Ethyne (C₂H₂) is used as fuel in welding torches where it is burned with pure oxygen rather than air. Using your understanding of combustion of carbon compounds, explain: (i) why pure oxygen is preferred over air, and (ii) what type of combustion occurs when sufficient oxygen is available versus when it is insufficient, and what the observable difference would be.
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
(i) Pure oxygen is preferred over air because air contains about 78% nitrogen, which dilutes the oxygen and lowers the flame temperature. Pure oxygen ensures a more complete and intense combustion of ethyne, producing a much hotter flame — sufficient for welding metals.
(ii) When sufficient oxygen is available, complete combustion occurs, producing CO₂ and H₂O with a clean blue flame. When oxygen is insufficient, incomplete combustion occurs, producing carbon (soot) and CO, with a yellow, sooty flame. The observable difference is a clean blue flame vs. a smoky, yellow, sooty flame.
Source: Chapter 4, Section 4.3.1 Combustion
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Explanation
- The key contrast examiners expect: complete vs. incomplete combustion tied to oxygen supply.
- Mention flame colour (blue vs. yellow/sooty) — this comes directly from Activity 4.4 and the textbook paragraph on saturated/unsaturated combustion.
- For part (i), the point about nitrogen diluting oxygen and reducing flame temperature is the core reason stated in the textbook question ("why a mixture of ethyne and air is not used?").
- Don't forget to name the products of both types of combustion for full marks.