Earth & Atmospheric ScienceEdexcel GCSE Chemistry: Revision notes
Section 1
How did the Earth's early atmosphere form?
The Earth's early atmosphere came almost entirely from volcanic activity. Volcanoes released large amounts of gas, and scientists think this early atmosphere contained:
- Little or no oxygen
- A large amount of carbon dioxide
- Water vapour
- Small amounts of other gases (e.g. methane, ammonia)
Much of the evidence for this comes from studying present-day volcanic activity, though scientific opinion on the exact make-up of the early atmosphere is still uncertain because there is limited direct evidence from four billion years ago.
If asked to 'interpret evidence' about the early atmosphere, say explicitly that our knowledge is based on inference from volcanic gases today, not direct measurement.
Section 2
How did the oceans form and why did carbon dioxide levels fall?
As the Earth cooled, water vapour condensed to form the oceans.
Once oceans existed, carbon dioxide gas dissolved into the ocean water. Some of this dissolved carbon dioxide was later used by sea organisms to form carbonate sediments and, eventually, sedimentary rocks (e.g. limestone) and fossil fuels, locking the carbon away.
This is why atmospheric carbon dioxide fell steadily over billions of years even though volcanoes kept releasing more.
Section 3
How did oxygen build up in the atmosphere?
Around 2.7 billion years ago, primitive plants and algae evolved in the oceans. These organisms carried out photosynthesis:
carbon dioxide + water → glucose + oxygen (using light energy)
Photosynthesis removed carbon dioxide from the atmosphere and released oxygen. Over a very long period, oxygen built up, eventually allowing animal life to evolve.
Chemical test for oxygen: insert a glowing splint into the gas — if oxygen is present, the splint relights.
A glowing splint relighting is the standard positive test result you should quote for oxygen in any practical or written question.
Section 4
What is today's atmosphere made of, and what is the greenhouse effect?
Today's atmosphere is roughly:
| Gas | Approximate % |
|---|---|
| Nitrogen | 80% |
| Oxygen | 20% |
| Carbon dioxide, argon and other gases | small amounts (less than 1%) |
Greenhouse gases — including carbon dioxide, methane and water vapour — absorb heat radiation emitted by the Earth's surface, then release that energy in all directions, some of it back towards the Earth. This keeps the planet warmer than it would otherwise be: the greenhouse effect.
Do not describe the greenhouse effect as gases 'trapping' heat like a physical lid — the correct mark-scheme wording is that gases absorb radiated heat energy and re-release/re-radiate it, warming the Earth.
Section 5
What is the evidence for human activity causing climate change?
Scientists have measured a correlation between:
- rising atmospheric carbon dioxide concentration
- increasing consumption of fossil fuels
- rising global average temperature
Human activities that increase greenhouse gases include burning fossil fuels (releases CO2) and livestock farming (releases methane).
However, there are uncertainties: measurements are taken at different locations (which may not represent the whole atmosphere), and historical temperature/CO2 data (e.g. from ice cores) has some inaccuracy. Correlation alone does not prove causation, but the pattern is strong and consistent with known chemistry.
Possible ways to mitigate climate change must be considered in terms of scale, risk and environmental implications.
For 'evaluate' questions, always mention BOTH the supporting correlation AND at least one source of uncertainty (location bias, historical accuracy) to reach full marks.
Must Know
- The early atmosphere came from volcanic activity: mostly CO2 and water vapour, little/no oxygen
- Condensation of water vapour formed the oceans; oceans then dissolved CO2, lowering its atmospheric level
- Photosynthesis by early plants/algae removed CO2 and released O2 over billions of years
- Chemical test for oxygen: a glowing splint relights
- Today's atmosphere is about 80% nitrogen, 20% oxygen, with small amounts of CO2 and other gases
- Greenhouse gases (CO2, methane, water vapour) absorb and re-release radiated heat, causing the greenhouse effect; there is a correlation between fossil fuel use, CO2 levels and temperature, but measurement uncertainties remain
That's the notes covered.
Carry on to the next subtopic.