Atmospheric pollutants from fuels – GCSE Chemistry exam questions with mark scheme

1Grade 4–54 marks

Complete the table about atmospheric pollutants from burning fuels. [4]

Pollutant	How it forms	Effect
carbon monoxide	(i) ......	(ii) ......
sulfur dioxide	(iii) ......	(iv) ......

(i) incomplete combustion (of a fuel containing carbon) allow burning fuel in a limited / insufficient supply of oxygen(1)
(ii) toxic / reduces the blood's ability to carry oxygen allow binds to haemoglobin; ignore colourless / odourless (not an effect)(1)
(iii) sulfur (impurities) in the fuel reacts with oxygen when the fuel burns allow burning fuels that contain sulfur(1)
(iv) acid rain / respiratory problems ignore global warming / greenhouse effect(1)
Answer: CO: incomplete combustion; toxic, reduces oxygen carried by blood. SO₂: sulfur in fuel oxidised; acid rain / breathing problems
Examiner tip: Sulfur dioxide comes from sulfur in the fuel, not from the air.

2Grade 1–3Foundation5 marks

Burning fuels releases pollutants into the atmosphere.

(a) Draw one line from each pollutant to how it is formed. [3]

Pollutant		How it is formed
carbon particulates (soot)		nitrogen and oxygen from the air react at the high temperatures in an engine
oxides of nitrogen		sulfur in the fuel reacts with oxygen
sulfur dioxide		incomplete combustion of a fuel
		thermal decomposition of the fuel to make smaller molecules

(b) Which pollutant causes global dimming?

Tick (✓) one box.

☐ carbon dioxide ☐ carbon particulates ☐ oxides of nitrogen ☐ water vapour [1]

(a) carbon particulates (soot) → incomplete combustion of a fuel(1)
(a) oxides of nitrogen → nitrogen and oxygen from the air react at the high temperatures in an engine(1)
(a) sulfur dioxide → sulfur in the fuel reacts with oxygen if more than one line is drawn from a pollutant, no mark for that pollutant(1)
(b) carbon particulates (only box ticked)(1)
(c) (cause / worsen) lung damage / breathing (respiratory) problems / asthma allow heart disease / lung disease / cancer; ignore 'bad for you' / 'makes you ill' unqualified; ignore poisonous / toxic unqualified(1)
Answer: (a) particulates → incomplete combustion; oxides of nitrogen → N₂ + O₂ from air at high temperature; sulfur dioxide → sulfur in the fuel (b) carbon particulates (c) breathing / lung problems
Examiner tip: Oxides of nitrogen come from nitrogen in the air, not from the fuel — linking them to the fuel is the most common wrong line.

3Grade 1–3Foundation6 marks

A gas boiler burns methane to heat a house.

(a) Complete the word equation for the complete combustion of methane. [2]

methane + oxygen → ______________ + ______________

(b) A faulty boiler in a room with poor ventilation burns methane in a limited supply of oxygen.

Balance the equation for this reaction. [1]

____ CH₄ + 3O₂ → 2CO + ____ H₂O

Explain why an alarm is needed to detect carbon monoxide. [2]

(d) Give one sign, that a person could see, that a boiler is burning methane in a limited supply of oxygen. [1]

(a) carbon dioxide allow CO$_2$(1)
(a) water allow H$_2$O / steam / water vapour (either order); do not accept hydrogen(1)
(b) 2CH$_4$ + 3O$_2$ → 2CO + 4H$_2$O (both numbers correct)(1)
(c) carbon monoxide is toxic / poisonous allow reduces the amount of oxygen the blood can carry(1)
(c) (because it is) colourless and / or odourless so it cannot be seen or smelled allow people cannot tell it is there; ignore 'invisible' alone for this mark unless linked to cannot detect it(1)
(d) yellow / orange (smoky) flame or black soot / sooty marks (around the boiler) allow flame not blue; ignore smell(1)
Answer: (a) carbon dioxide + water (b) 2, 4 (c) CO is toxic; colourless and odourless so cannot be seen or smelled (d) yellow/orange flame or soot marks
Examiner tip: In (c) 'carbon monoxide is dangerous' scores nothing — say it is toxic and that it cannot be seen or smelled because it is colourless and odourless.

4Grade 4–57 marks

A testing company measured the mass of three pollutants released by a petrol car and a diesel car. The bar chart shows the results.

(a) Which pollutant did the petrol car release the greatest mass of? [1]

(b) How many times greater is the mass of oxides of nitrogen released by the diesel car than by the petrol car? [2]

Calculate the mass of carbon monoxide released. Give your answer in grams. [2]

(d) The temperature inside a diesel engine is higher than inside a petrol engine.

Explain why the diesel car releases a greater mass of oxides of nitrogen. [2]

(a) carbon monoxide(1)
(b) values read from the chart: 150 (mg per km) and 50 (mg per km)(1)
(b) 3 (times) allow 3 with no working shown for 2 marks(1)
(c) 450 × 30 = 13 500 (mg) allow 450 read from chart and multiplied by 30(1)
(c) 13.5 (g) allow ecf from an incorrect chart reading × 30 ÷ 1000; allow 13.5 with no working for 2 marks(1)
(d) nitrogen and oxygen (from the air) react (in the engine) do not accept nitrogen from the fuel(1)
(d) (reaction happens / more reaction happens) at high(er) temperatures(1)
Answer: (a) carbon monoxide (b) 150 ÷ 50 = 3 times (c) 13.5 g (d) N₂ and O₂ from air react; more at higher temperatures
Examiner tip: Read bar heights against the gridlines carefully (each gridline is 50 mg per km) and remember 1 g = 1000 mg.

5Grade 6–7Higher5 marks

A power station burns 5000 tonnes of coal each day. The coal contains 1.6% sulfur by mass.

The sulfur burns to form sulfur dioxide:

$$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$$

(a) Calculate the mass of sulfur dioxide released each day. Give your answer in tonnes.

Relative atomic masses ($A_r$): O = 16 S = 32 [3]

(b) The power station changes to a coal that contains less sulfur.

A student says: 'Now the power station will not cause any acid rain.'

Explain why the student is wrong. [2]

(a) mass of sulfur = 1.6/100 × 5000 = 80 (tonnes)(1)
(a) $M_r$ of SO$_2$ = 64 allow mass of SO$_2$ is double the mass of S / ratio 32 : 64(1)
(a) 160 (tonnes) allow ecf from incorrect mass of sulfur × 2; correct answer with no working scores 3(1)
(b) (the coal still contains some sulfur so) some sulfur dioxide is still produced allow less sulfur dioxide, not none(1)
(b) oxides of nitrogen are still produced (from nitrogen and oxygen in the air at high temperature) and these also cause acid rain(1)
Answer: (a) 160 tonnes (b) some SO₂ is still made; oxides of nitrogen (from the air) also cause acid rain
Examiner tip: Find the mass of sulfur first (80 t), then scale by 64/32 — many students multiply 5000 t of coal by 64/32 and forget the 1.6%.

6Grade 8–9Higher8 marks

A camping stove burns butane, C₄H₁₀.

When the stove is used inside a closed tent, the butane burns in a limited supply of oxygen. The only products are carbon monoxide and water.

(a) Write a balanced symbol equation for this reaction. [2]

(b) Calculate the mass of carbon monoxide produced when 14.5 g of butane burns in this way.

Relative atomic masses ($A_r$): H = 1 C = 12 O = 16 [4]

$$2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}$$

Use this equation and your answer to (a) to explain why carbon monoxide is produced when the stove is used in a closed tent. [2]

(a) correct formulae: C$_4$H$_{10}$ + O$_2$ → CO + H$_2$O(1)
(a) balanced: 2C$_4$H$_{10}$ + 9O$_2$ → 8CO + 10H$_2$O allow multiples / C$_4$H$_{10}$ + 4½O$_2$ → 4CO + 5H$_2$O(1)
(b) $M_r$ of C$_4$H$_{10}$ = 58(1)
(b) moles of butane = 14.5 ÷ 58 = 0.25 (mol)(1)
(b) moles of CO = 0.25 × 4 = 1.0 (mol) allow ecf from an incorrect mole ratio in (a)(1)
(b) mass of CO = 1.0 × 28 = 28 (g) correct answer with no working scores 4; allow ecf from moles of CO × 28(1)
(c) complete combustion needs more oxygen: 13 mol O$_2$ per 2 mol butane compared with 9 mol O$_2$ for incomplete combustion allow 6½ vs 4½ per mole of butane(1)
(c) in a closed tent the oxygen is used up / not replaced, so there is not enough oxygen for complete combustion (so carbon is only oxidised to CO)(1)
Answer: (a) 2C₄H₁₀ + 9O₂ → 8CO + 10H₂O (b) 28 g (c) complete combustion needs 13 O₂ per 2 butane, not 9; oxygen in a closed tent runs low
Examiner tip: Use the 2 : 8 (= 1 : 4) mole ratio from your balanced equation — students often assume 1 mol butane gives 1 mol CO.

7Grade 6–76 marks

In some homes, food is cooked indoors on open fires that burn coal.

Coal contains mainly carbon, with some hydrogen and small amounts of sulfur. The air supply to an open fire indoors is often limited.

Explain how the pollutants released by burning coal on an indoor fire are formed, and the problems the pollutants cause. [6]

carbon monoxide is formed by incomplete combustion of the carbon in coal(1)
incomplete combustion happens because the supply of oxygen / air is limited(1)
carbon monoxide is toxic / reduces the oxygen-carrying capacity of the blood(1)
carbon monoxide is colourless and odourless so people do not know it is there / it is difficult to detect(1)
carbon particulates / soot are formed by incomplete combustion(1)
particulates cause health problems, e.g. lung damage / breathing problems(1)
particulates (released into the atmosphere) cause global dimming (by reducing sunlight reaching the Earth's surface)(1)
sulfur dioxide is formed when the sulfur in the coal reacts with oxygen(1)
sulfur dioxide causes respiratory problems / breathing problems(1)
sulfur dioxide (dissolves in rain water and) causes acid rain(1)
unburned hydrocarbons may also be released (from the hydrogen-containing compounds in coal)(1)
carbon dioxide (from complete combustion of carbon) and water vapour (from hydrogen) are also released(1)
problems are worse indoors because the pollutants are concentrated in a small space / are breathed in directly(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed, coherent explanation covering at least three pollutants. For each, the way it forms is correctly linked to the fuel or the limited air supply, and a specific problem it causes is explained. Uses scientific terms accurately.
Level 2	3–4	Explains how at least two pollutants form and gives problems they cause. Some links between formation and the conditions (limited oxygen, sulfur in coal) are made, but the explanation may be incomplete or uneven.
Level 1	1–2	Simple statements naming one or more pollutants, or giving a source or a problem, with few or no links between them.
No relevant content	0	Nothing worthy of credit.

Answer: Answers should link each pollutant to how it forms and to the problem it causes: CO and soot from incomplete combustion (limited oxygen) — CO is toxic and undetectable; soot damages lungs and causes global dimming; SO₂ from sulfur in the coal — respiratory problems and acid rain.
Examiner tip: A 6-mark 'explain' needs linked chains — pollutant → how it forms → the problem it causes — for at least three pollutants; a list of pollutant names stays in Level 1.

8Grade 4–57 marks

A teacher uses the apparatus in the diagram to investigate the products formed when candle wax burns. A pump draws the gases from the flame through the apparatus.

(a) Give the change seen in the limewater. Name the product of combustion that causes this change. [2]

(b) A colourless liquid collects in the U-tube. The liquid is water.

(i) Why is the U-tube surrounded by ice? [1]

(ii) Which element in candle wax forms the water? [1]

Name the black solid and explain why it forms. [2]

(d) Another product of combustion is likely to be released, but this apparatus does not show it.

Name this toxic gas. [1]

(a) (limewater) turns milky / cloudy allow white precipitate forms; ignore white unqualified(1)
(a) carbon dioxide allow CO$_2$; mark independently(1)
(b)(i) to cool and condense water vapour (into a liquid) allow to turn the steam into liquid water(1)
(b)(ii) hydrogen allow H; do not accept H$_2$O / oxygen(1)
(c) carbon / soot allow carbon particulates(1)
(c) incomplete combustion (of the wax) allow not enough oxygen for complete combustion / limited air supply(1)
(d) carbon monoxide allow CO; ignore sulfur dioxide / oxides of nitrogen(1)
Answer: (a) turns milky; carbon dioxide (b)(i) to condense the water vapour (ii) hydrogen (c) carbon / soot, from incomplete combustion (d) carbon monoxide
Examiner tip: In (b)(ii) the question asks for an element in the wax — 'water' or 'H₂O' scores zero; the answer is hydrogen.

9Grade 6–7Higher8 marks

Engineers measured the concentration of oxides of nitrogen (NOx) in the exhaust gases of an engine at different combustion temperatures. The graph shows the results.

(a) Describe how the concentration of NOx changes as the temperature increases. Use data from the graph. [2]

(b) Determine the increase in the concentration of NOx when the temperature rises from 1800 °C to 2200 °C. [2]

Write balanced symbol equations for:

(i) the formation of NO from nitrogen and oxygen [1]

(ii) the reaction of NO with oxygen to form NO₂. [1]

(d) Some engines mix a small amount of exhaust gas into the air going into the engine. This lowers the combustion temperature from 2000 °C to 1800 °C.

Use the graph to explain how this reduces the release of NOx. [2]

(a) as temperature increases the concentration of NOx increases(1)
(a) the increase gets faster / steeper at higher temperatures (not proportional), with data quoted, e.g. it rises by 900 ppm from 1800 to 2000 °C but by 1500 ppm from 2000 to 2200 °C allow correct values read from the graph at any two temperatures(1)
(b) values read from the graph: 600 (ppm) at 1800 °C and 3000 (ppm) at 2200 °C(1)
(b) 2400 (ppm) allow 2400 with no working for 2 marks(1)
(c)(i) N$_2$ + O$_2$ → 2NO allow multiples; ignore state symbols(1)
(c)(ii) 2NO + O$_2$ → 2NO$_2$ allow multiples / NO + ½O$_2$ → NO$_2$(1)
(d) at the lower temperature, less nitrogen and oxygen (from the air) react (so less NOx forms)(1)
(d) concentration falls from 1500 ppm to 600 ppm allow falls by 900 ppm / to less than half / by 60%(1)
Answer: (a) increases, and increases more steeply at higher temperature (b) 3000 − 600 = 2400 ppm (c)(i) N₂ + O₂ → 2NO (ii) 2NO + O₂ → 2NO₂ (d) less N₂ and O₂ react at lower temperature; NOx falls from 1500 to 600 ppm
Examiner tip: 'Use data from the graph' means you must quote numbers with units — a trend statement alone caps (a) at 1 mark.

10Grade 6–78 marks

The table shows the mass of pollutants released when electricity is generated in a coal-fired power station and in a natural-gas-fired power station.

Pollutant	Mass released by coal in g per kWh	Mass released by natural gas in g per kWh
carbon dioxide	900	400
sulfur dioxide	1.2	0.01
oxides of nitrogen	1.0	0.4
particulates	0.10	0.005

(a) Calculate the percentage decrease in the mass of carbon dioxide released per kWh when natural gas is used instead of coal. Give your answer to 3 significant figures. [2]

(b) Suggest why natural gas releases much less sulfur dioxide than coal. [1]

(d) A newspaper headline says: 'Switching from coal to gas will end air pollution from power stations.'

Evaluate this headline. Use data from the table. [3]

(a) (900 − 400) ÷ 900 × 100(1)
(a) 55.6 (%) allow 55.6 with no working for 2 marks; allow 55.5 / 55.56 for 1 mark only(1)
(b) natural gas contains (almost) no sulfur (compounds) allow sulfur is removed from natural gas before it is burned; ignore 'gas burns cleaner'(1)
(c) particulates (are tiny solid particles that) reflect / absorb / block sunlight(1)
(c) so less sunlight reaches the Earth's surface allow less light / light intensity at the ground decreases(1)
(d) supporting: sulfur dioxide (1.2 → 0.01) and particulates (0.10 → 0.005) are almost completely removed allow all pollutants decrease(1)
(d) against: gas still releases carbon dioxide (400 g per kWh) and / or oxides of nitrogen (0.4 g per kWh) allow some sulfur dioxide / particulates are still released(1)
(d) conclusion: the headline is wrong / exaggerated because pollution is reduced but not ended dependent on at least one data-based point(1)
Answer: (a) 55.6% (b) natural gas contains little or no sulfur (c) particulates reflect / absorb sunlight so less reaches the ground (d) SO₂ and particulates almost removed, but CO₂ (400 g/kWh) and NOx (0.4 g/kWh) still released, so pollution is reduced, not ended
Examiner tip: 'Evaluate' needs both sides and a judgement: quote numbers that support the headline, numbers that contradict it, then say whether you agree.

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