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Why is it when I turn on the stove the flame comes out blue? Also sometimes it gets orange on the top? So why's that?
Question Date: 1998-11-12
Answer 1:

Blue flames come from very hot, complete burning of air and fuel. Orange color comes from cooler spots in the flame where fuel isn't being totally burned and these produce a little bit of soot and smoke. The color of the flame is related to the heat of the flame. Think about the colors of different hot things: the Sun is white or yellow, a hot stove is red, a camp-fire is orange, and a gas flame is usually blue. These colors come from different kinds of a light and different places in the electromagnetic spectrum. This spectrum goes from relatively cool red things to very hot blue and violet things:

Red - Orange - Yellow - Green - Blue - Indigo - Violet (think "ROYGBIV")
relatively cool very hot

a stove top or toaster a gas flame

When the fuel in your gas flame doesn't burn completely, it doesn't get as hot and the flame is red/orange instead of blue. So, as a question for you, will food cook more quickly over a roaring orange campfire or a quiet blue gas jet? Why? Hope that helps.

Answer 2:

A flame is a region of strong chemical reaction. For example, a fuel, like say propane gas, a hydrocarbon can react with diatomic oxygen to produce carbon dioxide plus water vapor and lots of heat (and some light also). This is called a COMBUSTION reaction. (LOOK THIS WORD UP IN A SCIENCE ENCYCLOPEDIA) So when you see a flame you are looking at this reaction zone where the fuel is being converted to the products of COMBUSTION. Now the rate of the reaction, and hence the temperature and COLOR of the flame, is a function of the REACTION rate...that is, how fast the oxygen can combine with the fuel (natural gas or propane or gasoline or oil , etc.).

So at the edges of the reaction region where oxygen is more abundant (since air in the room is 21% by volume of oxygen) is where the temp is highest.

Answer 3:

Blackbody radiation is the emission of light from a hot body; higher the temperature larger the frequency of light emitted (light emitted will shift from infrared, red, green, blue towards UV as temperature increases). This is what the answer on science line points to: more complete the combustion process, more the heat released, and higher the temperature, which makes it look blue.

Although the above reasoning of the answer and the physics is correct, this is not typically what happens in gas flames. There is another mechanism by which gases/molecules or even solids emit light (LED: light-emitting diode is a great example of solid emitting light). This is a quantum mechanical phenomena, where the molecule gets into an excited state with the energy supplied by the combustion process and subsequently relaxes to a lower energy state by releasing the extra energy in the form of light. The color of the light emitted in this process is characteristic of the molecule excited.

Both, blackbody radiation and molecular excitation should be discussed in the context of color of the flame. Orange flame is due to blackbody radiation coming out from an incomplete combustion or slightly cooler area, where the released energy is not enough to excite the molecules. When the combustion process is complete and produces enough heat to excite molecules, the molecular excitation produces a distinct bluish-green spectrum.

A simple and beautiful experiment to see molecular excitation in play is to hold a salt in flame and see the color emitted. This is often used to identify a salt, since the color is characteristic of the molecule. Calcium salts emit brick-red color, Copper compounds emit greenish blue flame. Here is a youtube link: click here to watch

Answer 4:

Experiment suggested by a reader:
I just thought of an experiment to suggest to your audience. This can be carried out in an undergraduate lab.

Get a Bunsen burner. Take some small (small compared to the size of the burner flame) piece of something that will not melt or burn, ceramic, metal, a piece of rock, it doesn’t make much difference. Fashion some sort of support out of very thin platinum wire (thin to reduce the heat transfer through the support). Hold the object in the hottest part of the flame for a minute or so until it reaches thermal equilibrium with the flame. Note color. If the color of the flame is due to black body radiation, then the radiation of an actual body in thermal equilibrium (just to say the same temperature) should radiate the same, that is, the object should glow blue (In fact, it might actually disappear against the blue background; this is the idea behind a radiant

pyrometer ). It will not. It will, at best, glow reddish. One cannot argue that the object is cooler precisely because it is radiating since (1) the premise of the flame color is that it is radiating black body as well (although it could be less efficient, i.e., less “black”) (2) the surface area through which it is absorbing heat is the same as the surface area radiating, same as for a soot particle, and those do glow yellow-orange.

If anyone does this experiment, please let me know. I would like to know the result.

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