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

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.

The appearance of a wood flame being yellow is due to the incomplete combustion of the wood. When wood burns, it releases a mixture of gases, predominantly carbon dioxide and water vapor, as well as other byproducts such as carbon monoxide and small particles of soot. The yellow color in the flame comes from these glowing soot particles, which emit light as they heat up.

On the other hand, the blue color of a gas flame, such as one fueled by natural gas or propane, is an indicator of more complete combustion. Gas burns more efficiently because it provides a consistent and controlled fuel source. Unlike wood, gas is already in a gaseous state, and it can mix more thoroughly with oxygen in the air before being ignited.

To explain why a gas flame appears blue, we need to understand the chemistry behind it. When gas burns in the presence of sufficient oxygen, the combustion reaction is more complete, resulting in fewer byproducts and less soot. The blue color in the flame comes from excited molecular radicals, primarily carbon monoxide, hydroxyl (OH), and molecular nitrogen (N2). These radicals emit light in the blue region of the electromagnetic spectrum as they release energy while transitioning from higher energy states to lower energy states.

To conclude, the yellow appearance of a wood flame is due to the presence of glowing soot particles resulting from incomplete combustion, while the blue color of a gas flame is a result of more complete combustion, where excited molecular radicals emit blue light as they transition to lower energy states.