The stability of atmospheric composition Revision Notes | GCSE AQA Higher Chemistry

1. Why Our Atmosphere Stays the Same

For 200 million years, while dinosaurs evolved and continents drifted, one thing has remained remarkably constant: the air we breathe. The proportions of gases in our atmosphere have stayed virtually unchanged since the Jurassic period.

This stability isn't accidental. It's the result of perfectly balanced natural processes that act like a giant recycling system, constantly moving gases between the atmosphere, oceans, rocks, and living things.

Think of the atmosphere as a massive bank account where deposits and withdrawals must balance perfectly. When photosynthesis removes CO₂ and adds O₂, respiration does the exact opposite. When nitrogen gets locked into soil compounds, other processes release it back to the air.

2. The Great Balancing Act

The stability comes from three major cycling systems that work together like clockwork. Each system has processes that remove gases from the atmosphere perfectly balanced by processes that add them back.

The carbon-oxygen cycle is the most familiar. During photosynthesis, plants absorb CO₂ and release O₂. During respiration (in both plants and animals), the reverse happens - O₂ is consumed and CO₂ is released. These processes occur simultaneously across the planet, maintaining the balance.

Notice how these equations are exact opposites. What photosynthesis removes, respiration replaces. For most of the past 200 million years, these processes kept oxygen near 21% and carbon dioxide at a much lower level, although human activity is now disrupting carbon dioxide levels.

3. The Nitrogen Story

Nitrogen makes up 78% of our atmosphere, but here's the puzzle: most living things can't use atmospheric nitrogen directly. It's chemically unreactive - almost inert. So how does it cycle through ecosystems while maintaining its atmospheric dominance?

The answer lies in specialised bacteria. Nitrogen-fixing bacteria in soil and plant roots can convert atmospheric nitrogen into compounds like nitrates that plants can absorb. When organisms die and decompose, other bacteria break down nitrogen compounds and release N₂ gas back to the atmosphere.

Worked example: The Nitrogen Cycle

Trace how nitrogen moves from the atmosphere through living organisms and back again.

1. Bacteria convert $N_2$ gas into nitrates $(NO_3^-)$ — Only certain bacteria can break the strong N≡N bond
2. Plants absorb nitrates through their roots — Plants use nitrates to make proteins and DNA
3. Animals eat plants and build their own proteins — Nitrogen moves up the food chain
4. Dead organisms decompose, releasing nitrogen compounds — Bacteria break down proteins back to simpler compounds
5. Bacteria convert nitrates back to $N_2$ gas — Nitrogen returns to atmosphere, completing the cycle

Answer: Nitrogen cycles between atmosphere and living organisms through bacterial processes, maintaining 78% atmospheric concentration.

This cycling explains why nitrogen levels stay constant. The amount converted from gas to compounds equals the amount converted back to gas. It's a closed loop with no net gain or loss.

4. What Keeps It All Stable?

The key to atmospheric stability is that these cycles are self-regulating. When one gas increases, natural processes automatically work to bring it back down. When it decreases, other processes boost it back up.

For example, if CO₂ levels rise slightly, plants grow faster (more CO₂ available for photosynthesis), removing more CO₂ from the atmosphere. If O₂ levels drop, there's less respiration, so less CO₂ is produced and more O₂ accumulates.

The oceans also act as massive buffers. They can absorb excess CO₂ when levels are high and release it when levels drop. This ocean-atmosphere exchange helps smooth out any fluctuations.

5. Exam Success - Getting Full Marks

Exam questions on atmospheric stability often ask you to explain why composition has remained constant. The key is showing you understand the balance between opposing processes.

Exam precision note: In The stability of atmospheric composition - The proportions of different gases in the atmosphere - The composition and evolution of the Earth's atmosphere, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

The atmosphere is mostly nitrogen and oxygen, with much smaller amounts of carbon dioxide and noble gases such as argon. Argon makes up about 1% of the atmosphere and is the main noble gas present. Students should link this stable composition to the balance of processes such as photosynthesis, respiration, and decay.