Fire – Just a lot of Hot Air!
Nothing else that we see in nature is quite like it. It is essential to our survival and it is the Great Destroyer at the same time. It looks beautiful and magical; it’s ephemeral and yet leaves what it touches permanently changed. As a child I stared into fire wondering what it was or what the heck was going on there. When the ancient philosophers began to think about what all the stuff in the world was made of, it is unsurprising that many chose earth, water and air. But fire? Including fire as the fourth element seemed just the thing to keep the stuff in the world from being static, and in this they weren’t all that far off the mark. For a period of time, in the not too distant past, a popular theory about fire was that it consisted of something called phlogiston that existed in all combustible things, and that the flame and heat were the result of it’s being released. As strange as this may seem today, this idea was promoted by such great scientists as Joseph Priestley (1733-1804), the discoverer of oxygen. The truth is more interesting I think though, for fire is not a thing at all, but a process.
One of my great aims with this blog is to be able to explain science in a way that communicates my wonder at the natural world to non-scientists. The natural world is beautiful, and not just the way it appears and plays with our senses, but in understanding it. This beauty of understanding can reach down into our soul and help sustain us in an often difficult life. What appears below may appear technical at points, but if you stay with it, these concepts are enlightening (literally in this case). You may get excited when you see something later, in a totally different context, and you understand it because of a few simple and basic concepts. It’s worth the effort!
The ABCs of Fire
In order to underestand fire, there are three physical concepts that you will need to understand: light (a part of “electromagnetic radiation”), blackbodies, and a very simple bit of chemistry. “Electromagnetic radiation” and “blackbodies” may sound over your head, but I hope it won’t be soon. I pleed that you inform me if and where I lose you in the comment section below. I will amend this essay until it is comprehensible to all!
Today, physicists know a great deal about the way light behaves. They can predict nearly exactly what it will do and use it for all kinds of amazing purposes that make our lives easier. What it actually is, however, is a bit more difficult to know because what is happening with light is so utterly small that it lives far below our senses and it is literally the fastest thing possible in the Universe. Sometimes light acts like a “wave” in that it oscillates back and forth in its tiny world, and sometimes it acts as a “particle,” in that it comes in small packages of energy we can call “photons”. Whether light is a wave or particle has been one of the most contentious debates in all of science history. Aristotle thought of it as a wave while Democritus thought it was a particle. Later, with more evidence in hand, Isaac Newton went with the particle idea, while Christian Huygens showed it acted like a wave. The wave theory then gained the upper hand as scientists saw they could predict how light would behave if they used math that presumed it was a wave. Then came Einstein who proved it was a package of energy in 1905 (a simple thing for which he won the Nobel Prize – he never won the prize for his revolutionary work on Relativity). Enter the quantum physicists stage left. Lots of math. Amibguity (which is often called “duality”) on whether the new math supports either side or something else entirely. For us, let’s be content for now to think of light as a particle went it fits our needs and as a wave when it fits other needs. That’s pretty much what scientists did when they made your Ipad.
A: Light and the “Electromagnetic Spectrum”
Perhaps the most interesting thing about light to take away from this essay on fire, is that what we see as visible light is just a tiny portion of a huge spectrum of “electromagnetic radiation”. DO NOT be afraid of that word. Electromagnetic radiation is just a small packet of energy, called a photon, that travels as a wave. The distance from one wave “top” to the next is logically called “wavelength”. It so happens that the shorter the wavelenth, the more energy the photon particle carries and the longer the wavelength the less energy the photon carries. We will call electromagnetic radiation “light” in the remainder of this essay, to sound less technical, and then say that “visible light” is that little portion of light (EM) that our eyes are sensitive to and that we can see. It is time to look at a picture of the Electromagnetic Spectrum (or spectrum of light).
From this diagram it is obvious that the light we see is only a small portion of the light there is. This little sliver is expanded above to show how the colors of the rainbow are really light waves of different wavelenths. This range is very small, – 375 to 750 billionths of a meter for visible light. Helpful for our ability to see objects, much of this part of light tends to bounce off the objects around us. If you see an orange chair, you are seeing the light in the orange wavelenth that is bouncing off; the rest of the light in the visible range hitting the chair is absorbed.
Not a coincidence, if you ask me, our eyes have evolved to “see” only that part of the light spectrum that is at the peak wavelength that our sun gives off. Aliens who evolved around a different sun might have eyes that aren’t sensitive to our sunlight and they might be blind here.
What about light at other wavelenths? Is that useless to us just because we can’t see it? Oh no!! Light at a bit longer length is absorbed by our skin and we feel it as warmth. This is infrared light. A bit longer and you find light that we called “microwaves” and this wavelength is useful to warming our food and sending your cell phone signal. A bit longer yet and we find light that transmits radar, TV, then FM and AM radio. Obviously, at the long wavelenths of these radio waves, the light passes right through things we know as solid objects like your house, otherwise, your radio would not pick up a signal. Also, our eyes are certainly not senstive to this light. Can you imagine if you could “see” cell phone, TV and radio signals? Our vision would be a chaotic unhelpful mess!
On the other side of visible light we find light that has shorter wavelength and is more energetic. First up, we find ultraviolet light which is responsible for giving us a sunburn. Much of this light from the sun is absorbed by the ozone layer high up in the stratosphere. If the ozone layer were to suddenly disappear, we would certainly need to be wearing lots of sunscreen or skin cancer for the damage it would do would be rampant. If the ozone layer had never existed, however, I suspect that our skin would have evolved some protective measure by now, probably in the form of more pigments like melanin that would absorb it before it could do damage. It’s no accident that people indigenous to the equatorial regions have darker skin. More energetic than ultraviolet-rays, light is then called X-rays. This light is useful at finding cavities, where bones are broken, and where the bomb is hidden in that terrorist’s luggage. Too much X-ray light can cause internal damage to our DNA, not just to the skin layer. Finally, gamma rays are the most energetic form of light, given off by such high intensity sources of energy as the explosion of supernovae. They can be lethal to humans in a short time, but luckily, our atmosphere absorbs almost all of it. In space, however, we do not have this protection. The space suits and space stations humans live in out in space have to be specially designed to absorb this really high intensity light before it gets to the body.
So just how does light (EM) arise in the first place? Well, first, did you know that there is such a thing as the coldest temperature a thing can get? The temperature that a thing is depends on how fast the molecules, atoms, and even electrons in a thing are moving. There is no exact limit to how much movement that these things at the atomic level can have, so there is no one highest temperature. But there is a limit to the coldest temperature: this is simply where all atomic motion comes to a stop. This is called Absolute Zero which is defined as 0° Kelvin, -273.15° Celcius, or -459.67° Fahrenheit. There is nothing we encounter here on Earth that is that cold, however (except possibly approaching it in a laboratory). Everything in our world that we know of as “stuff” jiggles around at the atomic level, and so, it is above Absolute Zero. One type of movement that happens at this level is that electrons jump up and down from different energy levels. If an atom receives a photon of light energy at a specific wavelength, an electron jumps outward to a higher energy level. If the electron then jumps back down to where it was, it releases the same wavelength of light back out. Now, as I described in my essay on “Water,” each element has electrons that can only live at specific energy levels, or “orbitals”. So, if you were looking at an oxygen atom in isolation, there would be only specific wavelengths of light it could absorb or emit. This is how it is possible for astronomers, looking at light emitted from the farthest reaches of star’s atmosphere, can tell what elements are there: the elements present emit light in only specific wavelength bands. But in our world (and deeper in a sun) the atoms are not in isolation. They are constantly bouncing off each other, and the energy that is absorbed and emitted when this happens varies all over the map. So for substantial matter, not isolated atoms, the light energy gets smeared out into a spectrum that depends more on the object’s temperature. Any object that acts this way is called a blackbody. The sun is a blackbody with a surface temperature of 5778° Kelvin and it give off light that is largely in the visible spectrum as noted at the beginning of this essay. Here is a diagram which shows the amount (intensity) of electromagnetic radiation (light) for a blackbody at different temperatures.
At the temperature of the earth and the human body, most of the light given off is in the infrared range. Although you and I don’t glow when the light is turned off, if we had special glasses that were sensitive to infrared light then we could see each other. The army has developed special glasses that are designed to see exactly the spectrum of light that something about our body temperature gives off; a good way to see the enemy at night! Here are two pictures of a man with a plastic bag around his arm. On the top we see him with the lights turned on, as normal. On the bottom, however, the lights are off and it is pitch dark. The camera taking this picture is sensitive to light that a blackbody the temperature of 73.6°F to 93.4°F gives off (near that of the man). Notice that the infrared light from his arm travels right through the bag. His right temple is the warmest thing in the picture.
So I hope this discussion of blackbodies helps to understand where light comes from. Before I put these ideas together to create fire, we need to understand one final concept, and this I promise to keep simple.
The Chemistry of Fire
When two substance react with one another to create new substances, heat can either be absorbed or given off. Let’s look at these.
1. Substance A + Substance B + HEAT ENERGY = Substance C + (maybe other substances)
We can see that heat energy is taken from the environment when this type of reaction occurs. The environment becomes colder. If you held the substances in your hand while this was occurring, the Substance C would feel cold because it has taken heat from your skin. This type of chemical reaction is called an endothermic reaction (endo = in, within; therm = heat). Endothermic reactions cannot occur spontaneously because work must be done to gather the heat into the reaction. Consider that you plug your refrigerator into the electrical socket to make the machinery do this work so it can take the heat from the food.
2. Substance A + Substance B = Substance C + (maybe other substances) + HEAT ENERGY
In this case heat energy is released into the environment. This is called an exothermic reaction (exo = out). Exothermic reactions CAN occur spontaneously when the two substances touch, they may even explode!, or they may need a bit of energy to get them started.
To the right is a diagram of an exothermic reaction where a little energy is needed to get it started. An example of such a reaction would be when the oxygen in the air reacts with carbon compounds in paper (which comes from wood). One of the most famous novels by American author Ray Bradbury was titled “Fahrenheit 451,” which is the temperature at which paper spontaneously bursts into flames (book burning featured prominently in the story). At this temperature there is enough energy for the reaction between oxygen in the air and the carbon to just go. At lower temperatures, however, you will need something to get it started. Once the reaction is started, however, remember that it is releasing energy (the ∆H in the diagram) into its immediate surroundings, and as long as the temperature on the surface of the paper is over 451°F, the reaction will keep going. Do you see where this is headed?
Putting it All Together: Fire!
Now we have all the elements in place to explain what I wanted to know when I looked at fire as a boy. You probably think we have too much information, but I hope you will see why I explained the first bits.
Let’s consider a candle flame and a wood match. Neither the match or candle catch on fire at room temperature. If you rub a match just right, however, the friction adds a bit of heat to the tip which ignites at much less than the 451°F of paper because the chemicals there ignite more easily. The fire from this tiny explosion then provides enough energy to start a reaction between the oxygen in the air and the carbon in the wood. (Notice how this energy is called “activation energy” in the above diagram.) Being exothermic, this reaction keeps releasing heat, which then causes more oxygen to react with the wood, and on and on, until the wood is used up. That is, it becomes self sustaining. The energy released as the match stick reacts with air can then be used to make the candle wick (a combination of cotton and wax) spontaneously ignite by placing the burning match near it. This temperature is not all that far from the 451°F where paper ignites. Now just like the match, the reaction in the wick also releases heat, and that reaction also becomes self sustaining. It takes much longer for all the material in the candle to react with the air than the match does, and so candles are more useful to us; matches exists only to provide the activation energy for other things to catch fire.
OK, so now we have an ongoing reaction between the wick and the air occurring at the surface of the candle. As this exothermic reaction continues, the heat released has to go somewhere, so where does it go? Simply into the air in the immediate vicinity. Now let’s recall what a blackbody is. A blackbody emits more or less energy at different wavelengths depending on its temperature. Normally things like a candle and the air are cool enough that they don’t give off enough light at a wavelength we can see. As they get hotter, however, they will eventually get to the point where they begin emitting light in the part of the electromagnetic spectrum that is visible. This is why the flame shines and lights a room; because it is hot. The reason the flame flickers is because YOU ARE ACTUALLY SEEING THE AIR THAT IS USUALLY INVISIBLE, and that’s how air moves. Normally, we can feel the air blow across our skin, but we don’t usually see it.
You might wonder why the air does not simply glow in a sphere around the place on the wick surface where the reaction is happening and the heat is being released. This is because hot air expands and becomes lighter. That hot air, hot enough for us to see, rises up, and the surrounding cooler air rushes in to replace it providing both fresh oxygen for the ongoing exothermic reaction and at the same time molding the air into the familiar birthday candle flame shape shown in the picture at the start of this essay. About an inch above the reaction process happening on the wick surface, the air cools off enough that it becomes invisible to us again.
And that’s it folks. It is possible that many pyromaniacs, be they lovely experts in fireworks displays or heinous arsonists, don’t really understand what they are doing. But for me, at least, it is beautiful to know.