Read the following and answer the questions that follow:
Rahim noticed that the bottom of his cooking vessel was turning black on the outside. His science teacher explained that the colour of the flame from a gas stove depends on the air supply. When sufficient air enters through the inlets of the burner, the fuel burns completely giving a clean blue flame. If the air holes are blocked, the flame becomes yellow and sooty, and unburnt carbon is deposited. The teacher also told him that fuels such as coal and petroleum contain small amounts of nitrogen and sulphur, and their combustion produces additional harmful gases.
(i) Why does a yellow sooty flame form when air supply is limited? (1 mark)
(ii) Name the two additional harmful gases released when coal or petroleum containing nitrogen and sulphur burns. (1 mark)
(iii) Write a balanced chemical equation for the complete combustion of methane. (1 mark)
(iv) What should Rahim do to fix the sooty flame problem in his stove? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
(i) When air supply is limited, combustion is incomplete. The fuel does not burn fully, and unburnt carbon particles are released, producing a yellow sooty flame with black smoke (carbon/soot deposition).
(ii) The two harmful gases released are oxides of sulphur (SO₂) and oxides of nitrogen (NOₓ).
(iii) Balanced chemical equation for complete combustion of methane:
$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat and light}$$
(iv) Rahim should clean and unblock the air holes/inlets of the burner so that sufficient air enters and the fuel burns completely, giving a clean blue flame.
Source: Chapter 4, Section 4.3.1 Combustion
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Explanation
- (i) Key phrase: "incomplete combustion" → unburnt carbon → soot/yellow flame. Examiners expect this cause-effect link.
- (ii) Must name both gases specifically — "oxides of sulphur and oxides of nitrogen." Simply writing "SO₂ and NO₂" also acceptable.
- (iii) Balancing is critical: CH₄ needs 2O₂ on the left and 2H₂O on the right. Unbalanced equation loses the mark.
- (iv) The passage directly states: blackened vessel bottoms mean air holes are blocked. The fix is to clear those holes — state this clearly.