Saturated hydrocarbons burn with a clean blue flame while unsaturated hydrocarbons burn with a sooty yellow flame. (i) Why do unsaturated hydrocarbons produce a sooty flame during combustion? (ii) What does this tell us about their carbon-to-hydrogen ratio compared to saturated hydrocarbons?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i) Unsaturated hydrocarbons have a higher proportion of carbon relative to hydrogen. During combustion, the available oxygen is insufficient to burn all the carbon completely. This results in incomplete combustion, releasing unburnt carbon particles as soot, producing a yellow, smoky flame.
(ii) This shows that unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio compared to saturated hydrocarbons. The higher carbon content means more carbon remains unburnt, whereas saturated hydrocarbons, having relatively more hydrogen, burn completely with a clean blue flame.
Source: Chapter 4, Section 4.3.1 Combustion
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Explanation
- Examiners expect you to link "sooty flame" directly to incomplete combustion and higher C:H ratio — these are the two scoring points.
- Part (i) is worth ~2 marks: mention high carbon proportion → insufficient oxygen → incomplete combustion → unburnt carbon = soot.
- Part (ii) is worth ~1 mark: a direct comparative statement about the C:H ratio is enough.
- Never just say "they have double/triple bonds" without connecting it to why that causes soot — the examiner wants the combustion logic.