Gases in the Atmosphere
8 marks · HardThe table below shows the main gases present in dry, unpolluted air:
| Gas | Percentage by volume (%) |
|---|---|
| Nitrogen | 78 |
| Oxygen | 21 |
| Argon | 1 |
| Carbon dioxide | 0.04 |
Read the data carefully. Then refer to the information provided to answer the following questions about atmospheric composition and environmental impact.
State the two most abundant gases in dry air and their approximate percentages by volume.
Suggest a reason for the very low percentage of carbon dioxide, even though its environmental impact is significant.
Describe, with reference to an experiment involving copper, how you could show that air contains about one-fifth oxygen by volume.
Explain what is meant by the term 'greenhouse gas', and why carbon dioxide is an important greenhouse gas.
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) Nitrogen: about 78%; Oxygen: about 21% · must show nitrogen ~78% stated, oxygen ~21% stated
(b) Carbon dioxide makes up a tiny fraction of the air but can trap heat and affect climate; even small changes have large effects. · must show CO2 is small fraction, CO2 traps heat / impacts climate
(c) Pass air repeatedly over heated copper in a sealed system, measure original and final gas volume after all oxygen reacts; volume decrease shows oxygen used up is ~1/5 of air. · must show air passed over excess heated copper, measure initial and final volume, calculate % decrease (~1/5 oxygen)
(d) A greenhouse gas absorbs infrared radiation, warming the atmosphere. CO2 is important because it absorbs IR radiation and its rising level contributes to global warming. · must show greenhouse gas absorbs IR radiation, CO2 absorbs IR / causes warming
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States nitrogen as most abundant gas, ~78%
B2 (a) · 1 mark · States oxygen as second most abundant, ~21%
B3 (b) · 1 mark · Mentions even low % CO2 has strong environmental (climate/greenhouse) impact
M1 (c) · 1 mark · Mentions passing air over excess heated copper in sealed system
M2 (c) · 1 mark · Mentions measurement of initial and final air volume
A1 (c) · 1 mark · States 1/5 volume decrease shows oxygen is about 21% of air
B4 (d) · 1 mark · Defines greenhouse gas as one that absorbs IR radiation (or traps heat)
B5 (d) · 1 mark · Explains CO2 is a greenhouse gas; absorbs IR, rising levels cause warming
Worked solution
a) The two most abundant gases in dry air are:
- Nitrogen, about 78% by volume.
- Oxygen, about 21% by volume.
b) Carbon dioxide makes up only about 0.04% of air, but it has a significant environmental impact because it absorbs infrared radiation and traps heat, leading to global warming. Even small increases in its concentration can strongly affect the climate.
c) To show that air contains about one-fifth oxygen using copper:
- Pass a known volume of air repeatedly over excess heated copper in a sealed tube.
- The copper reacts with oxygen to form black copper(II) oxide.
- Measure the volume of air before and after the reaction, allowing the apparatus to cool first.
- The decrease in volume is due to the oxygen reacting. Comparing the decrease to the starting volume shows oxygen makes up about 21% (one-fifth) of air.
d) A greenhouse gas is a gas that absorbs infrared radiation emitted by the Earth, trapping heat and warming the atmosphere. Carbon dioxide is important because it is effective at absorbing infrared radiation, and increasing its level in the atmosphere leads to more global warming.