A student burns two different hydrocarbons — one saturated and one unsaturated — and holds a metal plate above each flame. What difference would she observe on the metal plate, and why does this difference occur?
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
Observation: A sooty (black) deposit will appear on the metal plate held above the unsaturated hydrocarbon's flame. The plate held above the saturated hydrocarbon's flame will remain comparatively clean (little or no soot).
Reason: Unsaturated hydrocarbons contain multiple bonds (C=C or C≡C) and have a higher carbon-to-hydrogen ratio. They undergo incomplete combustion, producing a yellow, sooty flame with unburnt carbon particles that deposit as soot on the plate. Saturated hydrocarbons combust more cleanly, giving a blue flame with no sooty deposit.
Source: Chapter 4, Section 4.3.1 – Combustion
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Explanation
- Examiners award marks for: (1) correct observation for each hydrocarbon, (2) linking unsaturated compounds to sooty/yellow flame and incomplete combustion, and (3) explaining why (higher C:H ratio / multiple bonds → incomplete combustion → carbon deposit).
- Use the textbook's exact language: "saturated hydrocarbons give a clean flame; unsaturated carbon compounds give a yellow flame with lots of black smoke / sooty deposit."
- Do not write a lengthy essay — two short paragraphs covering observation + reason is ideal for 3 marks.