Part 1: Multiple choice
The atmosphere is
The most abundant gas in the atmosphere is
Oxygen makes up approximately what percentage of the air by volume?
The percentage of argon in the air is about
The percentage of carbon dioxide in the air is about
Which process adds oxygen to the atmosphere?
Which process removes CO₂ from the atmosphere?
Nitrogen oxides are found in very small quantities because
The bond between the two atoms in a nitrogen molecule is
Which gas is a noble gas?
A greenhouse gas among the following is
Which is NOT an importance of the atmosphere?
The layer of the atmosphere closest to Earth's surface is the
Most weather phenomena occur in the
In the troposphere the temperature decreases with altitude at about
If the temperature at ground level is 5 °C, the temperature at the top of a 2 km mountain is
The temperature falls in the troposphere with height because
The tropopause is
The ozone layer is located in the
The ozone layer extends approximately from
Temperature increases with height in the ozone layer because ozone
Aircraft prefer to fly in the lower stratosphere because
The layer of the atmosphere with the lowest temperature (≈ −93 °C) is the
Most meteors burn up in the
The thermosphere extends from about
Temperatures in the thermosphere can exceed
The ionosphere is useful for
The ionosphere contains many charged particles because
The aurora is produced when
Solar-wind particles are directed toward the poles by Earth's
The highest layer of the atmosphere is the
Satellites are placed in the exosphere because
Which planet is the smallest in the solar system and has no gaseous atmosphere?
The escape velocity for Earth is about
A planet retains a gas if
vrms of gas molecules is greater when
Escape velocity depends on
At the same temperature hydrogen molecules (M = 2) move about ... times faster than oxygen molecules (M = 32).
The thickness of the thermosphere is approximately
Which layer contains most of the atmosphere's water vapour and mass?
Part 2: Written questions and problems
1. [Give reasons] Give a reason: the percentage of nitrogen oxides in the air is very low.
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N₂ has a strong triple bond and is largely inert; it reacts with oxygen only during lightning or at extremely high temperatures.
2. [Give reasons] Give a reason: carbon dioxide is important for plants.
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It is the raw material for photosynthesis, from which plants make glucose.
3. [Give reasons] Give a reason: the air temperature decreases with altitude in the troposphere.
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Pressure falls with altitude so air expands and cools; also the air is heated from the ground below.
4. [Give reasons] Give a reason: the temperature in the lower part of the troposphere is higher than in the upper part.
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The air is warmed by contact with the ground, which absorbs sunlight; the upper air is farther from this heat source and has expanded.
5. [Give reasons] Give a reason: the troposphere is the most biologically active layer.
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It has oxygen, carbon dioxide, water vapour, suitable temperature and pressure for life.
6. [Give reasons] Give a reason: the temperature rises with height in the ozone layer.
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Ozone absorbs the Sun's UV radiation and converts the energy into heat.
7. [Give reasons] Give a reason: the stratosphere is suitable for aircraft flight.
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Air movement is mostly horizontal, giving stability and little turbulence.
8. [Give reasons] Give a reason: the ozone layer is vital for life.
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It absorbs most harmful UV radiation that damages DNA and causes skin cancer and ecological damage.
9. [Give reasons] Give a reason: the mesosphere is the coldest layer.
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It has few gas molecules and little ozone to absorb solar radiation.
10. [Give reasons] Give a reason: most meteors do not reach the ground.
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They burn up and disintegrate by friction in the mesosphere.
11. [Give reasons] Give a reason: radio waves can travel over long distances around the Earth.
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The ionosphere reflects them back to the ground.
12. [Give reasons] Give a reason: the aurora is seen only near the poles.
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Earth's magnetic field guides charged solar particles toward the polar regions where they collide with ionospheric atoms.
13. [Give reasons] Give a reason: satellites remain in orbit in the exosphere for a long time.
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The extremely low density gives almost no air resistance.
14. [Give reasons] Give a reason: Mercury cools rapidly after sunset.
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It has no atmosphere to retain heat, so the heat radiates directly to space.
15. [Give reasons] Give a reason: Earth keeps its atmosphere while Mercury does not.
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Earth's higher escape velocity (11.2 km/s) is greater than the gas molecular speeds, but Mercury's weak gravity (4.3 km/s) and hot surface let gases escape.
Part C — Define / compare
16. [Define] Define the atmosphere.
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The layer of gases surrounding the Earth.
17. [Define] Define the tropopause.
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A thin transition layer at the top of the troposphere separating it from the stratosphere.
18. [Define] Define the ionosphere.
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The lower part of the thermosphere (about 80–550 km) with a high concentration of ions produced by high-energy solar radiation.
19. [Define] Define escape velocity.
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The minimum speed an object needs to escape a planet's gravity, independent of the object's mass.
20. [Define] Define biodiversity.
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All living organisms found in a specific place, from microbes to the largest plants and animals.
21. [Compare] Compare the troposphere and stratosphere in height, temperature trend and use.
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Troposphere: 0–10 km, temperature falls, weather and life. Stratosphere: 10–50 km, constant then rising (ozone), stable air for aircraft.
22. [Compare] Compare the mesosphere and thermosphere.
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Mesosphere: 50–80 km, coldest, burns meteors. Thermosphere: 80–700 km, very hot (>2000 °C), contains the ionosphere.
23. [Compare] Compare oxygen and nitrogen in abundance and reactivity.
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Nitrogen 78 %, inert; oxygen 21 %, reactive and essential for respiration and combustion.
24. [Compare] Compare the ozone layer and the ionosphere.
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Ozone layer: stratosphere, absorbs UV. Ionosphere: thermosphere, ionised by X- and gamma rays, reflects radio waves and produces aurora.
25. [Compare] Compare the retention of the atmosphere by Earth and Mercury using ve.
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Earth ve = 11.2 km/s, high; Mercury ve = 4.3 km/s, low and strongly heated, so it cannot keep gases.
Part D — Problems
26. [Problem] If the ground temperature is 20 °C, find the temperature at 3 km height in the troposphere.
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0.5 °C.
20 − 3 × 6.5 = 0.5.
27. [Problem] The ground temperature is 15 °C. At what height is the temperature −20 °C?
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≈ 5.4 km.
35/6.5 = 5.38 km.
28. [Problem] An airliner flies at 9 km. If the ground temperature is 25 °C, find the outside temperature.
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−33.5 °C.
25 − 9 × 6.5 = −33.5.
29. [Problem] A room of 60 m³ is full of air. What volume of oxygen does it contain, assuming 21 %?
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12.6 m³.
0.21 × 60.
30. [Problem] The air in the room (Q69) has density 1.2 kg/m³. Find the mass of air and of oxygen (23 % by mass).
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72 kg air; 16.6 kg oxygen.
60 × 1.2 = 72; 0.23 × 72 = 16.56.
31. [Problem] Calculate vrms for helium (M = 0.004 kg/mol) at 300 K.
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≈ 1368 m/s.
√(3 × 8.314 × 300/0.004) = √(1.871 × 10⁶).
32. [Problem] Compare vrms of hydrogen and oxygen at the same temperature.
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Hydrogen is 4 times faster.
√(MO₂/MH₂) = √(32/2) = 4.
33. [Problem] Calculate the escape velocity from Mars (M = 6.42 × 10²³ kg, R = 3.39 × 10⁶ m).
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≈ 5.0 km/s.
v = √(2 × 6.67 × 10⁻¹¹ × 6.42 × 10²³ / 3.39 × 10⁶) = √(2.53 × 10⁷).
34. [Problem] What fraction of Earth's escape velocity (11 200 m/s) is the vrms of nitrogen at 300 K (517 m/s)?
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4.6 %.
517/11 200.
35. [Problem] Find the thickness of the thermosphere and its share of the 1000 km atmosphere.
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620 km; 62 %.
700 − 80 = 620.
36. [Problem] Atmosphere mass is 5.1 × 10¹⁸ kg and 75 % is in the troposphere. Find the mass in the troposphere.
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3.8 × 10¹⁸ kg.
0.75 × 5.1 × 10¹⁸.
37. [Problem] CO₂ is 420 ppm. Express this as a percentage and as litres of CO₂ in 1 m³ of air.
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0.042 %; 0.42 L.
420 × 10⁻⁶ = 4.2 × 10⁻⁴; 1000 L × 4.2 × 10⁻⁴.
38. [Problem] A radio signal reflects from the ionosphere at 300 km height and returns to a point near the transmitter. How long does the round trip take?
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2 ms.
600 000 m / 3 × 10⁸ m/s.
39. [Problem] Find the energy of a photon of green aurora light (557.7 nm).
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3.57 × 10⁻¹⁹ J.
E = hc/λ = 6.63 × 10⁻³⁴ × 3 × 10⁸/557.7 × 10⁻⁹.
40. [Problem] A meteor moves at 20 km/s through the 30 km thick mesosphere. How long does it spend in it?
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1.5 s.
30/20.