The Composition and Evolution of the Earth’s Atmosphere

1. What is the fraction of nitrogen in the atmosphere today?

Answer: 4/5

2. Approximately how long has the proportion of gases in the atmosphere been the same as today?

Answer: 200 million years

3. Approximately how old is the Earth?

Answer: 4.6 billion years

4. What made oxygen in the Earth’s atmosphere?

Answer: Plants and/or algae produce oxygen by photosynthesis.

5. The figure below shows the composition of the Earth’s atmosphere.

a) State the name of the gas that makes up most of the Earth’s atmosphere.

Answer: nitrogen

b) Give the molecular formula of the gas that makes up approximately 1/5 of the Earth’s atmosphere.

Answer: O2 (oxygen)

c) Suggest a gas that is found in the 0.1% of gases in our atmosphere.

Answer: carbon dioxide, water vapour and any noble gas i.e. He, Ne, Ar, Kr, Xe or Rn.

6. The evolution of the Earth’s atmosphere is not known. Evidence is collected from the Earth, as well as observing planets in our solar system to suggest how our own atmosphere may have been created. One theory is that the early atmosphere was like Venus or Mars today.

a) Use your knowledge of this theory to explain how the concentration of carbon dioxide has naturally changed over the last 4.6 billion years.

Answer: Intense volcanic activity created the early atmosphere made of mainly carbon dioxide. This level of carbon dioxide reduced, as the Earth cooled, and the oceans formed some carbon dioxide dissolved and formed carbonate rocks. Then as algae/plants developed photosynthesis occurred and the carbon dioxide levels reduced to present levels (0.04%).

b) There is more than one theory about how the Earth’s atmosphere formed. Explain why it is difficult to know exactly how the atmosphere developed.

Answer: It happened a very long time ago and it is difficult to collect evidence.

7. Evidence collected from rocks and analysis of air bubbles trapped in Arctic ice are used by scientists to support their theory that the Earth’s atmosphere has changed over time.

a) State the main gas in Earth’s early atmosphere.

Answer: carbon dioxide

b) Describe the theory of how nitrogen became part of Earth’s atmosphere.

Answer: Volcanoes released gases which included nitrogen. The levels of nitrogen gradually increased.

c) Explain the role of the oceans in reducing the amount of carbon dioxide in Earth’s atmosphere.

Answer: Carbon dioxide dissolved in the water, this increased the amount of dissolved carbon dioxide. Animals that contain shells take in carbon dioxide in the air to make carbonate. When they die they sink and form sedimentary rocks. These processes reduce and store the carbon dioxide.

8. Oxygen is essential for life. Oxygen is released into the Earth’s atmosphere by a process called photosynthesis.

a) Write a word equation for photosynthesis.

Answer: carbon dioxide + water  →  glucose + oxygen

b) Cyanobacteria use photosynthesis to make glucose to live. State two other organisms that perform photosynthesis.

Answer: algae and plants

c) Air today is mainly made of nitrogen and oxygen. Give the ratio of these gases in the modern atmosphere.

Answer: The ratio is 4 : 1

9. The Earth’s atmosphere has evolved over about 4.6 billion years.

a) Write a balanced symbol equation that shows how oxygen was formed in Earth’s atmosphere from carbon dioxide and water.

Answer: 6CO2(g) + 6H2O(l)  →  C6H12O6(aq) + 6O2(g)

b) Carbon dioxide dissolved in the oceans to make carbonic acid (H2CO3 ). Write a balanced symbol equation for this reaction.

Answer: H2O(l)+ CO2(g)  →  H2CO3(aq)

c) Carbonic acid is a weak acid. Write a balanced symbol equation to show how carbonic acid is a weak acid.

Answer: H2CO3  ⇋  2H+ + CO32−

d) Suggest what the effect is on the pH of the oceans as the carbon dioxide dissolved, and explain why this happens.

Answer: pH decreased, because carbonic acid released H+(aq)