Why does ceramic turn black when in contact with a safety flame, and not with a blue flame?  

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The "safety flame" is a colloquial term for one of the settings of a Bunsen burner, where the adjustable collar that controls the amount of air entering the reaction is set to the closed position. The reduced amount of air entering into the combustion results in a lower temperature and...

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The "safety flame" is a colloquial term for one of the settings of a Bunsen burner, where the adjustable collar that controls the amount of air entering the reaction is set to the closed position. The reduced amount of air entering into the combustion results in a lower temperature and an incomplete reaction; insufficient O2 is available for all of the hydrocarbon fuel to react.

Using methane as an example, the complete reaction should be 2 O2 + CH4 -> 2 H2O + CO2. This is what takes place in the "blue flame" setting. However in the absence of enough O2, as in the safety setting, the carbon atom does not participate in a reaction with the O2. This causes a gradual buildup of carbon residue, more commonly called soot, which appears black. The soot may penetrate the microscopic pores in unglazed ceramic, rendering it very difficult to remove; otherwise it might be removed with a finger, sponge or steel wool. The ceramic itself should not turn black, as it (should) contain little or no carbon and not participate in the combustion reaction, nor chemically react at all.

 

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