Revision notes for OCR GCSE Chemistry Interpreting and interacting with earth systems. Open the guide for explanations and worked examples. Written against the OCR GCSE Chemistry (J248) specification, so the content matches what's examinable rather than general Chemistry background.
Interpreting and interacting with earth systems
What you'll learn
- How scientists interpret evidence for the early atmosphere.
- How oxygen built up and how greenhouse gases affect climate.
- How human activity can change the atmosphere, including climate change and air pollution.
- How potable water is produced from ground water, waste water and salt water.
Starting point: Earth systems
The atmosphere is the layer of gases surrounding Earth. It is not a huge, endless store of air: compared with the size of Earth, it is a very thin layer. That is why small percentage changes in gases such as carbon dioxide can still have important effects.
An Earth system is a part of Earth, such as the atmosphere, oceans, rocks, living things or ice, that interacts with the others. For example, carbon dioxide can move between the atmosphere, oceans, living organisms and rocks.
Definition
Climate
Climate means the long-term pattern of weather in a region or across Earth. It is different from weather, which is what happens over a short time, such as today’s temperature or rainfall.
The atmosphere today
Dry air is roughly:
- 78% nitrogen, N2
- 21% oxygen, O2
- about 0.04% carbon dioxide, CO2
- small amounts of other gases, including argon and water vapour
The exact amount of water vapour varies from place to place and day to day.
How the early atmosphere formed
Scientists cannot directly sample Earth’s first atmosphere, so they use evidence: rocks, volcanic gases, fossils, ice cores and comparisons with other planets.
It is thought that Earth’s early atmosphere was formed mainly by volcanic activity. Volcanoes released gases such as carbon dioxide, water vapour and nitrogen, with small amounts of methane and ammonia. There was little or no oxygen.
As Earth cooled, water vapour condensed to form liquid water. Rain collected in basins and formed oceans. Carbon dioxide dissolved in the oceans and became locked up in carbonate sediments, limestone and fossil fuels.
Definition
Sediment
A sediment is solid material that settles at the bottom of water. Over long times, sediments can become sedimentary rocks such as limestone.
Example
Interpreting evidence for the early atmosphere
Scientists find very old rocks containing minerals that only form when there is very little oxygen. They also find later carbonate rocks formed from dissolved carbon dioxide.
- The low-oxygen minerals suggest that the early atmosphere had little or no oxygen.
- The carbonate rocks suggest that carbon dioxide was removed from the atmosphere and dissolved in oceans.
- Combining both pieces of evidence supports the model that early Earth had high carbon dioxide and low oxygen compared with today.
How an oxygen-rich atmosphere developed
The key process was photosynthesis. This is the process where plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water, releasing oxygen.
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
Early photosynthetic algae and bacteria released oxygen into the oceans and atmosphere. At first, much of this oxygen reacted with dissolved substances such as iron compounds. Later, oxygen began to build up in the atmosphere.
At the same time, photosynthesis removed carbon dioxide. Some carbon from living things became trapped in fossil fuels, and some carbon dioxide became locked into carbonate rocks.
Key Idea
Gas changes over time
Over geological time, carbon dioxide decreased and oxygen increased because of cooling oceans, carbonate formation, fossil fuel formation and photosynthesis.
The greenhouse effect
The greenhouse effect is the warming of Earth’s surface and lower atmosphere because some gases absorb and re-emit infrared radiation.
The Sun emits mostly short-wave radiation, which passes through the atmosphere and warms Earth’s surface. The warm surface emits longer-wave infrared radiation. Greenhouse gases, including carbon dioxide, methane and water vapour, absorb some of this infrared radiation and re-emit it in all directions, including back towards Earth.
The natural greenhouse effect is essential: without it, Earth would be much colder. The problem is the enhanced greenhouse effect, where extra greenhouse gases cause extra warming.
Common Mistake
Not the ozone hole
Global warming is not caused by the ozone hole. The ozone layer issue involves ultraviolet radiation, while the greenhouse effect involves infrared radiation absorbed by greenhouse gases.
Example
Tracing radiation through the greenhouse effect
A student says: “More carbon dioxide can warm Earth because it traps heat.” Improve this explanation using radiation.
- Incoming short-wave radiation from the Sun passes through the atmosphere and warms Earth’s surface.
- The warm surface emits long-wave infrared radiation.
- Carbon dioxide molecules absorb some of this infrared radiation and re-emit it in all directions.
- If carbon dioxide concentration increases, a greater fraction of infrared radiation is returned towards Earth, increasing warming of the lower atmosphere.
Evidence for human-caused climate change
Anthropogenic means caused by human activity. Anthropogenic climate change is mainly linked to increased greenhouse gases from burning fossil fuels, agriculture, landfill, deforestation and some industrial processes.
A fossil fuel is a fuel formed from ancient living organisms over millions of years, such as coal, crude oil or natural gas. Burning fossil fuels releases carbon dioxide:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Scientists compare data from different sources, including direct atmospheric measurements, ice cores, fuel consumption records and global temperature records. A key piece of evidence is the correlation between fossil fuel use and atmospheric carbon dioxide concentration.
Definition
Correlation
A correlation is a relationship between two variables. If they both increase together, that is a positive correlation. Correlation alone does not prove causation, but it can support a causal explanation when there is a scientific mechanism too.
Example
Interpreting climate data
A graph shows that fossil-fuel carbon dioxide emissions rose from 9 billion tonnes per year in 1960 to 35 billion tonnes per year in 2020. Atmospheric carbon dioxide rose from 317 ppm to 415 ppm over the same period. ppm means parts per million.
- Compare the emissions: 359≈3.9\frac{35}{9} \approx 3.9935≈3.9, so emissions became about four times larger.
- Compare carbon dioxide concentration: 415 ppm − 317 ppm = 98 ppm increase.
- Both quantities increased over the same period, so the graph shows a positive correlation.
- The correlation supports the idea that fossil fuel burning is increasing atmospheric carbon dioxide, especially because combustion produces carbon dioxide.
Tip
Orders of magnitude
An order of magnitude is about a factor of 10. A change from 1 to 100 is two orders of magnitude because 100 = 10210^2102.
There are uncertainties in climate evidence. Older measurements are less complete than modern ones, natural factors also affect climate, and climate models must estimate complex interactions involving oceans, clouds and ice. However, uncertainty does not mean “no evidence”; it means scientists give a range of likely outcomes.
Effects of increased carbon dioxide and methane
Carbon dioxide and methane are both greenhouse gases. Methane, CH4, is released from sources such as farming, landfill and natural gas leaks. Increased levels can lead to:
- higher global average temperatures
- melting ice and rising sea levels
- more extreme weather events in some regions
- changes to rainfall patterns and food production
- habitat loss and increased extinction risk
- ocean acidification when carbon dioxide dissolves in seawater
Mitigation means reducing the seriousness of a problem. Climate change may be mitigated by using renewable energy, improving energy efficiency, reducing deforestation, planting trees, reducing methane emissions, using public transport, and capturing and storing carbon dioxide.
Example
Evaluating a mitigation method
A country is deciding whether to replace coal power stations with wind farms.
- Coal power stations burn fossil fuels, so replacing them reduces carbon dioxide emissions at the source.
- Wind farms generate electricity without combustion, so they produce much less carbon dioxide during use.
- The decision still needs scale and risk considered: wind farms need suitable locations, materials, land or sea space, and backup or storage when wind speeds are low.
- Overall, the method is strong mitigation if it replaces a large amount of fossil-fuel electricity.
Other atmospheric pollutants
A pollutant is a substance released into the environment that can cause harm. Some pollutants are greenhouse gases, but many air pollutants mainly harm health or cause local environmental problems.
| Pollutant | Major sources | Problems caused |
|---|---|---|
| Carbon monoxide, CO | Incomplete combustion in car engines, gas boilers and fires | Poisonous; reduces the blood’s ability to carry oxygen |
| Sulfur dioxide, SO2 | Burning fossil fuels containing sulfur impurities | Acid rain; breathing problems |
| Oxides of nitrogen, NOx | High-temperature car engines and power stations | Acid rain; photochemical smog; breathing problems |
| Particulates | Soot, smoke, diesel engines, industry and dust | Lung and heart disease; global dimming |
Incomplete combustion can produce carbon monoxide:
2C(s) + O2(g) → 2CO(g)
Sulfur impurities form sulfur dioxide when burned:
S(s) + O2(g) → SO2(g)
At high temperatures, nitrogen and oxygen in air can react:
N2(g) + O2(g) → 2NO(g)
Common Mistake
Carbon dioxide vs carbon monoxide
Carbon dioxide, CO2, is a greenhouse gas. Carbon monoxide, CO, is a poisonous gas made by incomplete combustion. Do not mix up their formulas or effects.
Increasing the availability of potable water
Potable water is water that is safe to drink. It does not have to be pure water; it just needs low enough levels of dissolved substances and microorganisms.
Different water sources need different treatment because they contain different impurities.
Ground water
Ground water often contains fewer harmful substances than waste water. It may be treated by:
- filtration to remove insoluble solids
- sterilisation using chlorine, ozone or ultraviolet light to kill microorganisms
Waste water
Waste water needs more treatment. It may involve screening, sedimentation, biological treatment, filtration and sterilisation.
Biological treatment uses microorganisms to break down organic matter. Sedimentation allows suspended solids to settle.
Salt water
Salt water contains dissolved salts, so simple filtration is not enough. It must be desalinated, meaning salts are removed.
Two main desalination methods are:
- distillation, where water evaporates and then condenses, leaving salts behind
- reverse osmosis, where pressure forces water through a partially permeable membrane that prevents many dissolved ions from passing through
Example
Choosing a water treatment route
An island has plenty of seawater but very little fresh water. Decide which treatment is needed.
- Seawater contains dissolved salts, so filtration alone will not make it potable.
- A desalination method is needed: either distillation or reverse osmosis.
- Reverse osmosis may use less energy than distillation, but it needs high pressure and suitable membranes.
- The treated water may still need sterilisation before drinking if microorganisms could be present.
Exam technique
In the exam
- Use the word correlation carefully: say what increases or decreases together, then explain whether the evidence supports a cause.
- For greenhouse effect questions, mention infrared radiation being absorbed and re-emitted by greenhouse gases.
- For water treatment questions, match the method to the impurity: filtration removes insoluble solids, sterilisation kills microorganisms, and desalination removes dissolved salts.
Self review
Check yourself
- Why did carbon dioxide decrease as Earth’s atmosphere changed?
- How does the greenhouse effect involve both short-wave and infrared radiation?
- Which treatment methods would be needed to make seawater potable?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
Global challenges
Guide 3 of 3
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?