Gases in the Atmosphere
7 marks · HardThis question is about the composition of air and the environmental impact of greenhouse gases.
State the approximate percentage by volume of oxygen in dry air.
Explain, using your knowledge of greenhouse gases, why increasing the amount of carbon dioxide in the atmosphere matters.
A student claims that the Earth’s atmosphere could safely be made up almost entirely of carbon dioxide, so long as there is still about 21% oxygen for breathing. Explain, using scientific reasons, whether you agree with this statement.
State one major human activity that increases the amount of carbon dioxide in the atmosphere and explain how it contributes to environmental change.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 21%
(b) Increasing CO_2 traps more infrared radiation, warming the planet and leading to climate change.
(c) Disagree; if air is mostly CO_2, Earth would be too hot for life because CO_2 is a greenhouse gas. The percentage of oxygen is not enough—too much CO_2 would cause extreme climate change.
(d) Burning fossil fuels releases CO_2, increasing the greenhouse effect and contributing to global warming.
Common mistakes: Confusing greenhouse effect with ozone layer; Incorrect percentages of gases in air
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states approximate percentage of oxygen in dry air as 21%.
B1 (b) · 1 mark · Explains CO_2 is a greenhouse gas that traps infrared radiation / heat.
B2 (b) · 1 mark · Mentions global warming or climate change due to increased CO_2.
B1 (c) · 1 mark · Rejects the claim; CO_2 as main gas leads to overheating/extreme climate change.
B2 (c) · 1 mark · Reason is based on the properties of CO_2 as a greenhouse gas, not just oxygen for breathing.
B1 (d) · 1 mark · Identifies a relevant human activity (e.g., burning fossil fuels, deforestation).
B2 (d) · 1 mark · Explains link to release of CO_2 and enhancement of greenhouse effect/global warming.
Worked solution
a) The approximate percentage by volume of oxygen in dry air is 21%.
b) Carbon dioxide is a greenhouse gas, which means it absorbs infrared radiation emitted from the Earth's surface and traps heat in the atmosphere. As more CO_2 is added, more heat is retained, causing Earth's average temperature to rise. This leads to global warming and climate change.
c) I do not agree with the student. If the atmosphere mostly contained carbon dioxide rather than nitrogen, the planet would become extremely hot because CO_2 is a powerful greenhouse gas. The high concentration of CO_2 would trap too much heat, resulting in an environment that is too hot for most life forms, even if there is enough oxygen to breathe.
d) One major human activity that increases CO_2 in the atmosphere is burning fossil fuels (such as coal, oil, and gas). This releases CO_2, which enhances the greenhouse effect, and contributes to an overall increase in global temperatures and climate change.