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How does plasma make a campfire flame orange?
Category: Chemistry
Published: June 21, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
The de-excitation of plasma (charged gas) is not the source of the light given off by a campfire's flame. The incandescence of solid soot particles billowing up on an updraft of hot air is what creates the light seen as a flame. Let us look at the typical ways materials emit light and then apply it to the campfire.
Electrons in molecules can exist in various orbital states. When electrons absorb energy, they are lifted to a higher energy state in the molecule. This is roughly similar to lifting a ball onto a high shelf where it can fall back down from. The absorbed energy of the electron is stored as potential energy as it sits in the higher energy state. Many things can excite an electron in a molecule to a higher energy state, including chemical reactions, collisions with other particles, and absorption of light. Eventually, excited electrons fall back down to their lower states, like a ball on a shelf rolling and falling back to the ground. When the electrons fall, their potential energy is lost and given off. There are many ways that a falling (de-exciting) electron can give off its energy, but the most common way is for it to emit a bit of light called a photon. As a result, any process that excites electrons leads to light being created when they de-excite. Often, this light has a non-visible color or is too dim to be seen by human eyes, but the light is still being created.
Because of the law of conservation of energy (which states that energy cannot be created or destroyed), all of the energy that the electron loses in transitioning from a higher state to a lower state must be given to the photon that it creates. Electron states only exist at fixed levels that are constant for a given molecule, so a certain isolated molecule can only emit photons of specific energies. Because a photon's frequency (the light's color) is directly proportional to its energy, certain isolated molecules can only emit certain colors of light. The specific set of colors that a molecule emits (its spectrum) is unique to the molecule and acts like a fingerprint. Astronomers can tell what chemicals a star is made out of by simply looking at what colors are present in its starlight. By plotting the few, distinct colors present in the light given off by a single molecule, a "line spectrum" is formed, which looks like a collection of thin lines. In addition to electrons getting excited, the whole molecule itself can get excited by spinning more or vibrating more. There are therefore three ways a molecule can emit light: an electron falls to a lower state (electron transition), the molecule spins less (rotational transition), and the molecule vibrates less (vibrational transition).
Now something interesting happens if you have more than one molecule involved. If you have a collection of molecules, they tend to bump into each other. When they collide, some of the energy in the excited electrons, excited spinning states, and excited vibrating states is converted simply to movement (kinetic energy). As a result, there is less energy present for the photon that is emitted when the molecule transitions, leading to a photon with a color that is different from if there had been no collision. Because the collisions are random, the color changes of the light given off are random. Where there was just one color (a line) in a certain portion of the spectrum of the light emitted, there are now many colors. Collisions between molecules therefore tend to smear the nice crisp lines of the light's color spectrum into bands with many colors. The more collisions there are and the harder they are, the more colors there are in the light given off.
If there is a very high amount of strong collisions between molecules, all of the light given off by molecular transitions gets smeared into one continuous band of colors. In such a case, all colors of the rainbow are present in the light and the light is therefore white. The light is typically not pure white, but is whitish red, whitish orange, etc. depending on the nature of the collisions. Light with this broad arrangement of colors is called "thermal radiation" or "blackbody radiation" and the process that creates this light is called "incandescence". In every day life, we refer to incandescence as "glowing hot". A material with zero collisions therefore emits a line spectrum, which is a collection of a few perfectly defined, unsmeared colors. On the other extreme, a material with an infinite number of collisions emits a blackbody spectrum, which is a perfectly smooth collection of all colors in a very distinct distributional shape called a "blackbody curve". The two opposite extremes; the line spectrum and the blackbody spectrum; are idealizations. In the real world, each spectrum is somewhere between the two extremes. When we say that the color distribution of light is a line spectrum, we mean that it is close to a line spectrum, and not that it is exactly a line spectrum.
There are two things that determines how fast molecules collide with each other. The first is the density of the molecules. The closer the molecules are together, the more chance they have to collide. Solids have their molecules very close together and therefore collide enough to emit all colors of light. Solids typically emit a spectrum that is close to a blackbody spectrum. On the other hand, a dilute gas has its molecules much farther apart, so the color distribution of its emitted light looks more like a line spectrum. The other thing that affects the collision rate is the temperature of the object. As an object gets hotter, its molecules shake around more and move faster, which leads to more collisions. More collisions means more spectral line spreading, and it also means that more energy is being exchanged in each collision. The thermal spectral curve shifts to higher energies for higher temperatures, which is the same as higher frequencies. Room temperature objects are constantly emitting light in a color we can't see (infrared). As we heat up an object such as an electric stove element, the collisions between molecules become more intense and more frequent. The light given off therefore has a spectral thermal distribution that shifts to higher frequencies. The peak of the light given off by a heated object shifts from infrared, to red, to orange, and so on as it gets hotter. But remember, that thermal radiation contains all colors, so that hot metal does not glow red, it actually glows whitish red, and then whitish orange as it gets hotter, etc. (Remember that "white" is just how we experience an equal mixture of all colors.)
Let us look at some examples. A neon sign contains a gas at normal temperature getting excited by electricity. The light emitted by a neon sign is therefore close to a line spectrum (just a few, sharply defined colors). In contrast, the sun contains gas, but the gas is at such a high temperature that the light emitted is close to a blackbody spectrum. The metal elements in a toaster, electric stove, and incandescent light bulb are all solid and therefore glow when heated such that the light they produce is close to a blackbody spectrum, which contains all colors and therefore is whitish.
Now let us apply this to a campfire's flame. At first thought, you may think that the space where the flame exists only contains gases, therefore the light emitted should be a line spectrum with only a few distinct colors. This is clearly not the case. The flame has a broad range of colors, from red to orange to yellow and all the colors in between. So there must be more going on than heated gases. The best general description of a campfire flame's color is: whitish orange. This description matches exactly what we would expect from the thermal radiation of incandescence. It turns out that a campfire flame contains small solid particles of half-burnt wood called "soot" that are so hot that they glow. They glow in the exact same way that electric stove elements glow. When you look at a whitish-orange flame, you are looking at a cloud of little, hot, glowing, solid bits of half burnt fuel. The hot air that is flowing upwards catches this glowing soot and carries it upwards. As the soot rises, it cools and its thermal spectrum shifts to ever lower colors. That is why the bottom of a flame is whitish-yellow, and as you look higher, it changes to whitish-orange, and then whitish-red, and then to whitish-infrared (which we can't see, but can feel with our hands). The flame of a campfire actually extends meters into the air, we just can't see the top portion because the soot has cooled enough to emit mostly infrared colors.
Interestingly, the presence of half-burnt soot in a flame, and therefore the flame's whitish color, is a result of there not being enough oxygen. If oxygen is pumped fast enough into the fire, or is premixed properly with the fuel, all of the fuel gets burned all the way and there is no soot. Adding enough extra oxygen therefore makes the whitish-orange flame go away. The color that remains in the flame (usually blue) is from the electron transitions involved in the chemical reaction itself. Fuels that don't need much oxygen to burn, such as the natural gas in a gas stove, can burn without giving off whitish-orange glowing half-burnt particles.