It is the hottest part of the flame and undergoes complete combustion.
Explanation
A flame, such as that of a candle or a Bunsen burner, consists of three distinct zones based on the availability of oxygen and the extent of combustion: the innermost zone (dark zone), the middle zone (luminous zone), and the outermost zone (non-luminous zone). The temperature varies significantly across these zones due to differences in combustion efficiency.
Detailed Analysis
- Outermost Zone (Non-Luminous Zone): This zone receives an ample supply of atmospheric oxygen, leading to complete combustion of the fuel. Consequently, it is the hottest part of the flame. Goldsmiths use a metallic blowpipe to direct this specific zone onto gold and silver to achieve the high temperatures required for melting.
- Middle Zone (Luminous Zone): In this zone, the supply of oxygen is limited, resulting in partial combustion. It contains unburnt carbon particles that glow, producing a yellow light. It is moderately hot but not the hottest.
- Innermost Zone (Dark Zone): This area surrounds the wick and contains unburnt wax vapours. It is the least hot region of the flame.
Therefore, the primary reason for utilizing the outermost zone is its high temperature resulting from complete combustion, which is necessary to melt metals with high melting points like gold and silver.
Key Takeaway: The outermost zone of a flame is the zone of complete combustion; it is blue in colour (non-luminous) and represents the hottest region of the flame.