Why Do We Like Staring Into Fire?

The Rest Is Science

The episode explores the scientific, historical, and psychological dimensions of fire, defining it as a chemical process rather than a singl

Key takeaways

• Fire is defined by the fire tetrahedron: fuel, heat, oxygen, and a self-perpetuating chemical chain reaction.
• The blue portion of a flame results from chemiluminescence, while the orange glow comes from blackbody radiation of heated soot particles.

Main topics

• Chemistry of combustion
• Evolutionary history of fire

Notable quotes

Fire is the great-grandmother of social media.

Conclusion

The episode highlights how fire acted as a fundamental catalyst for hu

Transcript preview

Speaker 3 (0:00) Welcome to the Rest of Science. I'm Hannah Fry. And I am Michael Stevens. Today, I want to talk about fire. Yes, please. Okay. A lot of people ask, what is fire? Speaker 2 (0:11) And Speaker 3 (0:11) today, I'm going to argue that it's not a gas. It's not a plasma. It's not a liquid. Fire is the great-grandmother of social media. Speaker 4 (0:24) Michael, you are genuinely one of the best people on earth. Wait until you listen to the Speaker 3 (0:31) episode before you make any kind of judgment. All Speaker 4 (0:34) right, deal. Speaker 3 (0:39) This episode is brought to you by Cancer Research UK. Speaker 4 (0:42) As if they weren't cool enough already, some astronauts have now added cancer scientists to their CV. They are making use of microgravity conditions in space to carry out experiments. That's because the same thing that Speaker 3 (0:54) lets astronauts experience weightlessness also affects how cancer cells grow, behave, and respond to drugs. And Speaker 4 (1:02) back down on Earth, Cancer Research UK is working with astronomers to explore tumours with technologies that are typically used to map the Milky Way, which I think... Gives you an idea of just how complex this disease is. Speaker 3 (1:14) That's right. There are over 200 different types of cancer. But by pushing boundaries and embracing the latest technologies, Cancer Research UK has helped double cancer survival in the UK over the past 50 years. And Speaker 4 (1:26) today it is continuing to save and improve lives around the world. For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org slash rest of science. Speaker 3 (1:45) I mean, let's just begin with like what fire is. I've made a lot of shorts about fire. And this is my chance to really clarify, solidify, and calcify my stance on fire. First of all, what would you say fire is? Like your daughter asks you, one of your daughters asks, what is fire? Is it a gas? Is it electricity? Speaker 4 (2:08) No, I mean, I think it's... It's the energy given off by a chemical reaction. Yeah. Speaker 3 (2:12) Yeah. It's a chemical reaction. Yeah. So it isn't like one thing the way gold is one thing. It's one kind of atom. Fire is a whole lot of stuff going on. And what exactly is in it depends on what's burning. So it's it's that that's kind of the key. I think asking what is fire is like asking what. Speaker 4 (2:36) Like foaming or fizzing? Speaker 3 (2:38) Yeah, like when you mix vinegar and baking soda and you shake it up and you look at the whole thing and Speaker 4 (2:43) you Speaker 3 (2:44) say, what is that? We don't say, oh, that's blooba. You know, we don't need to give it one noun. We say that it's a process. It's a thing that's happening. We've got bubbles of gas and we've got the vinegar in the baking soda mixing and then there's some water being produced, whatever. And everyone leaves it at that. But fire. We Speaker 4 (3:00) still see it as a verb rather than a noun. Speaker 3 (3:02) That's right. But we see fire as a noun. Fire is a noun. It can also be a verb. You can fire something. You can. You can fire someone. You Speaker 4 (3:11) can. Speaker 3 (3:12) Okay, let's walk through the steps, okay? Because fire is at least. Two different things, depending on where you think the fire begins and ends. For those of you that aren't, that haven't watched every single short I've ever made or don't already know what the fire triangle is, it's the three things fire needs to happen. Because fire is a chemical reaction known as combustion. And you need to have three things for there to be a fire. Or do you? We'll get to that later. Okay, so you need fuel, you need heat, and you need oxygen. Take away any of the three, just one of the three. No combustion. Which is how putting out fires works. Exactly. So you've got some fuel. Let's say it's your house. It gets hot enough. There's oxygen around. There's a house fire. How do you put it out? Well, you need to get rid of one of those three points of the triangle. You get rid of the fuel. We do that when there's a wildfire and we start like clearing the brush and creating a perimeter around the fire where the flames just are going to run out of fuel because they're going to hit like dirt. If you want to get rid of the heat, water is perfect for that. That's why we put water on fires because the water absorbs so much heat. The fuel goes. ah, there's not enough left for me to react with oxygen. Guess I'll stop burning. Speaker 4 (4:28) So Speaker 3 (4:28) that's what, even hot water will put out a fire. But remember we talked about like wet water? And Speaker 4 (4:34) wait, unless it's like many thousands of degrees hot. Let's just, let's put limits on this. Yeah, Speaker 3 (4:40) so steam, if you had, if you used steam that was at a thousand degrees Celsius, it wouldn't put out the fire. Speaker 4 (4:49) Thank you. Worth clarifying. Carry Speaker 3 (4:51) on. Isn't this the most practically useful podcast you guys have ever listened to? Speaker 4 (4:56) Guys, guys, guys, your house is burning down. Right, let me just... Don't use steam at 1,000 degrees. Yeah. Speaker 3 (5:01) You can also get rid of the oxygen. You can do that by smothering a fire in sand with a blanket. Stop, drop, and roll. You can also use fire extinguishers that shoot carbon dioxide foam on the fire. Carbon dioxide is heavier than oxygen. It's going to go down low. It's going to starve, suffocate the fire. But there's actually a third way to stop a fire. Wait, a fourth way? There's a fourth way. There's a fourth point on the... The fire triangle. You're going Speaker 4 (5:29) to try and tell me now that this is actually a fire square and has been all along. It's Speaker 3 (5:32) never been a square because squares don't have the beautiful geometric property where every single vertex touches. Speaker 4 (5:42) Sure. So Speaker 3 (5:43) instead, they call it the fire tetrahedron. Oh, Speaker 4 (5:46) much nicer. Speaker 3 (5:47) Okay. The fire pyramid. And so before we talk about this fourth point on the... the fire pyramid, the fire tetrahedron. Let's talk about what happens when something burns, like a candle, okay? So strike a match. Now I've got a big source of heat. I bring that towards the candle, and the candle is a source of hydrocarbons, and they're in this solid-ish form, right? It's a wax. And when that heats up, the hydrocarbon breaks apart. It chemically breaks apart into what are called free radicals. These are basically... If you're familiar with molecules, when you look at these, you go, that's not a molecule. That's like it's incomplete. It's like half a molecule. It is ready to combine with stuff. So we've got diatomic carbon, just like two carbon atoms. Give me a break. They've got free electrons. They need to combine with something, but they aren't yet. You've got things like, I don't know, carbon and hydrogen. Just those two atoms bonded together. They're extremely reactive. They're radical. Don't Speaker 4 (6:48) you think that sort of, I always think the phrase free radical describes, I don't know, people in San Francisco in the 1970s, you know? Yes, very Speaker 3 (6:55) similar. So whichever analogy is going to help you, just grab on to it. Speaker 4 (6:59) As I've mentioned before on this podcast a number of times, chemistry, my chemistry knowledge is pretty limited, right? Like maths and physics, I'm all there. Biology, I can make my way around. Chemistry is ropey at best. But I remember in chemistry lessons, there was this talk about the states of the electron, right? That it's like you have these shells, which I think is actually an incorrect model. Yeah, Speaker 3 (7:27) but I think it works for kind of understanding and explaining chemical reactions. Speaker 4 (7:30) That the electrons are at different levels, energy levels. some farther and some closer in towards the nucleus of the atom. And when the electrons move between those states, move between those energy levels, that's when they start emitting certain colors of light. Speaker 3 (7:47) Yeah, when they absorb electromagnetic radiation, there's a lot of ways to describe what's happening. They're vibrating more. They go up an energy level. My chemistry, my atomic chemistry, is also pretty limited. But when they fall back down... they usually emit something like a photon. And that might be a photon with a wavelength we can see or it might not. It might be visible light and we go, wow, it's glowing. Or it might be thermal light, in which case we can feel warmth and a snake can say, wow, I see it. But these radicals are coming out in kind of two main areas. One is right out of the wick where they actually don't have any access to oxygen. So when you look at a candle flame, you can sometimes see that right by the wick, it's kind of darker. It's like it's very beautiful. There's like this darker region in the flame. And that's where you've got this vaporized wax primarily made out of radicals that haven't combined with anything yet. Down near the base, those radicals combine with oxygen and they have so much energy that they. literally release light through a chemical process called chemiluminescence, where their electrons are just like so energized, they drop down, release photons, and it happens to be in the blue wavelength. Speaker 4 (9:02) So Speaker 3 (9:03) the blue part of a flame and the dark inner part of the flame are gases. They're just a mixture of gases, primarily gases that are vaporized radicals, vaporized wax. But higher up, you get the classic red-orange flame of a fire. And that is where the really hot radicals are interacting with oxygen and they're finally getting a companion to share electrons with. And they're forming carbon dioxide. They're forming water vapor. That process creates a lot of energy. Like to break a bond, you've got to put in energy. But when you form a bond, energy is produced. And that's why the flame is so bright. The heat produced by the radicals interacting with oxygen is a self-perpetuating process. That's why fire kind of like persists. You touch the match to the flame. That's enough heat to get it started. But then what happens to those radicals and the oxygen produces enough heat to keep it going. So it just keeps eating. It's almost like alive in that sense. It preserves itself. Right. Speaker 4 (10:11) So this is what I was thinking as you were talking, because actually, when you try and come up with a definition for what life is, consumption is a big part of it. Being able to sort of self-sustain is another part of it. And actually, by a few different measures, fire sort of is alive. It's like an Speaker 3 (10:32) almost life. Speaker 4 (10:33) So Speaker 3 (10:33) if you really don't want fire to count, you can say life needs DNA. It's like. Come on. Wet blanket. Come on. Come Speaker 4 (10:40) on. But Speaker 3 (10:41) fire, you know, fire. A flame is made of totally different molecules every second. Speaker 4 (10:47) But Speaker 3 (10:47) I'm made of different molecules every five, six years. So, like, I'm just a slow flame. You're just a slow flame. We can discuss what life is in another episode. I Speaker 4 (10:59) mean, there's all sorts of chemistry going on inside of you. There's all sorts of combustion even. Speaker 3 (11:02) There's slow combustion going on inside me. You're just Speaker 4 (11:05) a slow... Yeah, Speaker 3 (11:07) it's like Pajero was like, guys, let's just chill out Speaker 4 (11:10) and Speaker 3 (11:11) become traditional life. Speaker 4 (11:12) I think it's Speaker 3 (11:13) the Speaker 4 (11:13) other way around. I think fire looks at humans like, guys, can we speed this up? Speed it up. What Speaker 3 (11:17) are you doing? Funny enough, when I say that joke, I realize that it's entirely not true because fire on Earth doesn't really predate life. Fire is able to exist. Combustion is able to happen on this planet because life was here first. There was a time before... wildfires of any kind could even happen. Obviously, there was a time before there were plants on the dry land, but you need to have free oxygen. Those radicals need something to react with. Well, they need oxygen to react with to produce what we call fire. Hold Speaker 4 (11:50) on. Is that true, then, that you that you only get? I mean, of course, you only get fire when you have oxygen. Fine. But you only get. Fire where there is life. Do you not see fire in the rest of the solar system? You don't need Speaker 3 (12:02) life, but you do not see fire, except I'm sure in some crazy situations that are extremely rare. You do not see fire anywhere else in this solar system. There just isn't free oxygen on any other planet where a fire can just start happening. Even early Earth had no fire. We needed to wait until life, like very early. algaes and bacterias came about that were like, hey, I'm going to poop out oxygen. And they gradually, over millions of years, put oxygen into the atmosphere. Once that reached about 13%, which was more than 2 billion years ago, which is like halfway through Earth's life. So for the last couple Speaker 4 (12:42) of billion years, there was no fire. There Speaker 3 (12:44) was no fire. Yeah. So there were living things on Earth before there was fire. Speaker 4 (12:50) So hold on. Does that mean that fire has a birthday? Speaker 3 (12:54) The possibility of the chemical reaction happening has always existed. And I'm sure fires were happening elsewhere in the universe. On Earth. But Speaker 4 (13:05) on Earth. There would have been one day, one single day when the first fire happened. Speaker 3 (13:11) Yes. I mean, that's true. But it depends on. How we define fire, because I'm sure we could find an isolated incident of combustion, very, very quick combustion in the early days of Earth. There happened to be enough oxygen in some little environment that it happened. But evidence for like a fire fire, a wildfire is much harder to find. The earliest evidence we have found of a wildfire was 420 million years ago. Speaker 4 (13:40) Right. So actually much later. Yeah. Speaker 3 (13:42) Much, much later. Because you needed plants to evolve on dry land that could burn, that could be a fuel source. Yeah. Speaker 4 (13:49) And that Speaker 3 (13:49) evidence, by the way, was found in Wales. Speaker 4 (13:52) Really? Okay. Speaker 3 (13:53) Right in this country, the United Kingdom. And then we're going to jump forward a lot just because this fact is also very cool. The earliest evidence we have of humans making fire on purpose were Neanderthals in Suffolk. What? They found this fire pit, not Homo sapiens, but it looks like Neanderthals. had