In a well-controlled flame, which color typically indicates incomplete combustion and potential soot formation?

• A. Blue flame
• B. Green flame
• C. Yellow/orange flame
• D. White flame

Answer: (C)

Flame color tracks how completely the fuel is burning and whether soot is forming. A blue flame signals clean, complete combustion with plenty of oxygen—the flame is hot and the fuel oxidizes fully, so there’s little to no soot. When combustion is incomplete, soot particles form and glow as they are heated, which produces a yellow to orange color in the flame. That glowing soot is the telltale sign of potential soot formation and less efficient burning. A white flame isn’t the typical indicator of incomplete combustion in this context and can come from other effects that aren’t the same soot-related issue. So the yellow/orange glow is the color that best indicates incomplete combustion and potential soot formation.