Which of a following can be understood when a bottom of a vessel blackens on a outside during cooking?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Combustion is incomplete.
The black deposit is soot, that is unburnt carbon. It settles on the vessel when the fuel does not get enough oxygen and burns only partly, giving a yellow, luminous flame instead of a clean blue one.
Complete combustion produces only carbon dioxide and water with a blue flame and leaves no soot, so a blackened vessel is the sign that combustion is incomplete — the reason the air holes of a gas burner have to be kept clear.