Composition & evolution of the atmosphere – GCSE Chemistry exam questions with mark scheme

1Grade 1–3Foundation3 marks

(a) Give the approximate percentage of nitrogen and of oxygen in dry air today. [2]

(b) Calculate the volume of oxygen in a 250 cm³ sample of dry air. [1]

(a) nitrogen about 80% accept 78–80% / four-fifths(1)
(a) oxygen about 20% accept 20–21% / one-fifth(1)
(b) 50 cm³ allow 52.5 / 52 / 53 cm³ if 21% used; allow correct use of their oxygen percentage from (a)(1)
Answer: (a) N₂ ≈ 80%, O₂ ≈ 20% (b) 50 cm³
Examiner tip: AQA uses 'about four-fifths nitrogen and one-fifth oxygen' — either the rounded or the precise values score.

2Grade 4–54 marks

Scientists believe that the Earth's early atmosphere was mainly carbon dioxide, with little or no oxygen.

Explain how the amount of carbon dioxide decreased and the amount of oxygen increased. [4]

algae and / or plants carried out photosynthesis allow either algae or plants alone(1)
photosynthesis takes in / uses carbon dioxide and gives out / produces oxygen allow correct word or symbol equation allow plants / algae absorbed CO₂ and released O₂ (this mark can be awarded even if photosynthesis is not named) ignore plants 'breathe in' CO₂(1)
any two from (one mark each):
• carbon dioxide dissolved in the oceans
• (dissolved CO₂) formed carbonates that precipitated as sediments / sedimentary rocks such as limestone allow marine organisms used it to make shells / skeletons that formed limestone
• carbon was locked up in fossil fuels (coal, crude oil, natural gas) formed from the remains of organisms(1)
second point from the list above(1)
Answer: Photosynthesis by algae and plants removed CO₂ and released O₂; CO₂ also dissolved in the oceans and formed carbonate sediments / sedimentary rocks such as limestone; carbon was also locked up in fossil fuels.
Examiner tip: Photosynthesis alone scores only 2 marks — for full marks also give a second way CO₂ was removed (dissolving in the oceans, carbonate sediments / limestone, or fossil fuels).

3Grade 1–3Foundation5 marks

Scientists think that during the first billion years of the Earth's existence there was intense volcanic activity.

(a) Volcanoes released gases that formed the Earth's early atmosphere.

Name two gases that volcanoes released in large amounts. [2]

(b) Describe what happened to the water vapour in the early atmosphere as the Earth cooled. [2]

Give one reason why. [1]

(a) any one from: carbon dioxide / water vapour / nitrogen allow correct formulae CO₂ / H₂O / N₂(1)
(a) a second gas from the list ignore methane / ammonia (only small amounts) do not accept oxygen; if oxygen is given as one of two answers, max 1 mark(1)
(b) (water vapour) condensed allow turned into liquid water / rain(1)
(b) formed the oceans allow seas(1)
(c) there were no plants / algae (yet) or no photosynthesis had taken place (yet) allow no living things to produce oxygen; allow volcanoes did not release oxygen; ignore 'no trees'(1)
Answer: (a) two of: carbon dioxide, water vapour, nitrogen (b) it condensed to form the oceans (c) no algae or plants yet to produce oxygen by photosynthesis
Examiner tip: In (b) 'it turned into the sea' only scores 1 mark: you must name the process, condensed, as well.

4Grade 4–55 marks

The bar chart shows the percentage of carbon dioxide in the atmospheres of Venus, Mars and the Earth today.

(a) The atmosphere of Venus contains only carbon dioxide and nitrogen.

Calculate the percentage of nitrogen in the atmosphere of Venus. [1]

(b) Calculate how many times greater the percentage of carbon dioxide is on Mars than on the Earth today. [2]

Use the bar chart and your own knowledge to suggest why. [2]

(a) 4 (%) allow 100 − 96(1)
(b) 95 ÷ 0.04(1)
(b) 2375 (times) allow 2400 / 2380 allow 1 mark for an answer of 0.00042 (inverted division)(1)
(c) (like Mars and Venus) the Earth's early atmosphere was mainly carbon dioxide allow Mars and Venus have (almost) no oxygen, like the early Earth(1)
(c) Mars and Venus have no plants / algae / life, so no photosynthesis has removed carbon dioxide or added oxygen allow their atmospheres also came from volcanic activity(1)
Answer: (a) 4% (b) 2375 times (about 2400) (c) the early Earth's atmosphere was also mainly CO₂ with little or no oxygen; Mars and Venus have had no photosynthesis to change their atmospheres
Examiner tip: In (b) divide the bigger percentage by the smaller one: 0.04 ÷ 95 gives a tiny number that cannot mean 'times greater'.

5Grade 4–56 marks

A student used the apparatus shown to find the percentage of oxygen in air.

At the start, one syringe contained 100.0 cm³ of air.
The air was pushed backwards and forwards over heated copper until the volume stopped changing.
The apparatus was left to cool and the final volume of gas was measured.

The student did the experiment three times.

Trial	1	2	3
Final volume of gas in cm³	79.2	78.8	79.4

(a) The copper turned black during the experiment.

Explain why. [1]

(b) Calculate the percentage of oxygen in the air. Use the mean final volume. Give your answer to 3 significant figures. [3]

(d) Name the gas that makes up most of the gas left at the end. [1]

(a) copper reacted with oxygen (in the air) to form copper oxide allow copper was oxidised ignore 'burnt' alone(1)
(b) mean final volume = (79.2 + 78.8 + 79.4) ÷ 3 = 79.1(33) cm³(1)
(b) oxygen removed = 100.0 − 79.13 = 20.87 cm³ allow ECF from their mean(1)
(b) 20.9 (%) allow ECF; 20.87 or 20.866… scores 2 marks only (not 3 s.f.); correct answer with no working scores 3 marks(1)
(c) hot gas has expanded / a larger volume, so the final volume (reading) would be too high or the percentage of oxygen would be too low allow gas contracts as it cools(1)
(d) nitrogen allow N₂ do not accept N / nitrogen oxide(1)
Answer: (a) copper reacted with oxygen to form (black) copper oxide (b) mean 79.13 cm³; 20.87 cm³ O₂; 20.9% (c) hot gas has a larger volume, so the reading would be too high (d) nitrogen
Examiner tip: Many students give the mean volume (79.1) as the answer: the question asks for the volume that was removed, 100.0 minus the mean.

6Grade 6–78 marks

The graph shows one model of how the percentage of oxygen in the Earth's atmosphere has changed since the Earth formed, 4.6 billion years ago.

(a) Give the percentage of oxygen in the atmosphere today. [1]

(b) Use the graph to find when oxygen first appeared in the atmosphere. [1]

Give your answer in standard form. [2]

(d) Explain why the percentage of oxygen started to increase. [2]

(e) Describe how the percentage of oxygen has changed over the last 0.3 billion years. Use data from the graph. [2]

(a) 21 (%) allow 20–21(1)
(b) 2.7 billion years ago allow 2.6–2.8(1)
(c) 4.6 − 2.7 = 1.9 (billion years) allow ECF from (b)(1)
(c) 1.9 × 10⁹ (years) allow ECF from (b); 1 900 000 000 (not in standard form) scores 1 mark(1)
(d) algae (and later plants) evolved / appeared allow plants(1)
(d) which produced / released oxygen by photosynthesis allow photosynthesis releases oxygen (this mark can be awarded even if algae / plants are not named); ignore 'made oxygen' with no reference to photosynthesis(1)
(e) (rose to a maximum of) 30% (at 0.3 billion years ago) then decreased to 21% (by 0.2 billion years ago) allow fell from 30% to 21%(1)
(e) stayed (about) the same / constant at 21% for the last 0.2 billion years / 200 million years(1)
Answer: (a) 21% (b) 2.7 billion years ago (c) 1.9 × 10⁹ years (d) algae evolved and produced oxygen by photosynthesis (e) fell from 30% to 21% between 0.3 and 0.2 billion years ago, then constant at 21% for 200 million years
Examiner tip: In (c) a 'billion' is 10⁹: write 1.9 × 10⁹ years, not 1.9 × 10⁶.

7Grade 6–7Higher6 marks

Algae in the early oceans removed carbon dioxide from the atmosphere by photosynthesis.

(a) Balance the equation for photosynthesis.

$$\ldots\,\text{CO}_2 + \ldots\,\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \ldots\,\text{O}_2$$ [1]

(b) Some algae made 45 g of glucose, C₆H₁₂O₆.

Calculate the mass of carbon dioxide removed from the atmosphere.

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

(a) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (all three 6s needed)(1)
(b) M_r glucose = 180 and M_r CO₂ = 44(1)
(b) moles of glucose = 45 ÷ 180 = 0.25 (mol)(1)
(b) moles of CO₂ = 0.25 × 6 = 1.5 (mol) allow ECF from their moles of glucose and ratio from (a)(1)
(b) mass of CO₂ = 1.5 × 44 = 66 (g) allow ECF; correct answer with no working scores 4 marks; allow mass-ratio method: (6 × 44 = 264) ÷ 180 × 45 = 66 g(1)
(c) 48 (g) allow ECF from moles in (b) × 32(1)
Answer: (a) 6, 6, 6 (b) 66 g (c) 48 g
Examiner tip: Students who forget the 6 : 1 ratio get 11 g of CO₂: always multiply the moles of glucose by the number in front of CO₂.

8Grade 6–76 marks

Much of the carbon from the carbon dioxide in the Earth's early atmosphere is now locked up in sedimentary rocks, such as limestone, and in fossil fuels.

(a) Describe how coal formed and how crude oil formed. [3]

(b) Limestone is mainly calcium carbonate, CaCO₃.

Calculate the mass of carbon locked up in 500 kg of calcium carbonate.

Relative atomic masses ($A_r$): C = 12 O = 16 Ca = 40 [2]

Suggest why this is changing the composition of the atmosphere much faster than the rate at which coal formed. [1]

(a) coal formed from the remains of (land) plants / trees allow forests(1)
(a) crude oil formed from the remains of (tiny) sea / marine organisms allow plankton / algae(1)
(a) remains were buried (in mud / sediment) and compressed / heated over millions of years ignore 'decayed'(1)
(b) M_r CaCO₃ = 100 or carbon is 12 ÷ 100 = 12% of the mass(1)
(b) 60 (kg) allow ECF from incorrect M_r; correct answer with no working scores 2 marks(1)
(c) coal took millions of years to form but the carbon is released in a (very) short time / only a few hundred years allow we are burning coal much faster than it can form(1)
Answer: (a) coal: remains of plants buried and compressed over millions of years; crude oil: remains of marine organisms (plankton) buried in mud and compressed over millions of years (b) 60 kg (c) carbon locked up over millions of years is being released in a few hundred years
Examiner tip: 'Dead plants turned into coal' scores only 1 mark: say the remains were buried and compressed over millions of years.

9Grade 8–96 marks

One theory about the Earth's atmosphere is:

“The early atmosphere was formed by volcanic activity and was mainly carbon dioxide with little or no oxygen. Oxygen was added much later by algae.”

Some evidence scientists have collected is given below.

A Gases from volcanoes erupting today are mainly water vapour and carbon dioxide, with some nitrogen.
B The atmospheres of Mars and Venus today are mainly carbon dioxide, with very little oxygen.
C Some very old rocks contain iron compounds that could not have formed if there was oxygen in the atmosphere.
D The oldest fossils of algae are found in rocks about 2.7 billion years old. No fossils of living things are found in rocks older than about 3.5 billion years.
E There are huge deposits of limestone and coal in the Earth's crust.
F The Earth formed 4.6 billion years ago. No samples of the Earth's first atmosphere exist, and very few rocks survive from its first few hundred million years.

Evaluate how well the evidence supports this theory.

In your answer, explain why scientists cannot be certain about the Earth's early atmosphere. [6]

Supports the theory:(1)
A: volcanoes release carbon dioxide (and water vapour / nitrogen) but not oxygen, so a volcanic atmosphere would be mainly CO₂(1)
B: Mars and Venus have no life / photosynthesis, so their atmospheres show what a volcanic atmosphere is like without algae and plants(1)
C: the iron compounds show that there was little or no oxygen when those rocks formed(1)
D: algae appeared about 2.7 billion years ago, so oxygen from photosynthesis could only have been added after this(1)
E: limestone and coal contain carbon that was removed from a CO₂-rich atmosphere (by dissolving / sediments and by photosynthesis), so the early atmosphere must have contained a lot of CO₂(1)
Limitations / why scientists cannot be certain:(1)
F: no samples of the early atmosphere exist, so the evidence is indirect / the theory cannot be tested directly(1)
evidence is limited because the time scale is so long (4.6 billion years) and early rocks have been destroyed / changed(1)
A and B are evidence from today / other planets, which may not be the same as conditions on the early Earth(1)
fossils of simple organisms are rarely preserved, so algae may have existed earlier than 2.7 billion years ago (D)(1)
evidence can be interpreted in different ways, so other theories have been suggested and the accepted theory has changed over time / may change with new evidence(1)
Conclusion: the evidence is consistent with the theory / several independent lines of evidence agree, but it is not proof(1)

Level	Marks	What it looks like
Level 3	5–6	Uses several pieces of evidence and explains how each supports (or does not fully support) the theory. Explains why the evidence is limited or uncertain. Reaches a reasoned judgement about how well the theory is supported. Logically linked throughout.
Level 2	3–4	Explains how at least two pieces of evidence support the theory, and gives at least one reason why scientists cannot be certain. A judgement may be missing or not fully justified.
Level 1	1–2	Refers to one or more pieces of evidence with little explanation of how they relate to the theory, or simply states that there is no direct evidence.
No relevant content	0	No relevant content.

Answer: Most evidence (A–E) is consistent with the theory: volcanoes give out CO₂ not O₂; Mars and Venus (no life) are mainly CO₂; old iron compounds show no O₂; algae and O₂ appear later; limestone and coal show CO₂ was removed. But the evidence is indirect and limited (no samples, rocks destroyed, 4.6 billion years), so the theory is supported, not proved.
Examiner tip: Top-level answers do more than list the evidence: they say what each piece shows, give at least one limitation, and finish with a judgement such as 'supported but not proved'.

10Grade 1–3Foundation5 marks

The table shows the percentages of the main gases in dry air today.

Gas	Percentage (%)
nitrogen	78.1
oxygen	20.9
argon	X
carbon dioxide	0.04
other gases	0.03

(a) Calculate the value of X. [1]

(b) Argon is in Group 0 of the periodic table.

What name is given to the elements in Group 0? [1]

Tick (✓) one box.

☐ $\frac{1}{5}$ ☐ $\frac{1}{2}$ ☐ $\frac{3}{4}$ ☐ $\frac{4}{5}$ [1]

(d) For about how long have the proportions of gases in the atmosphere stayed much the same as they are today? [1]

(e) Which gas in the table made up most of the Earth's early atmosphere? [1]

(a) 0.93 (%) allow 0.9(1)
(b) noble gases allow inert gases(1)
(c) $\frac{4}{5}$ only(1)
(d) (about) 200 million years allow 150–250 million years / 0.2 billion years(1)
(e) carbon dioxide allow CO₂(1)
Answer: (a) 0.93% (b) noble gases (c) 4/5 (d) about 200 million years (e) carbon dioxide
Examiner tip: In (a) remember to subtract every other gas from 100, including the small 0.04 and 0.03 values.

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