Q1. [3] medium initial-understanding
A student burns two different hydrocarbons in air and observes that one produces a clean blue flame while the other produces a sooty, yellow flame. (i) Which type of hydrocarbon — saturated or unsaturated — is more likely to produce each kind of flame? Give a reason for your answer. (ii) Under what condition could even the hydrocarbon that normally burns cleanly begin to produce a sooty flame?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i)
- Saturated hydrocarbons (e.g., alkanes) produce a clean blue flame because they have a higher proportion of hydrogen relative to carbon and burn completely in sufficient air.
- Unsaturated hydrocarbons (e.g., alkenes, alkynes) produce a yellow, sooty flame because they have a higher carbon content and a higher C:H ratio, leading to incomplete combustion and release of unburnt carbon particles (soot).
(ii) Even a saturated hydrocarbon that normally burns cleanly will produce a sooty, yellow flame if the supply of air (oxygen) is limited. Insufficient oxygen causes incomplete combustion, depositing unburnt carbon as soot.
Source: Chapter 4, Section 4.3.1 Combustion
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Explanation
- The key textbook line is: "Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke."
- The reason examiner expects: unsaturated compounds have higher carbon content → incomplete combustion → soot.
- For part (ii), the textbook explicitly states: "limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame." — quote or paraphrase this directly for full marks.
- Avoid writing lengthy chemistry; these two points, stated clearly, earn all 3 marks.