MadSci Network: Chemistry

Re: Why are flames often blue close to the fuel source?

Date: Tue Jan 5 19:40:34 1999
Posted By: Jeffrey Goldmeer, Post-doc/Fellow, Microgravity Combustion Science, NASA Lewis Research Center
Area of science: Chemistry
ID: 915473984.Ch

The flames that most people find familiar usually have two colors, blue and a yellowish-orange. The blue color comes from the burning of the fuel vapor, specifically a hydrocarbon fuel. (The color is indicative of a reaction involving a C-H molecule.)

The examples mentioned in the question, matches, wood logs, and gas stoves, all contain hydrocarbons. The matches and wood logs are made of cellulose, which is a long molecule that contains numerous hydrocarbons. Gas stoves use a hydrocarbon fuel, such as propane. The blue region can also be seen in a candle flame, or flames from certain plastics, both of which contain hydrocarbons.

If you can view the blue color in a flame you are actually seeing the 'reaction zone', the region in which the combustion reaction:

	Fuel + oxygen --> products
is actually occuring. As the fuel is heated, some portion of the fuel is vaporized (if a solid fuel), and the fuel vapor diffuses away from the fuel source towards the surrounding air. The region is which the fuel and air (oxygen) react is called the 'reaction zone'.

This reaction zone can be very close to the fuel source, or 'far away'. This depends on many factors, including the amount of oxygen in the air, the pressure, the size and strength of the flame, and whether the experiment is being conducted in normal gravity or in a weightless environment, such as on the space shuttle.

However, you can't always see this region. In many cases the reaction zone is hidden by the very bright yellowish-orange upper portion of the flame. This is the region in which unburned carbon particles are being heated. This unburned carbon is more commonly called soot. (It can be found all over the inside of a well used fireplace.)

Remember that this applies to hydrocarbon fuels. This is not true for a fuel such as hydrogen, which can burn with a flame that is invisible to the naked eye. Another example of a substance that burns invisibly is the fuel used in some racing cars. The car can be on fire with out a flame ever being seen by the naked eye!

In summary, the blue region in a flame is the reaction zone, the layer in which the fuel is consumed. The blue color has nothing to do with any impurities in the flame! The yellow color in a typical flame is from heeting of unburned carbon particles.

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