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Q & A: Blue Flame

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Most recent answer: 01/24/2011
Q:
I was wondering if there is a fuel out there that is good for starting fires with one that creates a blue flame
- Faye (age 13)
A:
Hi Faye --

The color of a flame depends on the material being burned. There are lots of fuels that will produce blue flames when burned; the most commonly available one in the household is natural gas. You can look at the flame on the stovetop (if your stove uses natural gas) or furnace to see a pretty blue flame (don't get too close! These are hot!). Natural gas, or any other pure hydrocarbon such as butane or propane will burn a light blue.

Some flames are yellow because of the presence of a small amount of sodium atoms in the fuel. Sodium atoms glow yellow very brightly when they are heated; yellow light is their very favorite color to emit (this particular shade of yellow is called the "sodium D line" because of the electron orbits involved in the sodium atoms before and after the light is emitted). Sodium street lights are very very yellow for this reason.

Other flames are yellow because of bits of soot, which don't get hot enough glow bluish. Most of the sources we've seen say that this is the main reason for wood fires being yellowish, not the presence of sodium.

Other chemicals make flames burn other colors. Here is a nifty web site listing what is added to commercial fireworks to make them glow all the different colors:



This site lists copper compounds as good sources of blue (in case the site goes away someday, here is the "blue list"):

copper compounds + chlorine producer
copper acetoarsenite (Paris Green),
Cu3As2O3Cu(C2H3O2)2 = blue
copper (I) chloride, CuCl = turquoise blue

The color of a fire won't depend on what you use to start it, however, just what is burning at the time.

Tom (mods by mbw)

(published on 10/22/2007)

Follow-Up #1: why are flames yellow?

Q:
I'm not a physicist, but I think you are wrong here. Most ordinary yellow flames are not yellow primarily because of the presence of sodium ions, but because of the incandescence of soot (carbon) particles in the flame, which is caused by incomplete combustion. Hence, things like camp fires and candle flames are not yellow/orange for the same reason that sodium street lights are. If you want to stick with your original answer, and think I am wrong here, please provide some additional info and citations to back up your reasoning. I am a layman researching incandescence, and everything I've read on the web so far would not support your original answer. Thanks in advance for your response. January 24, 2011
- Rick (age 51)
Ithaca, NY
A:
Thanks for this correction. For the time being, I'm just posting your note and a warning on the old answer. When we get time, we'll try to sort out which is closer to the truth. Do you have some useful references?

Mike W.

Got a little time to look at what other people say, and I think you're right. The changes are made.

(published on 01/24/2011)

Follow-up on this answer.