Revision notes · Chemistry of the atmosphere
The composition and evolution of the Earth's atmosphere
The proportions of different gases in the atmosphere4.9.1.1
Definition: Atmosphere is the layer of gases surrounding a planet, held in place by gravity.
For approximately 200 million years, the proportions of gases in the Earth's atmosphere have remained relatively constant.
- •Approximately 80% (about 78%) of the Earth's atmosphere is nitrogen.
- •Approximately 20% (about 21%) of the Earth's atmosphere is oxygen.
- •Small proportions of various other gases exist, including carbon dioxide, water vapour, and noble gases such as argon.
⚠️ Common mistake: Believing carbon dioxide makes up a large percentage of the modern atmosphere, when it actually accounts for less than 0.05%.
🧠 Remember: Nitrogen is Nearly 80%, Oxygen is Only 20%.
The Earth's early atmosphere4.9.1.2
Definition: Early atmosphere refers to the mixture of gases surrounding the Earth during its first billion years of existence.
Intense volcanic activity during Earth's early history released gases that formed the early atmosphere.
- •During the first billion years, volcanic activity released carbon dioxide, water vapour, and small amounts of nitrogen, methane, and ammonia.
- •The early atmosphere consisted mainly of carbon dioxide with little or no oxygen, similar to modern-day Mars and Venus.
- •As the Earth cooled down, the water vapour condensed to form the oceans.
⚠️ Common mistake: Assuming that oxygen was present in large amounts in the early atmosphere.
🧠 Remember: Early atmosphere = Volcanoes produced Carbon dioxide and Water vapour.
How oxygen increased4.9.1.3
Definition: Photosynthesis is the endothermic reaction in which plants and algae convert carbon dioxide and water into glucose and oxygen using light energy.
Photosynthesis by algae and plants caused oxygen levels in the atmosphere to increase gradually over time.
- •Algae first produced oxygen in the atmosphere about 2.7 billion years ago.
- •Over the following billion years, plants evolved and the percentage of oxygen gradually increased.
- •The chemical equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- •Once oxygen levels reached a sufficient level, animals were able to evolve.
⚠️ Common mistake: Thinking that animals evolved before plants increased the oxygen concentration in the atmosphere.
🧠 Remember: Algae came first to produce Oxygen for Animals to evolve.
How carbon dioxide decreased4.9.1.4
Definition: Sedimentary rock is rock formed from layers of mineral deposits or biological remains compressed over millions of years.
Carbon dioxide levels decreased as it dissolved in oceans, was absorbed by photosynthesis, and became locked in rocks and fossil fuels.
- •When oceans formed, carbon dioxide dissolved in the water to form soluble carbonate compounds and insoluble carbonate precipitates.
- •Photosynthesis by algae and plants removed carbon dioxide from the atmosphere.
- •Carbon became trapped in sedimentary rocks like limestone (calcium carbonate formed from marine shells/skeletons).
- •Carbon was also locked up in fossil fuels (coal, crude oil, and natural gas) formed from dead plant and animal matter.
⚠️ Common mistake: Forgetting that carbon dioxide dissolved in the oceans before forming carbonate sediments and sedimentary rocks.
🧠 Remember: Dissolved in oceans, Photosynthesis in plants, Locked in rocks and fossil fuels.
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