The non-luminous zone has a high temperature.
Explanation
The structure of a flame and the temperature variations across its different zones. A flame typically consists of three zones: the innermost zone (dark), the middle zone (luminous), and the outer zone (non-luminous).
Analysis of the Observation:
- The non-luminous zone contains unburnt carbon particles. is Incorrect: The presence of unburnt carbon particles is characteristic of the middle (luminous) zone. These particles glow due to partial combustion, giving the flame a yellow colour. The non-luminous zone undergoes complete combustion and does not contain unburnt carbon.
- The non-luminous zone has a high temperature. is Correct: The non-luminous zone is the outermost part of the flame where the fuel undergoes complete combustion due to a sufficient supply of atmospheric oxygen. Consequently, this is the hottest part of the flame. When a copper wire is introduced into this zone, the intense heat causes it to glow red hot, indicating a high temperature.
- Copper is a combustible substance. is Incorrect: While copper can oxidize, it is not a combustible substance in the context of acting as a fuel for the flame. The observation of the wire becoming red hot is a physical response to heat, not a chemical combustion process of the metal itself.
- The non-luminous zone is the coolest part of the flame. is Incorrect: The non-luminous zone is the hottest part of the flame. The innermost zone (near the wick), which contains unburnt wax vapours, is the coolest part.
Key Takeaway: The outer, non-luminous zone of a flame is the region of complete combustion and represents the highest temperature zone, often used by goldsmiths for melting metals.