Q1. [3] medium thorough-understanding
A student burns two gases: one produces a clean blue flame and the other a yellow, sooty flame. She also finds that the second gas decolourises bromine water while the first does not. (i) Identify whether each gas is a saturated or an unsaturated hydrocarbon, giving one reason for each identification. (ii) Explain, in terms of the types of bonds present, why unsaturated hydrocarbons decolourise bromine water while saturated hydrocarbons do not.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
(i)
- Gas 1 (clean blue flame): Saturated hydrocarbon. Saturated hydrocarbons burn with a clean blue flame due to complete combustion.
- Gas 2 (yellow, sooty flame): Unsaturated hydrocarbon. Unsaturated hydrocarbons burn with a yellow, sooty flame due to their high carbon content and incomplete combustion.
(ii)
Unsaturated hydrocarbons contain double or triple bonds (C=C or C≡C). These bonds break easily and react with bromine water in an addition reaction, decolourising it. Saturated hydrocarbons have only single bonds and are fairly unreactive; they cannot undergo addition reactions, so bromine water remains orange-brown.
Source: Chapter 4, Sections 4.2.2 and 4.3.1
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Explanation
- For (i), examiners want the correct label (saturated/unsaturated) and the reason linked to the type of flame — both parts needed for full credit.
- For (ii), the key terms examiners look for are double/triple bonds, addition reaction, and the contrast with single bonds in saturated hydrocarbons. Avoid just saying "they react" — name the reaction type.
- The bromine water test is the standard differentiation test between saturated and unsaturated hydrocarbons (Q14 in exercises).