GCSE Chemistry - Earth's Early Atmosphere
- Sallyann Clark
- 4 days ago
- 7 min read
A 4.6 Billion Year Story
You can complete this lesson in one go, or split it into 20–30 minute chunks.
You do not need to memorise everything at once; focus on understanding the story.
Before we start, I want you to picture something.
Imagine stepping out of a spacecraft onto the surface of Venus. It looks beautiful from space; bright, cloudy, almost glowing. But the moment you arrive, you wouldn’t survive. The temperature is hot enough to melt lead, the air pressure would crush you flat, and the clouds are full of sulfuric acid. Venus is a perfect example of how badly wrong a planet’s atmosphere can go.
Now pause for a second and think about Earth. We walk around breathing comfortably, with liquid water everywhere and life covering the planet. What makes this even more surprising is that Earth, Venus, and Mars formed at roughly the same time and in similar ways.
Big question:
If Earth, Venus and Mars formed in similar ways, why do their atmospheres look so different today?
Keep that question in your head; everything in this lesson helps answer it.
By the end of this session, you should be able to explain that story clearly, and that’s exactly what AQA wants you to do.
Task
Write one sentence predicting why Earth might be different. Don't worry about being right try to think of a logical and reasoned answer.
Misconceptions
One of the biggest obstacles students hit with this topic isn’t chemistry at all. It’s time.
We’re very good at understanding changes that happen quickly. You notice your phone battery draining. You notice the weather changing. You probably don’t notice a tree growing, because it happens on a time scale that doesn’t match your daily life.
Now stretch that idea massively. The Earth is about 4.6 billion years old. When we talk about changes in the atmosphere, we’re not talking about years, decades, or even thousands of years. We’re talking about hundreds of millions of years at a time.
This is where mistakes creep in. Many students subconsciously assume that the Earth has always looked more or less like it does now. In reality, the early Earth was almost unrecognisable.
Keep that in mind as we rewind the clock all the way back to the beginning.
So Where Did Earth’s Atmosphere Come From?
When Earth first formed, it wasn’t a calm, blue planet with oceans and clouds. It was violent. The surface was molten, constantly bombarded by asteroids, and riddled with volcanoes erupting almost continuously.
As the planet cooled, something important happened. Heavy elements like iron and nickel sank towards the centre of the Earth, forming the core. Lighter materials and gases moved upward. Those gases didn’t disappear, they collected around the planet, forming the very first atmosphere.
This is a key idea for your exam:
Earth’s early atmosphere was formed by gases released from volcanoes.
So let's learn a little about volcanoes.
Watch NGS - Types of Volcanoes.
If you want to go into a bit more detail watch this TedEd - Volcanic Eruptions Explained.
If you are also studying GCSE Geography or just have an interest in volcanoes, you may want to do a bit of research. Here is a link to a volcano lapbook where you can record all the information you find out.
Those volcanoes released large amounts of carbon dioxide, along with water vapour, methane, ammonia and nitrogen. At this stage, there was little to no oxygen at all. If you had somehow visited early Earth, you wouldn’t have been able to breathe.
What we think the early atmosphere looked like
Carbon dioxide: about 80–95%. This was the main gas released in huge amounts by volcanoes.
Nitrogen: around 5–15%. Present early on, but much lower than today.
Water vapour: variable but significant. This later condensed to form the oceans.
Methane and ammonia: tiny amounts (trace gases). Important chemically, but not a large percentage.
Oxygen: almost 0%. This is the key exam point — there was little or no oxygen.
At this point, Earth’s atmosphere probably looked quite similar to the atmospheres of Mars and Venus today. Which makes the next question unavoidable.
If Earth started out the same, why didn’t it stay that way?
When the Oceans Arrived, Everything Changed
As volcanic activity slowly decreased and the surface cooled, the water vapour in the atmosphere began to condense. Over a very long time, this led to the formation of the first oceans.
WatchNational Geographic - Origins of Oceans
This moment is crucial. Liquid water changes everything.
Once the oceans existed, carbon dioxide could dissolve into them. That alone started to reduce the amount of carbon dioxide in the atmosphere. But something even more important followed.
In those early oceans, simple life appeared. At first, this life was microscopic, bacteria and algae. They didn’t look impressive, but they triggered the most important chemical change in Earth’s history.
They could photosynthesise. You do not need to understand photosynthesis for this unit, but here is a quick rundown of this process.
Photosynthesis removes carbon dioxide from the atmosphere and releases oxygen. You need to know this equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
At first, the oxygen didn’t build up. It reacted with rocks and dissolved metals in the oceans. But over hundreds of millions of years, those reactions slowed down, and oxygen finally began to accumulate in the atmosphere.
This was not quick. If you imagine oxygen levels rising suddenly, stop and reset that picture. This was one of the slowest but most important changes the planet has ever experienced.
Getting Rid of Carbon Dioxide (And Locking It Away)
Photosynthesis wasn’t the only way carbon dioxide was removed from the atmosphere.
As carbon dioxide dissolved in the oceans, it formed dissolved carbonates. Marine organisms used these to build shells made of calcium carbonate. When those organisms died, their shells sank to the seabed and became buried in sediments. Over millions of years, this formed sedimentary rocks such as limestone.
Carbon that had once been part of the atmosphere was now locked away in rock.
Some of that buried organic material also formed fossil fuels like coal, oil and gas. Again, carbon was removed from the atmosphere and stored underground.
Watch Student Energy - Fossil Fuels 101
This explains something very important. Venus has an atmosphere that is still around 96% carbon dioxide. Earth does not. The difference is that Earth has oceans and life, and therefore ways to remove carbon dioxide. Venus does not.
Oxygen Changes the Rules of the Game
As oxygen levels increased, a small but vital fraction of it formed ozone high in the atmosphere. Ozone absorbs harmful ultraviolet radiation from the Sun. Before this layer existed, life could only survive in the oceans, where water provided protection.
Once the ozone layer formed, life could finally move onto land.
Oxygen also reacted with gases like methane and ammonia, converting them into nitrogen. Nitrogen is very unreactive, so once it entered the atmosphere, it stayed there. Over time, nitrogen slowly built up until it became the dominant gas.
By around 200 million years ago, the atmosphere had reached a composition very similar to what we have today: mostly nitrogen, plenty of oxygen, and small amounts of other gases.
The atmosphere is made up of:
Nitrogen — about 78%This is the main gas. It’s very unreactive, which is why it just sits there and builds up over time.
Oxygen — about 21%This is produced by photosynthesis and is essential for respiration and combustion.
Argon — about 0.9%A noble gas. Very unreactive and mostly ignored in everyday chemistry.
Carbon dioxide — about 0.04%A tiny percentage, but hugely important for climate and photosynthesis.
There are also trace amounts of other gases such as neon, helium, methane and krypton.
Water vapour is present, but it’s not given a fixed percentage because it changes depending on weather and location.
Bringing It All Together (This Is the Exam Bit)
If you’re asked to explain how the Earth’s atmosphere evolved, you’re telling a story, not listing facts.
It starts with volcanic activity producing a carbon-dioxide-rich atmosphere. As the Earth cooled, oceans formed, allowing carbon dioxide to dissolve.
Photosynthetic organisms then removed even more carbon dioxide and released oxygen.
Carbon became locked away in sedimentary rocks and fossil fuels. Oxygen levels rose, ozone formed, life spread, and nitrogen slowly accumulated.
That’s the journey.
Tasks
Draw a storyboard for the evolution of Earth's atmosphere.
Play Wordwall Evolution of Earths Atmosphere.
Answer the following questions.
Practice Questions (10)
State the main gas present in the Earth’s atmosphere today.(1 mark)
Give the approximate percentage of oxygen in the Earth’s atmosphere. (1 mark)
Name two gases that were present in the Earth’s early atmosphere.
(2 marks)
Why was there little or no oxygen in the Earth’s early atmosphere?
(2 marks)
Describe how volcanic activity contributed to the formation of the Earth’s early atmosphere. (3 marks)
Photosynthesis played a key role in changing the Earth’s atmosphere.
(a) Write the balanced chemical equation for photosynthesis.
(b) Explain how photosynthesis affected the amounts of carbon dioxide and oxygen in the atmosphere.(4 marks)
Explain two different ways carbon dioxide was removed from the atmosphere over time. (4 marks)
Nitrogen is now the most abundant gas in the atmosphere.
Explain why nitrogen built up in the atmosphere over time. (3 marks)
The atmospheres of Earth and Venus are very different today.
Explain why Earth does not have an atmosphere made mostly of carbon dioxide, but Venus does. (5 marks)
Describe how the Earth’s atmosphere has changed from its formation to the present day.
Your answer should include:
volcanic activity
oceans
photosynthesis
oxygen and nitrogen (6 marks)
Final Pause: Check You’ve Got It
Before you move on, try this without looking back:
Why did oxygen take so long to build up in the atmosphere?
Why does Venus still have so much carbon dioxide?
If you can answer those clearly, you’re in very good shape for AQA.
Well done on completing this unit of work. When you finish all the learning units check out the revision unit which focuses on answering past paper questions.
