combine with it. And once it starts to go it makes heat and that speeds up more of it.

The reason it gets the shape it does is because you’re heating up the gases. As they heat up they expand. And that makes them less dense than the surrounding air so they rise. You can say they float in the air, and it sort of pulls it all upwards, towards the ceiling or towards the sky. It’s the heat of the flame itself that makes the gases in the flame rise upwards and that pulls it all upwards. And then it tapers off because it’s cooling off up there, it's finished burning.

So it’s a chemical reaction basically.

It’s oxidation--just combining with oxygen--and that’s why you see an edge to it, too. It’s sort of a chimney effect, and it’s limited to that rising cone of hot gases.

Oxidation--is that like rusting?

Same thing as rusting. Actually iron will burn, too, just like wood--if you make little iron filings and you sprinkle them on a candle, it goes sparkle, sparkle. That’s the same thing as rusting, only sped up. Rusting is like a slow burn that releases the same amount of heat but just over a long period of time, so you don’t notice it, and it doesn’t glow.

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