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GCSE · Chemistry

C9a The Atmosphere: Evolution and Greenhouse Gases

AQA 8464 · Calculator allowed · about 70 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Piecing Together Earth's Early Atmosphere

Original text written for Revision Library.

The atmosphere that surrounds the Earth today, roughly four-fifths nitrogen and one-fifth oxygen, took billions of years to form, and scientists cannot observe this ancient history directly. Instead, the widely accepted theory is pieced together from indirect evidence: the composition of gases released by volcanoes today, the chemistry of very old rocks, and the fossil record of early life. During the Earth's first billion years, intense volcanic activity is thought to have released huge quantities of gas, forming an early atmosphere dominated by carbon dioxide, with little or no oxygen, alongside water vapour and nitrogen. As the young planet gradually cooled, water vapour condensed to form the oceans, into which much of the atmospheric carbon dioxide subsequently dissolved. Over enormous timescales, this dissolved carbon dioxide was converted into insoluble carbonates that settled as sediment, eventually forming sedimentary rocks such as limestone; later still, the buried remains of dead organisms were converted into the fossil fuels that lock away yet more carbon today. A second major change began around 2.7 billion years ago, when algae evolved in the oceans and began to photosynthesise, followed much later by plants on land. Because this process removes carbon dioxide and releases oxygen, it is thought to be the main reason oxygen levels climbed towards the 21% seen today, eventually allowing a protective ozone layer to form and enabling animal life to leave the oceans. Because these events happened so long ago and left only fragmentary evidence, scientists acknowledge considerable uncertainty over the precise timing and gas percentages involved at each stage, and the model continues to be refined as new evidence is found.

This question is about the composition of the Earth's atmosphere today.

(a)Which of these percentages is closest to the proportion of nitrogen gas in the Earth's atmosphere today?(1)

A) 21%
B) 78%
C) 95%
D) 0.04%

(b)Which of these percentages is closest to the proportion of oxygen gas in the Earth's atmosphere today?(1)

A) 0.04%
B) 21%
C) 50%
D) 78%

(c)Name the gas that makes up almost all of the remaining 'other gases' fraction (about 0.9%) of dry air today, and state whether this gas is reactive or unreactive.(2)

(Total for Question 1 is 4 marks)

Today's atmosphere is made up of several different gases in fixed approximate proportions.

(a)Name the two gases that together make up about 99% of the Earth's atmosphere today.(2)

(b)State the approximate percentage of carbon dioxide in today's atmosphere.(1)

(Total for Question 2 is 3 marks)

Earth formed about 4.6 billion years ago. For roughly the first billion years, intense volcanic activity released large amounts of gas, forming the Earth's early atmosphere.

(a)Name the gas that scientists believe made up most of the Earth's early atmosphere.(1)

(b)Name two other gases, besides carbon dioxide, that were released into the early atmosphere by volcanic activity.(2)

(c)State whether the Earth's early atmosphere is thought to have contained large amounts of oxygen gas.(1)

(Total for Question 3 is 4 marks)

As the young Earth gradually cooled during its first billion years, the huge amount of water vapour released by volcanic activity began to change state.

(a)Explain what happened to this water vapour as the Earth cooled, and name the feature of the Earth that this process eventually formed.(2)

(Total for Question 4 is 2 marks)

The five statements below describe key stages in the evolution of the Earth's atmosphere, but they are not in the correct order.
1. Green plants become widespread on land, further increasing the percentage of oxygen.
2. Volcanic activity releases a carbon-dioxide-rich atmosphere containing little or no oxygen.
3. Algae evolve in the oceans and begin to photosynthesise.
4. Water vapour condenses as the Earth cools, forming the oceans.
5. The atmosphere reaches approximately today's composition of 78% nitrogen and 21% oxygen.

(a)Write down the numbers 1 to 5 in the correct chronological order, starting with the earliest stage.(3)

(Total for Question 5 is 3 marks)

Since the Earth's early atmosphere formed, the percentage of carbon dioxide in the atmosphere has fallen enormously, from an estimated 95% to about 0.04% today. Two long-term processes helped to remove this carbon dioxide from the atmosphere.

(a)Describe how the formation of the oceans helped to reduce the amount of carbon dioxide in the atmosphere.(2)

(b)Describe how the formation of fossil fuels helped to reduce the amount of carbon dioxide in the atmosphere.(2)

(Total for Question 6 is 4 marks)

Around 2.7 billion years ago, algae evolved in the Earth's oceans and began to carry out photosynthesis, a process that changed the composition of the atmosphere forever.

(a)Complete the balanced symbol equation for photosynthesis, including state symbols: __CO2( ) + __H2O( ) -> C6H12O6( ) + __O2( )(3)

(Total for Question 7 is 3 marks)

Photosynthesis by algae, and later by plants, continued for hundreds of millions of years.

(a)Using the equation from the previous question, explain the effect that hundreds of millions of years of photosynthesis had on the levels of carbon dioxide and oxygen in the atmosphere.(3)

(Total for Question 8 is 3 marks)

As oxygen levels in the atmosphere increased, a layer of ozone (O3) began to form high in the atmosphere.

(a)State how the ozone layer is related to the rise in atmospheric oxygen described above.(1)

(b)Explain why the formation of the ozone layer was important for the evolution of life on land.(2)

(Total for Question 9 is 3 marks)

Nitrogen gas now makes up about 78% of the Earth's atmosphere, yet only a small amount of nitrogen was released directly by early volcanic activity.

(a)Suggest why nitrogen gas gradually built up in the atmosphere over billions of years, rather than reacting away or escaping.(2)

(Total for Question 10 is 2 marks)

The table below gives estimated percentages of three gases in the Earth's atmosphere at four points in its history. (Values are approximate; water vapour, also present throughout Earth's history, is not included in this table.)

Time (billion years ago): 4.6	2.7	1.0	0.2
Carbon dioxide (%): 95	40	5	0.04
Oxygen (%): 0	trace	10	21
Nitrogen (%): 4	10	60	78

(a)State the overall trend shown by the table for the percentage of carbon dioxide as time passed, from 4.6 billion years ago to today.(1)

(b)State the overall trend shown by the table for the percentage of oxygen over the same period.(1)

(c)Calculate the percentage decrease in carbon dioxide between 4.6 billion years ago and 1.0 billion years ago. Give your answer to 1 decimal place.(2)

(d)Suggest which biological process most likely explains the rise in oxygen levels between 2.7 billion years ago and 1.0 billion years ago.(1)

(Total for Question 11 is 5 marks)

A sealed sample of dry air has a volume of 500 cm^3.

(a)Assuming this sample contains 21% oxygen gas by volume, calculate the volume of oxygen gas in the sample.(2)

(Total for Question 12 is 2 marks)

Today's atmosphere contains approximately 78% nitrogen and 21% oxygen by volume.

(a)Calculate the ratio of nitrogen to oxygen in today's atmosphere. Give your answer in its simplest whole-number form.(2)

(Total for Question 13 is 2 marks)

Higher tier only. Some models estimate that atmospheric oxygen rose from about 15% to its present-day value of 21% over the final 200 million years of Earth's history.

(a)Calculate the percentage increase in oxygen concentration over this period. Give your answer to 1 decimal place.(3)

(Total for Question 14 is 3 marks)

Use the information in the source text above, and your own knowledge, to answer this question.

Describe how the Earth's atmosphere is thought to have changed from its earliest form to the atmosphere we have today, and evaluate how confident scientists can be in this account.

(Total for Question 15 is 6 marks)

The Earth's surface is warmed by energy transferred from the Sun.

(a)Name the three gases that contribute most to the natural greenhouse effect.(3)

(b)Describe how these gases keep the Earth's surface warmer than it would otherwise be.(4)

(Total for Question 16 is 7 marks)

The natural greenhouse effect has existed for billions of years.

(a)Explain why the natural greenhouse effect is essential for life on Earth.(2)

(Total for Question 17 is 2 marks)

Kwame and Isla investigated how the concentration of carbon dioxide affects the warming caused by the greenhouse effect. They set up two identical gas jars, A and B, each sealed with a rubber bung fitted with a thermometer. Jar A was filled with normal air. Jar B was filled with air containing a higher concentration of carbon dioxide, produced by reacting marble chips with dilute hydrochloric acid and collecting the gas by downward delivery. An identical lamp, representing energy from the Sun, was clamped at the same distance from each jar, and both lamps were switched on at the same time. The temperature inside each jar was recorded every two minutes for ten minutes. Their results are shown below.

Time (minutes): 0	2	4	6	8	10
Temperature in Jar A, normal air (deg C): 20.0	21.0	21.8	22.4	22.8	23.0
Temperature in Jar B, extra carbon dioxide (deg C): 20.0	22.0	23.6	24.8	25.6	26.0

Figure (to be drawn): Two identical gas jars, A and B, each sealed with a rubber bung fitted with a thermometer; an identical lamp is clamped at the same distance from each jar.

(a)Identify the independent variable and the dependent variable in this investigation.(2)

(b)Give one variable, other than the distance of the lamp from the jar, that should be controlled in this investigation.(1)

(c)Explain why it is important that the lamp is placed at exactly the same distance from each gas jar.(2)

(d)Suggest one advantage of using a data logger with a temperature probe, rather than reading a thermometer by eye, in this investigation.(2)

(e)Calculate the mean rate of temperature increase, in degrees Celsius per minute, for Jar A and for Jar B over the full ten minutes.(4)

(f)Use your answers to part (e) to explain what this investigation shows about the effect of carbon dioxide concentration on warming caused by the greenhouse effect.(2)

(g)Suggest one way in which this laboratory investigation is a limited model of the real atmosphere and the greenhouse effect.(2)

(Total for Question 18 is 15 marks)

Higher tier only. Some estimates suggest the Earth's very early atmosphere contained around 95% carbon dioxide, compared with about 0.04% in today's atmosphere.

(a)Calculate how many times greater the percentage of carbon dioxide was in the early atmosphere compared with today's atmosphere.(2)

(Total for Question 19 is 2 marks)

Mark scheme · C9a The Atmosphere: Evolution and Greenhouse Gases

Question 1

(a) B1 B (78%)
(a) Answer: B (78%)
(b) B1 B (21%)
(b) Answer: B (21%)
(c) B1 argon cao
(c) B1 unreactive/inert (it is a noble gas) oe
(c) Answer: Argon; it is unreactive (a noble gas)

Question 2

(a) B1 nitrogen cao
(a) B1 oxygen cao
(a) Answer: Nitrogen and oxygen
(b) B1 0.04% (accept any value between about 0.03% and 0.05%)
(b) Answer: About 0.04%

Question 3

(a) B1 carbon dioxide cao
(a) Answer: Carbon dioxide
(b) B1 water vapour oe
(b) B1 nitrogen oe (accept small amounts of methane or ammonia)
(b) Answer: Water vapour and nitrogen (small amounts of methane and ammonia may also have been present)
(c) B1 no - there was little or no oxygen gas oe
(c) Answer: No, there was little or no oxygen gas present

Question 4

(a) B1 the water vapour condensed (turned into liquid water) as the Earth cooled oe
(a) B1 forming the Earth's oceans cao
(a) Answer: The water vapour condensed into liquid water, forming the oceans

Question 5

(a) B1 2 and 4 correctly placed as the first two stages, in this order
(a) B1 3 correctly placed before 1
(a) B1 fully correct order: 2, 4, 3, 1, 5
(a) Answer: 2, 4, 3, 1, 5

Question 6

(a) B1 carbon dioxide dissolved into the oceans oe
(a) B1 where it was used by marine organisms to form insoluble carbonates (e.g. in shells), which were deposited as sediment and became sedimentary rock such as limestone oe
(a) Answer: Carbon dioxide dissolved in the oceans and was converted into carbonates (e.g. in shells), which formed sedimentary rock such as limestone
(b) B1 the remains of dead plants and animals were buried oe
(b) B1 and, over millions of years, were converted into fossil fuels such as coal, crude oil and natural gas, locking the carbon away oe
(b) Answer: Dead plants and animals were buried and, over millions of years, converted into fossil fuels, locking the carbon away

Question 7

(a) M1 correct coefficients: 6CO2 and 6H2O oe
(a) M1 correct coefficient: 6O2 oe
(a) A1 correct state symbols throughout: (g), (l), (aq), (g) cao
(a) Answer: 6CO2(g) + 6H2O(l) -> C6H12O6(aq) + 6O2(g)

Question 8

(a) B1 carbon dioxide is a reactant in photosynthesis, so it was removed/used up from the atmosphere, decreasing its percentage oe
(a) B1 oxygen is a product of photosynthesis, so it was released into the atmosphere, increasing its percentage oe
(a) B1 this continued over a very long period of time until oxygen levels reached roughly today's value of about 21% oe
(a) Answer: Carbon dioxide was used up (decreasing its percentage) and oxygen was released (increasing its percentage) as photosynthesis continued over hundreds of millions of years, until oxygen reached about 21%

Question 9

(a) B1 ozone (O3) forms from oxygen molecules (O2) high in the atmosphere oe
(a) Answer: Ozone forms from oxygen molecules high in the atmosphere
(b) B1 the ozone layer absorbs/blocks harmful ultraviolet (UV) radiation from the Sun oe
(b) B1 without this protection, UV radiation would have prevented organisms surviving out of water, so its formation allowed life to colonise the land oe
(b) Answer: The ozone layer blocks harmful UV radiation, which allowed organisms to survive out of water and colonise the land

Question 10

(a) B1 nitrogen is a very unreactive (inert) gas under normal atmospheric conditions, so once present it does not readily react further and simply accumulates oe
(a) B1 nitrogen was also gradually added by processes such as ammonia (from volcanic gases) reacting with oxygen, and by bacteria removing nitrogen compounds from soil and oceans and releasing nitrogen gas oe
(a) Answer: Nitrogen is very unreactive, so it accumulated over time rather than reacting away; it was also gradually released by processes such as ammonia reacting with oxygen and by bacteria in soil/oceans

Question 11

(a) B1 the percentage of carbon dioxide decreased/fell over time oe
(a) Answer: It decreased over time
(b) B1 the percentage of oxygen increased/rose over time oe
(b) Answer: It increased over time
(c) M1 (95 - 5) / 95 x 100 oe
(c) A1 94.7% cao (accept 94.6-94.8%)
(c) Answer: 94.7%
(d) B1 photosynthesis (by algae, and later plants) oe
(d) Answer: Photosynthesis, by algae and later plants

Question 12

(a) M1 500 x 0.21 oe
(a) A1 105 cm³ cao
(a) Answer: 105 cm³

Question 13

(a) M1 78 : 21 identified and divided by a common factor oe
(a) A1 26 : 7 cao
(a) Answer: 26 : 7

Question 14

(a) M1 (21 - 15) oe = 6 seen
(a) M1 divides by the original value (15) and multiplies by 100 oe
(a) A1 40.0% cao
(a) Answer: 40.0%

Question 15

Level 3 (5-6): A detailed and logically sequenced description of how the atmosphere changed, correctly covering its volcanic origin, the formation of the oceans and removal of carbon dioxide, and the role of photosynthesis in raising oxygen levels, together with a clear and well-linked evaluation of the uncertainty in the evidence for this account.
Level 2 (3-4): Accurate description of at least two or three stages in the change of the atmosphere, with some correct detail, and some comment (which may be limited or generic) on the uncertainty of the evidence.
Level 1 (1-2): Basic, isolated statements about how the atmosphere has changed, with little or no reference to the strength or limitations of the evidence.
Level 0 (0): No relevant content.
Indicative content:
- The early atmosphere formed from gases released by volcanic activity and is thought to have been mostly carbon dioxide, with little or no oxygen, alongside water vapour and nitrogen.
- As the Earth cooled, water vapour condensed to form the oceans.
- Much of the atmospheric carbon dioxide dissolved into the oceans and was converted into carbonates that formed sedimentary rock such as limestone; carbon was later also locked away in fossil fuels.
- Algae evolved around 2.7 billion years ago and began photosynthesising, followed later by land plants, removing carbon dioxide and releasing oxygen.
- This gradually decreased carbon dioxide to about 0.04% and increased oxygen to about 21%, allowing an ozone layer to form and block harmful UV radiation.
- Nitrogen gas, being largely unreactive, gradually built up to around 78% of the atmosphere.
- Evaluation: because these events happened billions of years ago, scientists rely on indirect evidence such as rock chemistry, present-day volcanic gases and the fossil record rather than direct measurement, so there is significant uncertainty about the exact timings and percentages involved, and the model continues to be refined as new evidence is found.

Question 16

(a) B1 water vapour cao
(a) B1 carbon dioxide cao
(a) B1 methane cao
(a) Answer: Water vapour, carbon dioxide and methane
(b) B1 radiation from the Sun (mostly short-wavelength) passes through the atmosphere and is absorbed by the Earth's surface, warming it oe
(b) B1 the warmed surface re-radiates energy as longer-wavelength infrared radiation oe
(b) B1 greenhouse gas molecules in the atmosphere absorb this infrared radiation oe
(b) B1 the greenhouse gases re-radiate this energy in all directions, including back towards the Earth's surface, keeping it warmer oe
(b) Answer: Short-wavelength radiation from the Sun warms the Earth's surface; the surface re-radiates longer-wavelength infrared radiation, which greenhouse gases absorb and re-radiate in all directions, including back to the surface, keeping it warm

Question 17

(a) B1 without the greenhouse effect, the Earth's average surface temperature would be much lower/far too cold oe
(a) B1 the natural greenhouse effect keeps the Earth at a temperature suitable to support liquid water and living organisms oe
(a) Answer: Without it, the Earth would be far too cold for life; the natural greenhouse effect keeps the planet at a temperature suitable for liquid water and living organisms

Question 18

(a) B1 independent variable: the concentration of carbon dioxide in the jar (which jar/gas mixture is used) oe
(a) B1 dependent variable: the temperature inside the jar oe
(a) Answer: Independent variable: carbon dioxide concentration in the jar. Dependent variable: temperature inside the jar
(b) B1 any one sensible control variable, e.g. starting temperature of the gas in each jar, power/wattage of each lamp, volume of each gas jar, time the lamps are left on oe
(b) Answer: Any one of: starting temperature, lamp power/wattage, jar volume, time the lamps are switched on
(c) B1 so that each jar receives the same intensity/amount of radiation from the lamp oe
(c) B1 making it a fair test, so that carbon dioxide concentration is the only factor affecting the temperature change oe
(c) Answer: So each jar receives the same intensity of radiation, making it a fair test where CO2 concentration is the only variable affecting the result
(d) B1 can take readings automatically/continuously/more frequently without needing to open the jar (which would disturb the results) oe
(d) B1 reduces human error in reading and timing the thermometer, e.g. avoids parallax error oe
(d) Answer: A data logger can take frequent, automatic readings without disturbing the set-up, and avoids human reading/timing errors
(e) M1 Jar A: (23.0 - 20.0) / 10 oe
(e) A1 Jar A = 0.3 deg C per minute cao
(e) M1 Jar B: (26.0 - 20.0) / 10 oe
(e) A1 Jar B = 0.6 deg C per minute cao
(e) Answer: Jar A: 0.3 deg C/min; Jar B: 0.6 deg C/min
(f) B1 Jar B, with the higher carbon dioxide concentration, warmed at twice the rate of Jar A (ft from their values in part e) oe
(f) B1 this supports the idea that increasing the concentration of a greenhouse gas increases the rate/amount of warming caused by the greenhouse effect oe
(f) Answer: Jar B warmed at twice the rate of Jar A, supporting the idea that a higher greenhouse gas concentration increases the warming caused by the greenhouse effect
(g) B1 any one sensible limitation, e.g. the carbon dioxide concentration used is far higher than in the real atmosphere oe
(g) B1 further development of the same or another sensible limitation, e.g. a sealed jar traps warmed air and prevents convection in a way the open atmosphere does not, or the investigation only runs for a short time compared with the real, long-term effect on the climate oe
(g) Answer: For example, the carbon dioxide concentration used is far higher than in the real atmosphere, and the sealed jar prevents convection/heat loss in a way the open atmosphere does not

Question 19

(a) M1 95 / 0.04 oe
(a) A1 2375 cao (accept awrt 2400, or 2.4 x 10³)
(a) Answer: 2375 times greater (approximately 2400)

The Atmosphere: Evolution and Greenhouse Gases - Worksheets, Questions and Revision