AQA GCSE Chemistry (8462), Higher tier · Chemistry of the atmosphere › Composition and evolution of the atmosphere
Practise How oxygen increased. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Complete the word equation for this process. carbon dioxide + .................... → glucose + oxygen
water
Balance the symbol equation for photosynthesis. ....CO2 + ....H2O → C6H12O6 + ....O2
6CO2 + 6H2O → C6H12O6 + 6O2
Name the gas that plants and algae take in during photosynthesis.
carbon dioxide
About how many billion years ago did algae first produce oxygen?
2.7 billion years ago
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
(a) Which process produced the oxygen that is now in the atmosphere? Tick (✓) one box.[1]
Combustion
Photosynthesis
Respiration
Volcanic activity
(b) Complete the word equation for this process. carbon dioxide + .................... → glucose + oxygen[1]
(c) Which organisms first produced oxygen, about 2.7 billion years ago?[1]
Show the answer and mark scheme
(a)Answer: Photosynthesis
(b)Answer: water
water
(c)Answer: algae
algae
Question 2Medium5 marks
Algae and plants produced the oxygen that is now in the atmosphere.
(a) Balance the symbol equation for photosynthesis. ....CO2 + ....H2O → C6H12O6 + ....O2[1]
(b) Where does the energy needed for photosynthesis come from?[1]
(c) Explain how the evolution of plants led to an increase in the percentage of oxygen in the atmosphere.[2]
(d) Suggest why animals could only evolve after the percentage of oxygen in the atmosphere had increased.[1]
Show the answer and mark scheme
(a)Answer: 6CO2 + 6H2O → C6H12O6 + 6O2
6CO2 + 6H2O → C6H12O6 + 6O2
(b)
light / sunlight
(c)
plants photosynthesise, taking in carbon dioxide and giving out oxygen
as plants evolved and spread over the next billion years, oxygen gradually built up
(d)
animals need oxygen for (aerobic) respiration
Question 3Hard7 marks
An area of forest uses 5.5 tonnes of carbon dioxide in photosynthesis each year. 6CO2 + 6H2O → C6H12O6 + 6O2 Relative formula masses (Mr): CO2 = 44 O2 = 32 C6H12O6 = 180
(a) Calculate the mass of oxygen released each year.[3]
(b) Calculate the mass of glucose produced each year.[2]
(c) Equal numbers of molecules of oxygen and carbon dioxide are involved in photosynthesis, but the mass of oxygen released is less than the mass of carbon dioxide used. Explain why.[2]
Show the answer and mark scheme
(a)Answer: 4.0 tonnes
amount of O2 = amount of CO2 / 6 : 6 ratio
5.5 × 32 ÷ 44
4.0 (tonnes)
(b)Answer: 3.75 tonnes
5.5 × 180 ÷ (6 × 44) / 5.5 × 180 ÷ 264
3.75 (tonnes)
(c)
6 molecules / moles of O2 are produced for every 6 molecules / moles of CO2 used
an oxygen molecule has a smaller mass than a carbon dioxide molecule (Mr 32 compared with 44)