Step 1: Understand the zones of a candle flame.
A candle flame typically has several distinct zones due to varying degrees of combustion:
- Innermost zone (Dark zone): Around the wick, contains unburnt wax vapors.
It is the least hot part.
Color: Dark/Black.
- Middle zone (Luminous zone): Incomplete combustion of wax vapors occurs here.
Unburnt carbon particles are present, which glow and emit yellow light, making this zone luminous.
It is moderately hot.
Color: Yellow.
- Outermost zone (Non-luminous zone): Complete combustion of wax vapors occurs here due to ample supply of oxygen from the surrounding air.
This is the hottest part of the flame.
Color: Blue (often faint, may appear almost invisible in bright light).
Step 2: Identify the color of the outer zone of complete combustion.
The outermost zone, where complete combustion takes place, is non-luminous and typically has a pale blue color.
This blue color is due to the chemiluminescence of radical species like CH and C formed during complete combustion.
Step 3: Compare with the options.
Option (1) "blue" matches the color of the outermost zone of complete combustion.
Option (2) "yellow" is the color of the middle, luminous zone (incomplete combustion).
Option (3) "black" refers to the innermost, dark zone (unburnt vapors).
Option (4) "red" is not a primary distinct zone color, though parts of the flame might appear orange-red due to temperature or soot incandescence.