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Why do unsaturated carbon compounds generally burn with a yellow, sooty flame?
ATheir double bonds release nitrogen which colours the flame yellow
BTheir higher carbon proportion leaves unburnt hot carbon particles that glow yellow
CThey contain extra oxygen that makes the flame brighter and yellow in colour
DThey burn faster than oxygen can supply, cooling the flame to yellow
Answer & Solution
Correct answer: B. Their higher carbon proportion leaves unburnt hot carbon particles that glow yellow
1. Comparing formulae, unsaturated compounds have a larger proportion of carbon.
2. During combustion some carbon particles remain unburnt.
3. These hot carbon particles glow and emit yellow light in the flame.
4. The unburnt carbon also forms the black soot seen as a deposit.
5. Rule out C: there is no extra oxygen; if anything oxygen supply is the limiting factor.
6. Rule out A: no nitrogen is released; hydrocarbons contain only carbon and hydrogen.
7. Rule out D: a cooler flame does not turn yellow; glowing carbon particles do.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 9 "Carbon Compounds", p.133_
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