The Colors of Fire and Flames: Why is Fire Orange?

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Why do flames have orange color? Photo by Oscar.

Why do flames have orange color? Photo by Oscar.

Question:

“Why is fire orange?”

To answer this question, we will discuss the process that generates a fire (combustion) and explain which are the parameters affecting the color of the fire / flame. We will talk about orange fire, but also about fires which have other colors.

What is Combustion?

Combustion is a reaction in which fuels react with an oxidant compound. In the majority of cases this is oxygen (O2) – We also call ‘combustion’ by a more common name – burning. During combustion, the process releases heat, as it is an exothermic reaction. Because of this, we use combustion reactions to generate energy; the most common fuels include natural gas, oil and wood.

Rapid combustion is a form of combustion in which a very high amount of energy is released; in these conditions, combustion generates a fire.

What is Flame?

Flame is the part of the fire made of burning gaseous compounds and fine suspended particles. Evaporation/gasification of the fuels (if these are liquid/solid) and subsequent thermal degradation into smaller molecules and/or reactive radical species forms the gaseous compounds. The composition of the flame can change depending on the nature of the fuel.

Flames emit energy in the form of light; for this reason we also refer to flame as the visible part of the fire.

Emitted Radiation

One important parameter affecting the color of the flame is its temperature; to understand this, we have to consider black body radiation emission.

Black body radiation emission is an electromagnetic radiation emitted by every object, whose characteristics depend on the temperature of the body itself. At relatively low temperatures (i.e. room temperature) this radiation does not fall into the visible region, so the human eye does not detect it. At higher temperatures, however, the radiation shifts into the visible range. As the temperature increases, the emitted colors will go from dark red to bright red, then to orange, yellow and finally to white-blue.

The Temperature of the Flame

Applying this concept to a flame, different parts of the flame will have different colors depending on the temperature. Normally the hotter parts, closer to the burning fuels, will be whitish-blue; the more distant parts, on the other hand, will be cooler and hence orange-reddish.

Considering in particular the orange color the question refers to, this generally corresponds to a temperature of 1000 – 1100 oC.

Fuel Composition

Lithium gives the flame a pink color. Photo by Herge.

Lithium gives the flame a pink color. Photo by Herge.

The nature of the fuel and its chemical composition can also affect the color of the flame. This is because, when excited, every element can emit light; the wavelength, and hence the color, of the emitted light is characteristic of each element.

In the case of the fuels mentioned above, they are made mainly of hydrocarbons, that is, molecules formed from carbon and hydrogen. These substances; wood, natural gas, and oil, will emit light which will fall in the blue region. An example of this is the fire from gas cookers, which is due to the combustion of methane (CH4).

Other elements, however, can give different colors. Lithium (Li), for instance, gives a pink flame (see picture).

The different colors which elements give to the flame can be used to identify them in a compound with unknown composition.

Oxygen Proportion

Another parameter which can have an influence on the flame color is the amount of oxygen present during the combustion.

If enough oxygen is available, a complete combustion will take place, with almost no unburnt material. If the oxygen quantity is smaller than the minimum required, or if the oxygen is not properly mixed with the fuel, the combustion will not be complete.

In an incomplete combustion, the flame temperature will be lower; this can affect the color. Moreover, unburnt material such as soot particles can emit radiation in the red, again affecting the color of the flame.

What Makes Fire Orange? Or Blue? Or Pink?

The color of a flame depends on a variety of conditions. Not only temperature, but chemical composition and the amount of oxygen present can change the color of a fire. So, why is a wood fire orange? Because it’s a hydrocarbon, and it burns at about 1000 -1100 degrees Celsius, with just the right amount of oxygen.

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