Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Aeris on 09/09/2021 22:26:44
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Ok so, unpunny titles aside, I love fire. It's one of the most beautiful things to look at and it's my favorite of the four classical elements coined by Greek philosophers. It's also a goldmine of science that singlehandedly taught me more about thermodynamics, plasma physics and chemical reactions then my time at secondary school. There's still a lot about the process of combustion that I still can't fully wrap my head around though, and since this website seems to be full of super smart people that know their stuff, I was wondering if you guys and girls could answer some red-hot questions I have about fire, flames and light.
Q1. Does the light of a flame come from the fuel reacting with heat and oxygen, the combustion byproducts radiating some of their energy away in the form of visible light or a combination of both?
Q2. Why does a candle burn with a (mostly) white flame, and why does a stack of wood and a patch of grass burn with an orangey yellow flame?
Q3. Why does complete combustion yield a blue flame as opposed to the standard orange-yellow flame? I know it has something to do with the perfect combination and amount of fuel, oxygen and heat, but how do all of these things come together to form a blue-colored flame?
Q4. Does a Hydrogen flame always burn blue regardless of incomplete combustion or complete combustion? Why is the flame itself barley visible to the human eye?
Q5. According to Wikipedia, Soot (a byproduct of incomplete combustion) can exist the form of a gas. Is this true? Does Soot have a sublimation-point? If so, how hot would it need to be to exit the solid state and become gaseous?
Q6. According to several sources I found on the internet, an object gains more mass when it gains more energy. Assuming this is true, would that mean getting punched by a fist that's on fire would hurt more than a normal punch?
Q7. What gives each of the flames shown in the videos below their color?
Link to the Wikipedia page on Soot https://en.wikipedia.org/wiki/Soot
Also, not at all related to the questions above, but what is YOUR favorite classical element? I'm curious to hear why.
Thanks :)
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Hi.
There are some articles and especially forum-like replies for questions similar to these.
Here's two links that seem to come up high on a Google search and also seemed sensible when I read them:
From Physics stackexchange:
https://physics.stackexchange.com/questions/44664/possible-colors-of-fire
From Chemistry stackexchange:
https://chemistry.stackexchange.com/questions/102280/how-is-the-light-from-a-fire-emitted
First of all, I'm not an expert on flames - these answers just seemed sensible to me. Anyway let's see how some of this information connects with your questions:
Q1. Does the light of a flame come from the fuel reacting with heat and oxygen, the combustion byproducts radiating some of their energy away in the form of visible light or a combination of both?
Depends on what you are considering as a "flame". The light of something undergoing combustion comes from a combination of the things you've described. However, we usually consider "a flame" to be something that rises above the actual fuel being burnt.
Let's consider a lump of coal that is being burnt. There is some red glow directly from the solid lump of coal. There are also some flames that rise a little above the coal. These flames are thought to be the result of a gaseous fuel being burnt. There are some volatile compounds contained in coal - volatile just means that they are easily turned to a gaseous state when they are warmed. It's also possible for combustion to be incomplete, or occur in multiple stages. A long chain hydrocarbon in coal can be broken into smaller chains, these small chains will then tend to be gases and will drift away to continue combustion elsewhere.
Anyway, a gaseous fuel can travel quite a distance in a convection current in the air before it actually reacts with Oxygen and is burnt. This explains how most of the light we see is from flames that seem to be above the solid fuel (coal) that you are burning.
Burning things in Zero-G environments produces unusual effects, flames don't tend to rise above the thing being burnt because there is no gravity. We need gravity to create a buoyancy effect on warm (less dense) air and hence to produce convection currents that can carry gaseous fuels (and particles of soot) upwards from something being burnt. In Zero-G environments the volatile gases (and all other particles like soot) spread out in all directions and we see symmetric balls of fire all around a solid lump of something that is being burnt.
Anyway, what is it that is producing the light? There seems to be two important sources or explanations:
1. Blackbody radiation. Materials get warm and radiate electromagnetic waves. The unburnt fuel is getting warm and therefore radiating light and infrared radiation. The products of the reaction are also getting warm and radiating for the same reason. Any soot in the flames is getting warm,.... everything in the region is getting warm and emitting e-m radiation, some of which will be in the visible spectrum if the temperature is high enough. There is a general correlation between the temperature at which something burns and the colour of the fire and any surrounding material. Warm objects tend to look red, while hotter objects tend to look white - this is exactly what blackbody radiation predicts.
2. Electron transitions within certain atoms. There is enough energy around and some atoms that are isolated enough will behave exactly as we would expect from models of these atoms. For example, if we add some copper to a flame then there are some copper atoms and we observe some blue/green light being emitted which is characteristic of certain electron transitions within those atoms. This is the basis for the "flame tests" you might have studied at school. Different metals produce flashes of different colours when inserted into a flame. Carbon atoms and short carbon chains tend to support electron transitions that produce light which is yellow-orange. Many of the fuels we burn (and indeed many of the things around us that we accidentally set fire to) contain carbon rich compounds and therefore we see a lot of yellow-orange flames.
If you burn non-carbon based fuels, like pure Hydrogen, you don't see yellow flames. Hydrogen burns with a blue tinted but almost colourless flame. You might have seen this in school when you tested for Hydrogen gas in a test tube by inserting a lit splint.
Personally, I would think that all light emitted is ultimately caused by electron transitions within some atom. However these transitions are complicated, they could involve a transition from an orbital that was not involved in a chemical bond to one that does sustain a chemical bond. These orbitals are also influenced by the close proximity of other atoms. There are millions of different configurations each with fine differences so we observe a more continuous spectrum of emitted frequencies. We see something that looks like "Blackbody radiation" rather than a discrete spectrum we might expect for an isolated atom. However, many answers separate "Blackbody radiation" from "emission due to excitation of atoms" and do not suggest that they are fundamentally the same.
First half of Q.2 Q2. Why does a candle burn with a (mostly) white flame,
The usual answer is that candles contain long-chain hydrocarbons (waxy stuff). When these burn they produce more soot (unburnt carbon and some short chains of hydrocarbons). This soot is extremely effective at absorbing the heat and radiating light in many different colours. All these different colours appear as white light to us human beings.
- - - - OK. I hope that's enough to start with and I'll leave some questions for others to answer - - - - -
Bye for now.
late editing: I've already noticed that there is some controversy over the colour of a Hydrogen flame. It'll be interesting to see what other people say.
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Did someone change the title of my question? I didn't even know you could do that.
@Eternal Student - Thank you for you reply. I'll check out those links you gave me right away :)
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Hello Aeris & Welcome to TNS!
🙏
Thanks for the Detailed Explanation Eternal.
👍
I had a fishy query not entirely related to the subject of the OP.
But, can a single Atom burn?
Like can it catch Fire?
& Then what residue would be left behind?
🤔
(Future Responders, Please stick to Answering the original subject raised in the OP & just sidenote your responses to my query.)
Ps - Airplanes or OPs, hijacking SuckZ!
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Hi again.
Did someone change the title of my question? I didn't even know you could do that.
I've only ever seen it when a moderator felt the original post wasn't phrased as a question. However, your original title was "Burning questions" which seemed OK to me.
Out of interest, I'll try to modify the title again for 2 minutes.
NOTES: the title was: Re: Where does firelight come from? And other burning questions...
I can copy and paste that back in later if I need to.
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Hi again,
OK, it seems that an ordinary person can change the title and it appears in their reply at the top. See my previous reply which has a slight variation on the title. However, this doesn't change the main title on the OP (Original Post).
I have to guess it was done by a moderator. There is a preference to have all forum posts presented as questions. However, your post does look like a perfectly sensible set of questions to me. Perhaps there is already an existing post with the same title and there was a need to avoid confusion.
Best Wishes.
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OK, it seems that an ordinary person can change the title and it appears in their reply at the top. See my previous reply which has a slight variation on the title. However, this doesn't change the main title on the OP (Original Post).
I have to guess it was done by a moderator. There is a preference to have all forum posts presented as questions. However, your post does look like a perfectly sensible set of questions to me. Perhaps there is already an existing post with the same title and there was a need to avoid confusion.
Best Wishes.
I see. Well, personally it doesn't really bother me much, I was just surprised that that's something you can even do. The reason why it was done though, makes perfect sense to me, and if my post really does need some improving here and there, by all means, don't be afraid to tell me so :)
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Hi.
if my post really does need some improving here and there, by all means, don't be afraid to tell me so
It looks good to me. Thanks for spending some time here. You're picking up the general rules for using the forum and the features of this interface really fast (for example, you can quote sections from other people's posts and insert smiley faces). The grammar and spelling is also good, so you're spending time. Your questions are quite challenging; you're obviously an intelligent person; you seem to be interested in science and learning and you're polite. I don't know what more anyone would be looking for in a forum member.
The only thing I haven't done is check too many of the video links you provided. Obviously people get paid for having more views of their YT videos and this becomes a grey area with respect to advertising. It's perfectly fine and often really helpful to include some YT videos if you are genuinely talking about the content and keeping in the spirit of the forum (which I expect you were). It's only a grey-area if you were deliberately picking videos from yourself and your friends.
...since this website seems to be full of super smart people that know their stuff, I was wondering if you guys and girls could answer some red-hot questions I have about fire, flames and light.
I do housework as my day job. I seek discussion here because otherwise I'd be doing the laundry. Don't overvalue my replies or opinions. The moderators (which will be identified just under their name) are described as "experts in one or more fields of study" according to the information given on this website. Their replies are usually quite solid and reliable. You'll make your own opinions of other members as you go. In this section, Chemistry, I would be expecting a member called "Bored_Chemist" to be contributing some high quality replies.
I had a fishy query not entirely related to the subject of the OP.
But, can a single Atom burn?
Like can it catch Fire?
& Then what residue would be left behind?
Firstly, single atoms are not very stable objects. You just don't tend to find them all alone and un-bonded to something in the real world. The noble gases are the obvious example of something that does seem to exist as individual atoms. These don't react with Oxygen (they don't burn), they are quite inert (which is why they float around as individual atoms in the first place).
However, if you did have a single atom of say Carbon, then it can react with Oxygen in the atmosphere. This is, by definition, burning or "combustion". It may not not "look" like it had caught fire because fire or flames are a multi-particle phenomena. One particle of Carbon reacting with one molecule of Oxygen would be unimpressive and we are assuming there are no other particles to get involved with producing a flame.
What residue would be left behind? Assuming complete combustion of 1 atom of Carbon with 1 molecule of Oxygen, we expect a molecule of Carbon Dioxide to be formed. This will tend to be a gas in ordinary conditions and would float away, no obvious residue left behind.
Best Wishes.
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Hi.
It looks good to me. Thanks for spending some time here. You're picking up the general rules for using the forum and the features of this interface really fast (for example, you can quote sections from other people's posts and insert smiley faces). The grammar and spelling is also good, so you're spending time. Your questions are quite challenging; you're obviously an intelligent person; you seem to be interested in science and learning and you're polite. I don't know what more anyone would be looking for in a forum member.
The only thing I haven't done is check too many of the video links you provided. Obviously people get paid for having more views of their YT videos and this becomes a grey area with respect to advertising. It's perfectly fine and often really helpful to include some YT videos if you are genuinely talking about the content and keeping in the spirit of the forum (which I expect you were). It's only a grey-area if you were deliberately picking videos from yourself and your friends.
Aw, thank you so much for saying that, that honestly just made my day :)
Also, thanks for letting me know about the potential issues of posting links to YouTube videos on the forum. I tried to insert them as links, but when I posted the list of questions above, they automatically transformed into videos (which I guess is convenient for answering the question without having to leave the forum). Don't worry though, I promise to only include videos in my questions that are related to the topic.
I do housework as my day job. I seek discussion here because otherwise I'd be doing the laundry. Don't overvalue my replies or opinions. The moderators (which will be identified just under their name) are described as "experts in one or more fields of study" according to the information given on this website. Their replies are usually quite solid and reliable. You'll make your own opinions of other members as you go. In this section, Chemistry, I would be expecting a member called "Bored_Chemist" to be contributing some high quality replies.
Understandable. I just like complimenting people honestly, lol :)
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Hi again.
I've read through the remaining questions you had. Let's pick another interesting one:
Q6. According to several sources I found on the internet, an object gains more mass when it gains more energy. Assuming this is true, would that mean getting punched by a fist that's on fire would hurt more than a normal punch?
Let's start with the light hearted comments first:
It would probably hurt the guy with his fist on fire more than it would make any difference to you.
Anyway, the basic idea is sensible. There are some minor technicalities and finally something surprising.
Let's start at the beginning:
A hotter object has more energy than a cooler object. This additional energy will increase the rest mass. So a hot fist would have more rest mass than a cold fist. Let's just consider this simple situation for a moment, the fist isn't on fire, it's just has more internal energy (it's hot).
The technicalities:
In assessing how much it hurts the person who is struck by the fist, let's also ignore any effect of being burnt by the fist. We are just considering the impact of the first on the target. We are also going to need to assume that the fist interacts the same way with target in each case. Let's assume the collision is inelastic and the fist is brought to a complete stop within 1 second of contact with the target. Then all of the momentum of the fist is transferred to the target in 1 second.
The total change in momentum would be called the "impulse". I've taken 1 second as the duration of the interaction because it makes everything simpler. We could calculate the average force exerted on the target during the impact event and it would be numerically the same as the impulse. Anyway, this impulse or average force could be taken as our measure of how much it is going to hurt and that's what I am going to do. If we were Biologists we could go further. How much something hurts is a biological, psychological or judgment call type of thing, not something that we can easily measure with a tape measure, a stop watch, Geiger counter, an accelerometer or anything a Physicist has in their lab. An impulse of 1 000 N for 1 second would hurt. It's like having the weight of a large person on you and supported on an area of skin the size of a fist for 1 second. So you can expect some bruising. Meanwhile an impulse of 10 000 N for 1 second wouldn't hurt 10 times as much, it would probably kill you. Similarly any larger impulse couldn't hurt more than this.
Anyway, we will just be considering the average force exerted on the target over the 1 second impact time as our measure of how much it hurts. It's a good measure of the mechanical damage that could be caused even if it isn't a measure of how much it hurts.
The physics appears
OK, we're all ready to go. So it's just about the change in momentum that occurs. Momentum is determined by the product of mass x velocity. So, we know the hot fist has more mass we only need to decide what velocity it might have. If we assume the hot and cold fists reached the same velocity, then the calculation is simple. Yes, of course, the hot fist has more momentum and causes more mechanical damage to the target. However, the real world situation is going to be less clear than this. When you punch something, the speed of your fist is determined by your muscle. A low mass fist can be accelerated faster than the same amount of muscle would accelerate a high mass fist. You can try this out yourself by going to the gym and seeing how fast you can punch with nothing in your hand compared to how fast you punch with a 10Kg weight held in your hand. In reality, the assumption that a heavy fist and a light fist would reach the same velocities is not a very good one.
That's enough about hot and cold fists. The main take away point is that the hot fist has more mass and under various other assumptions this would make it hurt more.
How much difference would it actually make? You've probably seen Einsteins famous formula E = mc2 before. This means that we determine the increase in mass by dividing the additional energy by c2. c2 is a very large number, while the energy difference between a hot fist and a cold fist is a small number. So the increase in mass is barely noticeable.
We want fire, not just heat: If we assume the fist was actually on fire instead of just being warm, we have to consider some chemistry. There is combustion going on and this is changing solid material in the fist into gaseous combustion products. So, quite a lot of the mass of the fist is being lost as a gas that just drifts away and does not strike the target. This loss of mass is going to be more important than the increase in mass due to energy considerations. So, in reality a burning fist will be less massive by the time it strikes our target.
Finally There is something slightly counter-intuitive happening. Burning something does not make energy out of nothing. By setting the fist on fire we haven't actually given it more energy. It's more like the opposite. There was chemical potential energy in the original combustion products. By allowing them to react we emit energy into the surrounding environment. So, the reaction products have less energy (which will show as less mass) than the original reactants. The only way that the fist could have more energy when it's on fire is if the fist can absorb the heat faster than the atmosphere and surrounding environment can absorb the heat. In that case it has effectively taken some energy that was previously bound up in Oxygen molecules floating around in the atmosphere. This seems unlikely since a fist is not a very good conductor of heat. The surface of the fist would get hot and radiation would quickly carry the energy away to the environment. Overall, I think that the fist would lose energy (and show a corresponding drop in mass) when it is burning.
This is complicated. A burning fist is not a very good example to discuss changes in mass of reaction products and reactants. There are better articles and learning resources to explain this issue.
Best Wishes.
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Hi again.
I've read through the remaining questions you had. Let's pick another interesting one:
Q6. According to several sources I found on the internet, an object gains more mass when it gains more energy. Assuming this is true, would that mean getting punched by a fist that's on fire would hurt more than a normal punch?
Let's start with the light hearted comments first:
It would probably hurt the guy with his fist on fire more than it would make any difference to you.
Anyway, the basic idea is sensible. There are some minor technicalities and finally something surprising.
Let's start at the beginning:
A hotter object has more energy than a cooler object. This additional energy will increase the rest mass. So a hot fist would have more rest mass than a cold fist. Let's just consider this simple situation for a moment, the fist isn't on fire, it's just has more internal energy (it's hot).
The technicalities:
In assessing how much it hurts the person who is struck by the fist, let's also ignore any effect of being burnt by the fist. We are just considering the impact of the first on the target. We are also going to need to assume that the fist interacts the same way with target in each case. Let's assume the collision is inelastic and the fist is brought to a complete stop within 1 second of contact with the target. Then all of the momentum of the fist is transferred to the target in 1 second.
The total change in momentum would be called the "impulse". I've taken 1 second as the duration of the interaction because it makes everything simpler. We could calculate the average force exerted on the target during the impact event and it would be numerically the same as the impulse. Anyway, this impulse or average force could be taken as our measure of how much it is going to hurt and that's what I am going to do. If we were Biologists we could go further. How much something hurts is a biological, psychological or judgment call type of thing, not something that we can easily measure with a tape measure, a stop watch, Geiger counter, an accelerometer or anything a Physicist has in their lab. An impulse of 1 000 N for 1 second would hurt. It's like having the weight of a large person on you and supported on an area of skin the size of a fist for 1 second. So you can expect some bruising. Meanwhile an impulse of 10 000 N for 1 second wouldn't hurt 10 times as much, it would probably kill you. Similarly any larger impulse couldn't hurt more than this.
Anyway, we will just be considering the average force exerted on the target over the 1 second impact time as our measure of how much it hurts. It's a good measure of the mechanical damage that could be caused even if it isn't a measure of how much it hurts.
The physics appears
OK, we're all ready to go. So it's just about the change in momentum that occurs. Momentum is determined by the product of mass x velocity. So, we know the hot fist has more mass we only need to decide what velocity it might have. If we assume the hot and cold fists reached the same velocity, then the calculation is simple. Yes, of course, the hot fist has more momentum and causes more mechanical damage to the target. However, the real world situation is going to be less clear than this. When you punch something, the speed of your fist is determined by your muscle. A low mass fist can be accelerated faster than the same amount of muscle would accelerate a high mass fist. You can try this out yourself by going to the gym and seeing how fast you can punch with nothing in your hand compared to how fast you punch with a 10Kg weight held in your hand. In reality, the assumption that a heavy fist and a light fist would reach the same velocities is not a very good one.
That's enough about hot and cold fists. The main take away point is that the hot fist has more mass and under various other assumptions this would make it hurt more.
How much difference would it actually make? You've probably seen Einsteins famous formula E = mc2 before. This means that we determine the increase in mass by dividing the additional energy by c2. c2 is a very large number, while the energy difference between a hot fist and a cold fist is a small number. So the increase in mass is barely noticeable.
We want fire, not just heat: If we assume the fist was actually on fire instead of just being warm, we have to consider some chemistry. There is combustion going on and this is changing solid material in the fist into gaseous combustion products. So, quite a lot of the mass of the fist is being lost as a gas that just drifts away and does not strike the target. This loss of mass is going to be more important than the increase in mass due to energy considerations. So, in reality a burning fist will be less massive by the time it strikes our target.
Finally There is something slightly counter-intuitive happening. Burning something does not make energy out of nothing. By setting the fist on fire we haven't actually given it more energy. It's more like the opposite. There was chemical potential energy in the original combustion products. By allowing them to react we emit energy into the surrounding environment. So, the reaction products have less energy (which will show as less mass) than the original reactants. The only way that the fist could have more energy when it's on fire is if the fist can absorb the heat faster than the atmosphere and surrounding environment can absorb the heat. In that case it has effectively taken some energy that was previously bound up in Oxygen molecules floating around in the atmosphere. This seems unlikely since a fist is not a very good conductor of heat. The surface of the fist would get hot and radiation would quickly carry the energy away to the environment. Overall, I think that the fist would lose energy (and show a corresponding drop in mass) when it is burning.
This is complicated. A burning fist is not a very good example to discuss changes in mass of reaction products and reactants. There are better articles and learning resources to explain this issue.
Best Wishes.
Yeah, I probably should've specified how exactly this would work, mechanically speaking. Realistically, you'd 'set' your fist on fire by covering it in some sort of flammable liquid that would stick to you for a long enough time to be delivered to the person you plan on punching. If you had magic/psychic fire powers though, you could just envelop the area around your fist in a small cloud of weakly ionized gas (Kim Possible's Shego anyone?). Still, I'm extremely impressed at just how detailed your answer was, given how vague I was. Thanks so much :)
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Hi again.
Are you (Aeris) happy enough with the answers. I can see there are a few outstanding questions but I believe some of them can be answered using what's already been said.
Let's take an example:
Q3. Why does complete combustion yield a blue flame as opposed to the standard orange-yellow flame? I know it has something to do with the perfect combination and amount of fuel, oxygen and heat, but how do all of these things come together to form a blue-colored flame?
I think it very much depends on what you are burning. For example, if you burnt something with a lot of copper in it then you would expect to get some of the characteristic blue/green flashes of copper in the flame. It doesn't matter how well oxygenated and how complete the combustion might be, there will be some greenish light.
For the more typical fuels that we burn (e.g. Hydrocarbons), it's already been stated that yellow/orange flames are characteristic of carbon and short carbon chains existing in the flame. If you can keep the fire well oxygenated, for example by mixing oxygen in with the gaseous fuel BEFORE combustion takes place then you can greatly reduce the amount of unburnt Carbon and short carbon chains persisting in the flame. You don't get the more conventional effects of some incomplete combustion happening low down in the flame and some unburnt carbon persisting into the upper parts of the flame (where they will either continue combustion or else escape as unburnt carbon). Instead any molecules that start combustion tend to undergo complete combustion almost immediately.
A school bunsen burner achieves this effect by opening an air hole on the shaft of the bunsen burner. Air is then drawn in and mixed with the fuel before the actual combustion reaction starts (which is only at the top of the shaft).
Anyway, if there are more questions you want answered, flag them up again (i.e. just write something here like "I still need an explanation for Q.10").
Best Wishes.
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Q5. According to Wikipedia, Soot (a byproduct of incomplete combustion) can exist the form of a gas. Is this true? Does Soot have a sublimation-point? If so, how hot would it need to be to exit the solid state and become gaseous?
I had a look at Wikipedia, and I think this is the sentence you are referring to?
Soot is...“Particles formed during the quenching of gases at the outer edge of flames of organic vapours, consisting predominantly of carbon..."
A few observations:
1) Soot is mostly nanoparticles of carbon, and carbon is solid to very high temperatures compared to the usual flammable hydrocarbons (eg methane, gasoline, candle wax). Wikipedia lists the sublimation point of macroscopic carbon as 3642 °C. This is much higher than the temperatures in a candle flame, so soot will not sublimate - it will remain a solid within the flame. I think the "vapours" refers to hydrocarbons that are above their boiling point, and are able to react readily with any oxygen that is around.
2) Carbon is quite flammable in an oxygen atmosphere, provided you get it hot enough (after all, black coal is mostly carbon). It is definitely hot enough in a flame, but the oxygen can only get to the outer atomic layers of the nanoparticles, so burning is slow.
3) As soon as the soot is carried outside the active flame, the temperature drops low enough that the soot stops burning (the flame is "quenched"). The nanoparticles of soot will persist in the atmosphere; it is a health hazard, being listed as a known carcinogen, and adversely affects lung and heart health.
4) The reason the soot forms from burning hydrocarbons is because of insufficient oxygen. The hydrogen atoms atoms readily react with oxygen to form water, and carbon atoms will react with oxygen to form carbon dioxide. Any "left over" carbon atoms tend to stick together to form solid soot, which burns more slowly. So adjust the oxygen flow so it doesn't produce soot! (When I was a child, we had a kerosene heater, and it often produced clouds of soot; the WHO has recognized the use of kerosene lighting as a major health risk in the third world - a solar panel + LED would greatly improve health in these countries.)
5) The affinity of carbon atoms for each other is great, and outer space is filled with soot. The reason we can't see into star-forming nebulas, or the center of our galaxy, is because the path is blocked by soot. Soot (carbon) is produced in medium-sized stars before they go supernova. As it cools, the common elements (hydrogen, carbon, oxygen, nitrogen, silicon) form compounds like water, methane, carbon monoxide/dioxide, ammonia - and soot. The JAXA mission Hayabusa2 has recently returned samples from carbonaceous asteroid Ryugu, and they are now being analyzed in labs around the world; I expect it will have a lot of soot.
See: https://en.wikipedia.org/wiki/162173_Ryugu
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Here's the definition in Wikipedia
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.[1][a] Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2); the bond energies of the fuel play only a minor role here.[2] At a certain point in the combustion reaction, called the ignition point, flames are produced. The flame is the visible portion of the fire. Flames consist primarily of carbon dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma.[3] Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.
https://en.wikipedia.org/wiki/Fire
Another type of fire that's interesting.
A cool flame is a flame having maximal temperature below about 400 °C (752 °F).[1] It is usually produced in a chemical reaction of a certain fuel-air mixture. Contrary to conventional flame, the reaction is not vigorous and releases very little heat, light, and carbon dioxide. Cold fires are difficult to observe and are uncommon in everyday life, but they are responsible for engine knock – the undesirable, erratic, and noisy combustion of low-octane fuels in internal combustion engines.
https://en.wikipedia.org/wiki/Cool_flame
And even more interesting fire, which doesn't burn.
In this video I show you how to make freezing fire that is so cold that it freezes things instead of burns them! I use cooled helium to flow past a high voltage electrode. This is the follow up video to the cold fire video I made last time. Here is that video if you haven't seen it yet:
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Hi again.
Are you (Aeris) happy enough with the answers. I can see there are a few outstanding questions but I believe some of them can be answered using what's already been said.
Oh yes, I'm very much happy with what's been said so far.
I think it very much depends on what you are burning. For example, if you burnt something with a lot of copper in it then you would expect to get some of the characteristic blue/green flashes of copper in the flame. It doesn't matter how well oxygenated and how complete the combustion might be, there will be some greenish light.
For the more typical fuels that we burn (e.g. Hydrocarbons), it's already been stated that yellow/orange flames are characteristic of carbon and short carbon chains existing in the flame. If you can keep the fire well oxygenated, for example by mixing oxygen in with the gaseous fuel BEFORE combustion takes place then you can greatly reduce the amount of unburnt Carbon and short carbon chains persisting in the flame. You don't get the more conventional effects of some incomplete combustion happening low down in the flame and some unburnt carbon persisting into the upper parts of the flame (where they will either continue combustion or else escape as unburnt carbon). Instead any molecules that start combustion tend to undergo complete combustion almost immediately.
A school bunsen burner achieves this effect by opening an air hole on the shaft of the bunsen burner. Air is then drawn in and mixed with the fuel before the actual combustion reaction starts (which is only at the top of the shaft).
Ok, but like, WHY are complete combustion flames blue? Are flames just blue by default when soot/carbon is taken out of the equation? Do the products of complete combustion emit light on the blue part of the spectrum due to their wavelengths?
Anyway, if there are more questions you want answered, flag them up again (i.e. just write something here like "I still need an explanation for Q.10").
I wouldn't mind getting an answer to questions 4 and 7, but I'm in no rush whatsoever. Take as long as necessary to get back to me on them.
Best Wishes.
Thanks :) You too.
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I had a look at Wikipedia, and I think this is the sentence you are referring to?
Soot is...“Particles formed during the quenching of gases at the outer edge of flames of organic vapours, consisting predominantly of carbon..."
I was actually referring to this line "Gas-phase soot contains polycyclic aromatic hydrocarbons (PAHs)."
1) Soot is mostly nanoparticles of carbon, and carbon is solid to very high temperatures compared to the usual flammable hydrocarbons (eg methane, gasoline, candle wax). Wikipedia lists the sublimation point of macroscopic carbon as 3642 °C. This is much higher than the temperatures in a candle flame, so soot will not sublimate - it will remain a solid within the flame. I think the "vapours" refers to hydrocarbons that are above their boiling point, and are able to react readily with any oxygen that is around.
Huh, that's interesting. So what would I get if I heated some soot (macroscopic carbon) up to its sublimation point? Soot usually glows orange-yellow when heated, but what color would it adopt once it becomes a gas? Will it stay orange/yellow, or will it change?
The rest of your answer was insightful. Gonna look at that link more thoroughly later today. Thank you :)
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And even more interesting fire, which doesn't burn.
Not really a fire.
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Soot is a powder-like form of amorphous carbon. Gas-phase soot contains polycyclic aromatic hydrocarbons (PAHs).
Soot as a "powder" refers to a solid phase of carbon nanoparticles.
- "Gas-phase soot" is the solid powder that is generated in the complex tangle of reactions in a burning gas.
- So the gas-phase refers to the burning gas
- The Gas-phase does not refer to the solid nanoparticles
Normal hydrocarbons have a formula like CnH2n+2 (although the number of hydrogens will be a bit lower if the molecule contains a carbon ring).
PAHs are mostly carbon rings, containing very many carbon atoms and few hydrogen atoms. To make a visible soot, the particles are around the wavelength of light (10-6m), while the distance between Carbon and hydrogen is a thousand times smaller, at 10-9m. So these PAHs may contain thousands to billions of carbon atoms, and relatively few hydrogen atoms.
See: https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon#Natural_fires
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And even more interesting fire, which doesn't burn.
Not really a fire.
Do you restrict the definition of fire to chemical reaction with oxygen?
Does it have to involve carbon?
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And even more interesting fire, which doesn't burn.
Not really a fire.
Do you restrict the definition of fire to chemical reaction with oxygen?
Does it have to involve carbon?
There is NO chemical reaction in that video.
It's not a fire.
Nor is a fluorescent lamp.
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Hi again.
Ok, but like, WHY are complete combustion flames blue? Are flames just blue by default when soot/carbon is taken out of the equation? Do the products of complete combustion emit light on the blue part of the spectrum due to their wavelengths?
The answer I would offer is a hybrid of things previously mentioned:
1. Yes, removing the soot / carbon is a large part of the explanation.
2. Complete combustion doesn't always produce a blue flame. Acetylene has the formula H-C≡C-H, it has a triple bonded Carbon - Carbon link and the entire molecule then has an equal portion of Carbon to Hydrogen. An acetylene torch is commonly used in metal work. It's hot enough to cut (melt) through metal.
With limited oxygen, the flame is yellowy, just as we would expect from burning a more typical Hydrocarbon and for much the same reasons - there will be unburnt Carbon in the flames.
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTbC6tiWbmrbIwi1kRCk5PEq-jBfZpzKpU_Wg&usqp=CAU)
However, when oxygen is mixed with the gaseous fuel we obtain a flame that is almost pure white. The image below shows regions tinted green and purple but a lot of this is due to the limits of the camera. Overall a good flame is almost white to the human eye.
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTLX9rZZ_ia44PysIz7cxFbV9DCAb3e_5vNxA&usqp=CAU)
Anyway, the main point is that the flame isn't especially blue. The exact colour you obtain in a flame very much depends on what you are burning.
It just so happens, that most fuels we burn contain more saturated hydrocarbons than acetylene. The proportion of Hydrogen to Carbon is much higher than in acetylene. Long chains of Hydrocarbons also behave like the smaller chains of saturated hydrocarbons under combustion because combustion is a complicated, multi-stage process and quite often a long chain hydrocarbon is broken apart and behaves like two smaller chain hydrocarbons anyway - or enough to explain the colour of flames produced from it. Overall, short chain saturated hydrocarbons do tend to burn with a bluish flame under a well oxygenated complete combustion situation.
3. Wien's displacement law: https://en.wikipedia.org/wiki/Wien%27s_displacement_law
This explains half of the reason we seek. It helps to explain why flames would look less red (less orange to be honest). It doesn't quite justify why flames turn blue instead of white.
Wien's law shows that for blackbody radiation the peak wavelength we expect from an object moves from red light wavelengths to blue light wavelengths as the temperature increases. To be quite honest, it's not usually so hot that the blue light would dominate and there would be hardly any red light. Instead, there just becomes a more even distribution of light of all colours and the overall light should then appear white rather than red.
However, we should note that yellow/orange is much closer to the red wavelengths while blue or an overall white would require higher temperatures. The temperature of the flames is dependant on how complete the combustion process can be. More complete combustion does produce more heat. Therefore, as the temperature of the fire increases, we would expect the flames to look less red (i.e. less yellow/orange). This is at least half the reason why the yellow-orange colour would disappear. Admittedly we might expect it to turn white rather than blue but, as previously noted above, there just does seem to be a more blue (acetylene being an obvious exception but there wasn't much hydrogen in those molecules).
Hydrogen is considered to burn with a bluish tint and if we simplify the combustion of a typical saturated hydrocarbon (recall that is where Hydrogen greatly outnumbers the carbon) then we can imagine that a lot of the reactions going on and many of the intermediate reaction species are much like Hydrogen undergoing combustion. This is probably enough to give us our blue tint.
4. Why does Hydrogen burn with a blue flame when it's well oxygenated? I'm really getting out of my depth here. It would seem likely that the intermediate reaction species favours electron transitions that will produce a blue light.
Did I mention that I am not an expert on flames?
Summary:
Not all flames associated with complete combustion taking place are blue. However, for many saturated hydrocarbons they would be.
Complete combustion of hydrocarbons removes most of the soot and carbon rich compounds from the flames. This is what caused less oxygenated flames to be yellow/orange.
The shift in Blackbody radiation with increasing temperature explains half of the change, more blue light is expected (it's just hard to explain why an overall white flame isn't seen).
The overall blue tint in complete combustion of saturated hydrocarbon fuels might be due to the proportion of Hydrogen atoms they contain and roughly explained by the species (particle types) that Hydrogen will provide during combustion.
Best Wishes.
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Essentially, the blue flame from a gas cooker or Bunsen burner is due to emissions from excited states of radicals like these.
https://en.wikipedia.org/wiki/File:Spectrum_of_blue_flame.png#/media/File:Spectrum_of_blue_flame_-_intensity_corrected.png
The temperature is somewhat beside the point.
If there's nothing in a flame that has an emission band in the yellow region, the flame will not emit yellow light.
On the other hand, an oxy acetylene torch is sufficiently hot that there's more ionisation; and ion/ electron pairs can produce a wider array of emission wavelengths, (and there's BB emission from transient soot particles too) so the flame looks more white.
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Soot as a "powder" refers to a solid phase of carbon nanoparticles.
- "Gas-phase soot" is the solid powder that is generated in the complex tangle of reactions in a burning gas.
- So the gas-phase refers to the burning gas
- The Gas-phase does not refer to the solid nanoparticles
Oh. So, there's no such thing as gaseous soot? It's just a name that describes the formation of soot particles during the process of incomplete combustion? That's a bit misleading. Gas-phase implies some level of sublimation/vaporization going on.
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Oh. So, there's no such thing as gaseous soot? It's just a name that describes the formation of soot particles during the process of incomplete combustion? That's a bit misleading. Gas-phase implies some level of sublimation/vaporization going on.
Soot is not actually in a gas phase since a gas is the phase of a material that consists of single atoms or molecules. Soot in the air is very small particles composed primarily of carbon that remain air borne, so it is not an actual gas phase.
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Great answers from both the Bored Chemist and the Eternal Student on my blue flame query. Thank you guys so much :)
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Soot is not actually in a gas phase since a gas is the phase of a material that consists of single atoms or molecules. Soot in the air is very small particles composed primarily of carbon that remain air borne, so it is not an actual gas phase.
I see. Still, is it possible to sublimate soot into a gas and if so, would it still glow orange-yellow?
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Soot as a "powder" refers to a solid phase of carbon nanoparticles.
What's the largest molecule which can still be considered as a gas?
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There is NO chemical reaction in that video.
It's not a fire.
Nor is a fluorescent lamp.
Let's draw the line between fire and non-fire.
Do you consider cool flame as fire? why or why not?
https://en.wikipedia.org/wiki/Cool_flame
Is Hydrogen and Chlorine Reaction considered as fire?
What about metal fire?
I think it would help in answering the question "Where does firelight come from?"
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There's still a lot about the process of combustion that I still can't fully wrap my head around though, and since this website seems to be full of super smart people that know their stuff, I was wondering if you guys and girls could answer some red-hot questions I have about fire, flames and light.
Good question if I can give a simple explanation, when light photons arrive from whatever the source they land on surface ore particles that then emit the return signal to our eyes the colour of what we see is a result of the chemical makeup from the emitting particle. I am sure that every chemical will have its own signiture.
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Is Hydrogen and Chlorine Reaction considered as fire?
Yes
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Do you consider cool flame as fire?
Yes.
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Do you consider glow stick as fire? why or why not?
https://en.wikipedia.org/wiki/Glow_stick
If someone invent hot glowstick,can it be called fire? what's the minimum temperature for it to be called fire?
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what's the minimum temperature for it to be called fire?
In principle, there isn't one.
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what's the minimum temperature for it to be called fire?
In principle, there isn't one.
Does it mean that any chemoluminescence are fire?
Do fireflies have fire in them?
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what's the minimum temperature for it to be called fire?
In principle, there isn't one.
Does it mean that any chemoluminescence are fire?
Do fireflies have fire in them?
No, and no.
A fire involves a reaction where at least one reactant is in the gas phase.
It's self sustaining
The mechanism by which it sustains itself is that the heat of reaction provides the activation energy to initiate reaction of further fuel.
The light comes from excited states of atoms, ions and molecules either heated to high energy states or released in excited states by reactions.
None of this is anything new.
http://www.engineerguy.com/faraday/pdf/faraday-chemical-history-complete.pdf
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None of this is anything new.
http://www.engineerguy.com/faraday/pdf/faraday-chemical-history-complete.pdf
Thanks, but the link doesn't seem to work.
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It's a reference to a rather old work.
Michael Faraday’s
The Chemic al History of a Candle
Google should find a copy for you , but the point is how long ago we recognised what fire is.
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The Chemic al History of a Candle
There's a dedicated page for this book in Wikipedia.
https://en.wikipedia.org/wiki/The_Chemical_History_of_a_Candle
The Chemical History of a Candle was the title of a series of six lectures on the chemistry and physics of flames given by Michael Faraday at the Royal Institution in 1848, as part of the series of Christmas lectures for young people founded by Faraday in 1825 and still given there every year.
The lectures described the different zones of combustion in the candle flame and the presence of carbon particles in the luminescent zone. Demonstrations included the production and examination of the properties of hydrogen, oxygen, nitrogen and carbon dioxide gases. An electrolysis cell is demonstrated, first in the electroplating of platinum conductors by dissolved copper, then the production of hydrogen and oxygen gases and their recombination to form water. The properties of water itself are studied, including its expansion while freezing (iron vessels are burst by this expansion), and the relative volume of steam produced when water is vaporized. Techniques for weighing gases on a balance are demonstrated. Atmospheric pressure is described and its effects demonstrated.
Faraday emphasizes that several of the demonstrations and experiments performed in the lectures may be performed by children "at home" and makes several comments regarding proper attention to safety.
The lectures were first printed as a book in 1861.
2016, Bill Hammack published a video series of the lectures supplemented by commentary and a companion book.[1] Faraday's ideas are still used as the basis for open teaching about energy in modern primary and secondary schools [2]
Contents of the six lectures
Lecture 1: A Candle: The Flame - Its Sources - Structure - Mobility - Brightness
Lecture 2: Brightness of the Flame - Air necessary for Combustion - Production of Water
Lecture 3: Products: Water from the Combustion - Nature of Water - A Compound - Hydrogen
Lecture 4: Hydrogen in the Candle - Burns into Water - The Other Part of Water - Oxygen
Lecture 5: Oxygen present in the Air - Nature of the Atmosphere - Its Properties - Other Products from the Candle - Carbonic Acid - Its Properties
Lecture 6: Carbon or Charcoal - Coal Gas Respiration and its Analogy to the Burning of a Candle - Conclusion
Unfortunately the book only covers a very specific type of fire, which is combustion of hydrocarbon with atmospheric oxygen. It doesn't say much about other kinds of fire, and where the boundary which separate them from other phenomena which are not fire.
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It's quite likely that, for hundreds or thousands of years, "fire" almost exclusively meant burning vegetation.
The interesting thing is that the definition has barely changed.
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It's quite likely that, for hundreds or thousands of years, "fire" almost exclusively meant burning vegetation.
The interesting thing is that the definition has barely changed.
Let's see what's the common definition of fire and flame.
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.[1][a] Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2); the bond energies of the fuel play only a minor role here.[2] At a certain point in the combustion reaction, called the ignition point, flames are produced. The flame is the visible portion of the fire. Flames consist primarily of carbon dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma.[3] Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.
https://en.wikipedia.org/wiki/Fire
A flame (from Latin flamma) is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone.[1] Very hot flames are hot enough to have ionized gaseous components of sufficient density to be considered plasma.
https://en.wikipedia.org/wiki/Flame
In sulfonation plants, we continuously burn sulfur to produce SO2 as one of main raw materials. Especially during startup, we must make sure that the fire presents, otherwise the furnace would be blocked by solidified sulfur.
It shows that the definition has been expanded.
By necessity, firefighters expanded the definition of fire even further.
Class D fires involve combustible metals - especially alkali metals like lithium and potassium, alkaline earth metals such as magnesium, and group 4 elements such as titanium and zirconium.[2]
Metal fires represent a unique hazard because people are often not aware of the characteristics of these fires and are not properly prepared to fight them. Therefore, even a small metal fire can spread and become a larger fire in the surrounding ordinary combustible materials. Certain metals burn in contact with air or water (for example, sodium), which exacerbates this risk. Masses of combustible metals do not usually represent great fire risks because heat is conducted away from hot spots so efficiently that the heat of combustion cannot be maintained. In consequence, significant heat energy is required to ignite a contiguous mass of combustible metal. Generally, metal fires are a hazard when the metal is in the form of sawdust, machine shavings or other metal "fines", which combust more rapidly than larger blocks due to their increased surface area. Metal fires can be ignited by the same ignition sources that would start other common fires.
https://en.wikipedia.org/wiki/Fire_class#Metal
Fires predominantly involving electricity have different classifications in each of the three systems. They are classified as a "Class E" fire under the Australian system, "Class C" under the American system,[3] and are classified based on the ignited fuel type under the European system (which previously shared the "Class E" classification with the Australian system). Electrical fires are fires involving potentially energized electrical equipment. This sort of fire may be caused by short-circuiting machinery or overloaded electrical cables.
https://en.wikipedia.org/wiki/Fire_class#Class_C_(US)/Class_E_(AU)_/_Unclassified_(EU)_-_Electrical
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(https://upload.wikimedia.org/wikipedia/commons/thumb/2/29/Anatomy_of_a_candle_flame.svg/330px-Anatomy_of_a_candle_flame.svg.png)
https://en.wikipedia.org/wiki/Flame
The picture shows that there's no obvious correlation between colors in parts of flame and their temperature.
(https://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Flametest--Na.swn.jpg/255px-Flametest--Na.swn.jpg)
A flame test for sodium. Note that the yellow color in this gas flame does not arise from the black-body emission of soot particles (as the flame is clearly a blue premixed complete combustion flame) but instead comes from the spectral line emission of sodium atoms, specifically the very intense sodium D lines.
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By necessity, firefighters expanded the definition of fire even further.
Not really.
The definition of fire is simply stuff that's burning.
The fact that you can burn some metals doesn't alter that.
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By necessity, firefighters expanded the definition of fire even further.
Not really.
The definition of fire is simply stuff that's burning.
The fact that you can burn some metals doesn't alter that.
It's quite likely that, for hundreds or thousands of years, "fire" almost exclusively meant burning vegetation.
The interesting thing is that the definition has barely changed.
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In the days before they had worked out how to make magnesium, magnesium fires were unknown.
But if you showed someone from that era some burning magnesium, they would have recognised it as fire because the definition is not dependent on the particular fuel.
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The video shows our current understanding on how fire occurs, especially in burning candles. The light is said to be produced by electrons falling back to lower energy orbitals after previously jump up to higher energy orbitals by absorbing energy. The problem is, the electrons in lower energy will absorb the light and jump to higher energy, which cancels out the light production by falling electrons, and leaving with the same light as the initial energy, analogous to Newton's craddle. But observations tell us that more fuel and oxygen mixture produce more light.
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The problem is, the electrons in lower energy will absorb the light and jump to higher energy, which cancels out the light production by falling electrons,
Since you can see the light from the candle, your "problem" is clearly imaginary.
Flames do absorb light.
https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#Flame_atomizers
but not particularly strongly, so much of the light escapes.
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The problem is, the electrons in lower energy will absorb the light and jump to higher energy, which cancels out the light production by falling electrons,
Since you can see the light from the candle, your "problem" is clearly imaginary.
Flames do absorb light.
https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#Flame_atomizers
but not particularly strongly, so much of the light escapes.
It means that the explanation in the video is incomplete. It's missing some important information in understanding how the light is produced. There must be some mechanism to get the electrons in the excited state other than absorbing visible light, which is not mentioned there.
Atomic absorption spectroscopy is based on absorption of light by free metallic ions.
https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy
I guess there won't be much metal in a candle flame.
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There must be some mechanism to get the electrons in the excited state other than absorbing visible light, which is not mentioned there.
Heat.
I guess there won't be much metal in a candle flame.
I doubt anyone ever thought there was.
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There must be some mechanism to get the electrons in the excited state other than absorbing visible light, which is not mentioned there.
Heat.
I guess there won't be much metal in a candle flame.
I doubt anyone ever thought there was.
How does an electron absorb heat? What happens to the heat source afterward?
Atomic absorption spectroscopy doesn't work on candle flame then.
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In this video I show you an amazing experiment where I actually make black fire! I show you how normally fire does not have a shadow since it is a source of light. Then I show you what happens when you shine a low pressure sodium vapor lamp on the fire. You still get no shadow, but something amazing happens when you put salt water on the fire! What how to make black fire!
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How does an electron absorb heat?
Hot molecules move quickly. When they hit eachother the electrons can get knocked into excited states.
What happens to the heat source afterward?
Nothing; why would it?
Atomic absorption spectroscopy doesn't work on candle flame then.
Yes it would.
AA doesn't only work with metals.
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Nothing; why would it?
Conservation of energy.
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Nothing; why would it?
Conservation of energy.
What do you think the "heat source" is?
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What do you think the "heat source" is?
Hot object.
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Well, it would get cooled, except that, since it's a flame, it stays at much the same temperature because it's heated by burning wax.