In the flame of a typical Bunsen burner or candle, which region is the hottest part of the flame?

Introduction / Context:
When studying combustion and flames, it is useful to know that temperature is not the same everywhere in a flame. Different coloured regions correspond to different temperatures and combustion conditions. In laboratory burners and candles, we often distinguish between a dark inner zone, a luminous bright zone and a non luminous blue outer zone. This question asks which of these regions is the hottest.

Given Data / Assumptions:

• We consider a steady, well adjusted Bunsen burner or candle flame.
• The flame has an inner dark zone, a luminous yellow region and an outer blue, non luminous region.
• Complete combustion occurs where fuel mixes well with air.
• We neglect special flames with unusual fuel mixtures.

Concept / Approach:
The hottest part of a flame is usually where the fuel has the best supply of oxygen and combustion is most complete. In a Bunsen burner with adequate air supply, the outer blue region is where gas and oxygen mix thoroughly and burn completely, reaching high temperatures. The luminous yellow region contains glowing soot particles and often indicates incomplete combustion, with lower temperature than the hottest outer zone. The dark inner zone near the burner mouth or wick is composed mostly of unburnt gas or vapour and is relatively cool.

Step-by-Step Solution:
Step 1: Identify the main regions of the flame: inner dark zone, middle luminous zone and outer non luminous blue zone. Step 2: Recognise that the inner dark zone contains unburnt fuel and little oxygen, so combustion is minimal and temperature is relatively low. Step 3: In the luminous zone, incomplete combustion produces glowing carbon particles, giving yellow light but not the highest temperature. Step 4: The outer blue non luminous zone is where fuel mixes efficiently with surrounding air and combustion is nearly complete. Step 5: Complete combustion releases more energy per unit mass of fuel and leads to higher temperatures. Step 6: Therefore, the hottest part of the flame is the blue outer non luminous zone.

Verification / Alternative check:
Practical tests support this explanation. When heating objects in a laboratory, instructions often recommend placing the object in the outer blue cone of a Bunsen burner flame for the greatest heating effect. If you hold a piece of wire in different parts of the flame, it becomes red hot quickest in the outer blue region. In a candle flame, experiments with thermocouples or heat sensitive paints similarly show higher temperatures near the outer edges compared with the luminous inner part.

Why Other Options Are Wrong:
Bright yellow luminous zone in the middle: Although bright and easy to see, this region contains incomplete combustion and hot soot particles; it is not the hottest region.

Inner dark zone: This zone has unburnt gas and relatively little combustion, so it is the coolest part of the flame.

A special non luminous zone that is cooler: This option contradicts the observation that the non luminous blue outer zone is hottest; there is no cooler non luminous region that is hotter than the blue outer zone.

Common Pitfalls:
Students often assume that the brightest or most luminous part of a flame must be the hottest. In reality, brightness depends on glowing particles and emission, not purely on temperature. Another confusion arises when comparing different flames; for example, a smoky yellow candle flame and a clean blue gas flame. It is important to focus on the internal structure of a single flame and remember that complete combustion in the blue outer region produces the highest temperatures.

Final Answer:
The hottest region of a typical laboratory or candle flame is the blue outer non luminous zone of the flame.