Subjects
Maths
Chemistry
Biology
English Language
History
Responding to change (a2 only)
Infection and response
Homeostasis and response
Energy transfers (a2 only)
Cell biology
Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
Organisation
Substance exchange
Bioenergetics
Genetic information & variation
Inheritance, variation and evolution
Genetics & ecosystems (a2 only)
Ecology
Cells
Show all topics
Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
The cold war
Inter-war germany
Medieval period: 1066 -1509
2d religious conflict and the church in england, c1529-c1570
2o democracy and nazism: germany, 1918-1945
1f industrialisation and the people: britain, c1783-1885
1c the tudors: england, 1485-1603
2m wars and welfare: britain in transition, 1906-1957
World war two & the holocaust
2n revolution and dictatorship: russia, 1917-1953
2s the making of modern britain, 1951-2007
World war one
Britain: 1509 -1745
Show all topics
Chemistry
5 Dec 2025
231
4 pages
The Earth's atmosphere is the gaseous layer surrounding our planet that enables all life. Its composition has changed... Show more
Our atmosphere today consists mostly of nitrogen (78%) and oxygen (21%), with small amounts of other gases including carbon dioxide, water vapour and noble gases. This composition has remained relatively stable for about 200 million years, but it wasn't always this way.
During Earth's first billion years, intense volcanic activity released gases that formed the early atmosphere, primarily water vapour and carbon dioxide. As the planet cooled, water vapour condensed to form oceans, but carbon dioxide remained dominant in the atmosphere—similar to what we see today on Mars and Venus.
Carbon dioxide levels decreased over time through several natural processes. The gas dissolved in oceans, creating a weak acid that reacted with minerals to form precipitates. These eventually became sediments of carbonate rock on the sea bed. Marine organisms like corals and mussels also used carbon dioxide to create their shells, which later formed limestone when they died.
Did you know? The limestone formations we see today represent carbon dioxide that was once in the atmosphere, showing how Earth naturally sequestered carbon over millions of years!
About 1.7 billion years ago, a major atmospheric shift began when photosynthetic algae evolved in the oceans. These simple organisms produced oxygen as a byproduct of photosynthesis. As plants later evolved on land, oxygen levels continued to increase until they reached a point where animal life could develop.
While oxygen levels rose, carbon dioxide was being trapped in what we now call fossil fuels. Coal formed when plant material in marshy wetlands didn't fully decompose due to lack of oxygen or acidic conditions. Under high pressure and temperature over millions of years, this plant matter transformed into coal.
Crude oil and natural gas formed similarly but from marine organisms. When plankton died and settled in mud on the sea floor, their remains were preserved without decomposition if oxygen was absent. Gradually, heat and pressure transformed these remains—plankton became crude oil, while natural gas (primarily methane) often formed nearby.
Important The greenhouse effect is actually natural and necessary! Without it, Earth would be too cold for life. Gases like carbon dioxide, water vapour and methane trap the sun's heat by allowing short-wavelength radiation in but absorbing the long-wavelength radiation that the Earth radiates back.
Human activities have significantly altered atmospheric composition, particularly through deforestation and fossil fuel use. Cutting down forests not only removes carbon-absorbing trees but also releases stored CO₂. Meanwhile, agriculture—especially cattle farming—produces large amounts of methane, another powerful greenhouse gas.
The impacts of these changes are already visible melting polar ice, rising sea levels, more severe weather patterns, and shifting distributions of plants, animals and diseases like malaria. While climate models are complex and sometimes lead to simplified predictions, the evidence for human-caused climate change is overwhelming.
Reducing our carbon footprint—the total amount of greenhouse gases produced through our activities—requires both individual and societal changes. Simple steps include better home insulation, using public transport instead of cars, switching to renewable energy sources, and reducing electricity consumption. Eating less beef and dairy also helps reduce methane emissions.
Take action Calculate your personal carbon footprint online to see which of your daily activities contribute most to greenhouse gas emissions. You might be surprised by what makes the biggest impact!
When we burn fossil fuels like coal and hydrocarbons, they release several harmful pollutants. The carbon and hydrogen in these fuels react with oxygen in the air to produce various compounds, some quite dangerous to human health.
Carbon monoxide is a toxic gas produced by incomplete combustion. It has no colour or smell, making it particularly dangerous—which is why carbon monoxide detectors in homes are so important. When coal containing sulfur is burned, it produces sulfur dioxide, which causes breathing problems and contributes to acid rain.
Vehicles create another set of pollutants. The high temperatures in car engines cause nitrogen and oxygen in the air to react, forming various nitrogen oxides (NOx). These, along with sulfur dioxide, dissolve in rainwater to create acid rain that damages trees and buildings. Cars also release particulates—tiny unburned hydrocarbon particles that increase the risk of heart and lung disease.
Health alert Air pollution isn't just an environmental issue—it's a major health concern. The World Health Organization estimates that air pollution contributes to millions of premature deaths annually, primarily from respiratory and cardiovascular diseases.
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
6
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Quotes from every main character
English LiteratureApp Store
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The Earth's atmosphere is the gaseous layer surrounding our planet that enables all life. Its composition has changed dramatically over billions of years, from a carbon dioxide-rich environment to the nitrogen-oxygen rich air we breathe today. Understanding these changes helps... Show more
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Our atmosphere today consists mostly of nitrogen (78%) and oxygen (21%), with small amounts of other gases including carbon dioxide, water vapour and noble gases. This composition has remained relatively stable for about 200 million years, but it wasn't always this way.
During Earth's first billion years, intense volcanic activity released gases that formed the early atmosphere, primarily water vapour and carbon dioxide. As the planet cooled, water vapour condensed to form oceans, but carbon dioxide remained dominant in the atmosphere—similar to what we see today on Mars and Venus.
Carbon dioxide levels decreased over time through several natural processes. The gas dissolved in oceans, creating a weak acid that reacted with minerals to form precipitates. These eventually became sediments of carbonate rock on the sea bed. Marine organisms like corals and mussels also used carbon dioxide to create their shells, which later formed limestone when they died.
Did you know? The limestone formations we see today represent carbon dioxide that was once in the atmosphere, showing how Earth naturally sequestered carbon over millions of years!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
About 1.7 billion years ago, a major atmospheric shift began when photosynthetic algae evolved in the oceans. These simple organisms produced oxygen as a byproduct of photosynthesis. As plants later evolved on land, oxygen levels continued to increase until they reached a point where animal life could develop.
While oxygen levels rose, carbon dioxide was being trapped in what we now call fossil fuels. Coal formed when plant material in marshy wetlands didn't fully decompose due to lack of oxygen or acidic conditions. Under high pressure and temperature over millions of years, this plant matter transformed into coal.
Crude oil and natural gas formed similarly but from marine organisms. When plankton died and settled in mud on the sea floor, their remains were preserved without decomposition if oxygen was absent. Gradually, heat and pressure transformed these remains—plankton became crude oil, while natural gas (primarily methane) often formed nearby.
Important: The greenhouse effect is actually natural and necessary! Without it, Earth would be too cold for life. Gases like carbon dioxide, water vapour and methane trap the sun's heat by allowing short-wavelength radiation in but absorbing the long-wavelength radiation that the Earth radiates back.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Human activities have significantly altered atmospheric composition, particularly through deforestation and fossil fuel use. Cutting down forests not only removes carbon-absorbing trees but also releases stored CO₂. Meanwhile, agriculture—especially cattle farming—produces large amounts of methane, another powerful greenhouse gas.
The impacts of these changes are already visible: melting polar ice, rising sea levels, more severe weather patterns, and shifting distributions of plants, animals and diseases like malaria. While climate models are complex and sometimes lead to simplified predictions, the evidence for human-caused climate change is overwhelming.
Reducing our carbon footprint—the total amount of greenhouse gases produced through our activities—requires both individual and societal changes. Simple steps include better home insulation, using public transport instead of cars, switching to renewable energy sources, and reducing electricity consumption. Eating less beef and dairy also helps reduce methane emissions.
Take action: Calculate your personal carbon footprint online to see which of your daily activities contribute most to greenhouse gas emissions. You might be surprised by what makes the biggest impact!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
When we burn fossil fuels like coal and hydrocarbons, they release several harmful pollutants. The carbon and hydrogen in these fuels react with oxygen in the air to produce various compounds, some quite dangerous to human health.
Carbon monoxide is a toxic gas produced by incomplete combustion. It has no colour or smell, making it particularly dangerous—which is why carbon monoxide detectors in homes are so important. When coal containing sulfur is burned, it produces sulfur dioxide, which causes breathing problems and contributes to acid rain.
Vehicles create another set of pollutants. The high temperatures in car engines cause nitrogen and oxygen in the air to react, forming various nitrogen oxides (NOx). These, along with sulfur dioxide, dissolve in rainwater to create acid rain that damages trees and buildings. Cars also release particulates—tiny unburned hydrocarbon particles that increase the risk of heart and lung disease.
Health alert: Air pollution isn't just an environmental issue—it's a major health concern. The World Health Organization estimates that air pollution contributes to millions of premature deaths annually, primarily from respiratory and cardiovascular diseases.
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
6
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Quotes from every main character
English LiteratureApp Store
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Sociology