Explain why the appearance of the wood flame is yellow whereas the gas flame most often appears blue.

Answer 1

There are at least two reasons why the wood flame is yellow. One is that the temperature of the burning wood is not high enough to completely burn some of the materials in the wood and you are seeing the "luminous" flame from the carbon that hasn't burned. The second reason is that the sodium in the wood (and there need not be but just small amounts) is yellow when burned. The color of the Na flame is actually due to two wavelengths of light. One is 589.0 nm and the other is 595.9 nm.