I have a question about striking back of Bunsen burner, regarding the reaction between methane and oxygen.

When methane gas reacts with oxygen completely, a clean non-luminous blue flame can be observed.

When methane gas reacts incompletely with oxygen, a sooty luminous flame will be observed instead.

Given the fuel of the Burner burner is methane, when the air hole of a burner burner is fully opened, striking back will occur. I observed that the strike-back flame is green in colour and produces loud noises. Are there any reasons for the green flame colour and noises?


1 Answer 1


The gas alimented to the Bunsen burner passes a hose, a valve, and a nozzle (where, if permitted, air can mix with the gas). Often, the nozzle is made of a material containing copper, like brass. And the green coloration of the flame you report can be due to this - sort of an unintended flame test, but the bright yellow of sodium by salt in the kitchen (e.g., cooking / roasting potatoes over a gas stove) could be familiar with you. If your lab still permits the use of Bunsen burners (borax beads, blow pipe test with char coal, digests from melt; some familiarity with glass blowing/bending, etc), chances are that the burner is a bit dirty within the barrel, too.

The origin of the noise is that the combustion doesn't (only) take place "on top" of this chimney (as it is designed), but right around the air holes, too. It is not the even combustion the burner was designed for; because of potential danger ahead, shut the gas the line. Once the burner is cold, you can lit it again; gently open the air holes.

Why all this? Coal gas and natural gas, both which a Bunsen burner can combust, are of different chemical composition. The former was/is up to half of the volume hydrogen, and 5 to 10 vol% CO; the later mostly methane. And the pressure these are delivered can differ. Hence gas stoves for cooking and heating need a different set of burners for one, or the other for safety, and efficiency of combustion. Depending on the work ahead, it might not matter this much for a Bunsen burner. (I have the impression, some of the tasks mentioned earlier do not work this well with natural gas and new burners, as they used to be in the old days with coal gas and the old ones.) So maybe you got hands on one of the older ones, where the nozzle still is the one about coal gas.


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