Which of the following can be understood when the bottom of a vessel blackens on the outside during cooking?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Combustion is incomplete.
When a fuel gets too little air (oxygen), it does not burn completely. Some of its carbon is left unburnt and rises as tiny black particles of soot, and the flame turns yellow and smoky. This soot settles on the cold bottom of the vessel and blackens it, so the black layer is a sign of incomplete combustion; it is usually corrected by opening the air holes of the stove.
Why the other options are incorrect
Option (1) The food is not fully cooked. — Incorrect. The soot comes from the flame, outside the vessel, and says nothing about the food inside.
Option (2) The fuel has a high moisture content. — Incorrect. Damp fuel is only one possible cause; the black layer shows incomplete burning, not its cause, which is often too little air.
Option (4) The fuel is combusting completely. — Incorrect. Complete combustion gives a clean blue flame and no soot.