Question

The hottest part of a gas flame is the:

• A. Luminous zone
• B. Dark zone
• C. Blue zone
• D. Non-luminous zone

Hint:

Identify the flame zone where combustion is most complete.

Answer 1

The Correct Option is D

Step 1: A gas flame, such as that from a Bunsen burner, has three main zones: the innermost dark zone of unburnt gas, the middle luminous zone where combustion is incomplete and produces soot, and the outer non-luminous zone where combustion is complete.

Step 2: In the outer non-luminous zone, the fuel gas gets a plentiful supply of oxygen from the surrounding air, so it burns completely to carbon dioxide and water vapour.

Step 3: Complete combustion releases the maximum amount of heat energy per unit of fuel burnt, because every carbon atom is fully oxidised to CO2 instead of being left as soot or partially burnt carbon monoxide.

Step 4: Since complete combustion is the most efficient way to release heat, the outer non-luminous zone reaches the highest temperature of the entire flame.

Step 5: Hence the hottest part of the gas flame is the non-luminous zone, option D.