AQA GCSE Chemistry 8462: Chemistry of the atmosphere – Study Guide

This study guide teaches Topic 9, Chemistry of the atmosphere (sections 4.9.1 to 4.9.3), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with GCSE exams from June 2018 (version 1.1). The topic is assessed on Paper 2, together with Topics 6 to 10, and Paper 2 can also draw on ideas from sections 4.1 to 4.3. Every statement in this topic is for both Foundation and Higher tier: none of it is marked “HT only”.

Use it with the Chemistry of the atmosphere revision notes and the Chemistry of the atmosphere practice questions. The course hub is AQA GCSE Chemistry and the printable checklist lists every statement. The next topic, Using resources, picks up sustainability, water and materials.

What this topic covers

Spec	What you must be able to do	Tier
4.9.1.1	State the proportions of gases in today’s atmosphere	Both
4.9.1.2	Describe one theory of the early atmosphere; interpret evidence and evaluate theories given information	Both
4.9.1.3–4.9.1.4	Explain how oxygen increased and carbon dioxide decreased; describe how limestone, coal, crude oil and natural gas formed	Both
4.9.2.1	Name the greenhouse gases; describe the greenhouse effect using short and long wavelength radiation	Both
4.9.2.2	Recall two human activities that increase carbon dioxide and two that increase methane; evaluate evidence about climate change	Both
4.9.2.3	Describe four potential effects of global climate change; discuss scale, risk and implications	Both
4.9.2.4	Define carbon footprint; describe ways to reduce it and why they may be limited	Both
4.9.3.1–4.9.3.2	Describe how pollutants form when fuels burn; predict combustion products; explain the problems each pollutant causes	Both

4.9.1 The atmosphere today and in the past

Today’s atmosphere (4.9.1.1)

For about 200 million years the proportions of gases in the atmosphere have been much the same as today:

• about four-fifths (about 80%) nitrogen
• about one-fifth (about 20%) oxygen
• small proportions of other gases, including carbon dioxide, water vapour and noble gases.

The specification links this to the maths skill of using ratios, fractions and percentages.

Worked example 1. A student pushes 100 cm³ of air backwards and forwards over heated copper, which reacts with oxygen. When the gas has cooled, 79 cm³ remains. Calculate the percentage of oxygen in the air sample.

volume of oxygen removed = 100 − 79 = 21 cm³
percentage oxygen = 21 / 100 × 100 = 21%

The answer, 21%, is close to “about one-fifth”. The gas left over is mostly nitrogen.

The early atmosphere (4.9.1.2)

The Earth is about 4.6 billion years old, so evidence about its early atmosphere is limited. Theories have changed over time as new evidence appeared. You need to know one theory only:

1. During the first billion years there was intense volcanic activity. Volcanoes released gases that formed the early atmosphere, and water vapour that condensed to form the oceans.
2. At the start, the atmosphere may have been like those of Mars and Venus today: mainly carbon dioxide, with little or no oxygen.
3. Volcanoes also released nitrogen, which gradually built up. There may have been small amounts of methane and ammonia.
4. When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated, forming sediments. This reduced the carbon dioxide in the atmosphere.

In an exam you may be given information about a different theory. You are not expected to know it in advance. Your job is to use the information: say what evidence supports each theory, what is missing, and why certainty is hard over such a long time.

How oxygen increased (4.9.1.3)

Algae and plants produced the oxygen now in the atmosphere by photosynthesis:

carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Algae first produced oxygen about 2.7 billion years ago, and oxygen soon appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that allowed animals to evolve.

How carbon dioxide decreased (4.9.1.4)

Three processes removed carbon dioxide:

• photosynthesis by algae and plants
• dissolving in the oceans, then forming sedimentary rocks (carbonates)
• forming fossil fuels that contain carbon.

You must describe how these deposits formed:

Deposit	How it formed
Limestone	Shells and skeletons of marine organisms, made of calcium carbonate, settled on the sea bed and were compressed into sedimentary rock over millions of years
Coal	Plant material (trees and ferns) was buried before it could decay fully, then compressed and heated over millions of years
Crude oil and natural gas	Plankton and other tiny marine organisms were buried in mud on the sea bed, then heat and pressure over millions of years changed them, without much oxygen present

In each case, carbon that was once in atmospheric carbon dioxide ends up locked in rock.

4.9.2 Greenhouse gases and climate change

The greenhouse effect (4.9.2.1)

Water vapour, carbon dioxide and methane are greenhouse gases. They keep the Earth warm enough to support life. Describe the effect in terms of wavelength:

1. The Sun emits short wavelength radiation. It passes through the atmosphere.
2. The Earth’s surface absorbs it and warms up.
3. The warm surface emits long wavelength (infrared) radiation.
4. Greenhouse gases absorb some of this long wavelength radiation and re-emit it in all directions, including back towards the surface.
5. So less energy escapes to space, and the lower atmosphere stays warmer.

The key difference is that greenhouse gases let short wavelength radiation through but absorb long wavelength radiation.

Human activities (4.9.2.2)

Learn two for each gas.

Gas	Human activities that increase it
Carbon dioxide	Burning fossil fuels (power stations, vehicles); deforestation (fewer trees for photosynthesis, and trees are often burned)
Methane	Cattle and other livestock farming (digestion); rice paddy fields; decay of waste in landfill sites

Based on peer-reviewed evidence, many scientists believe human activities will raise the temperature of the atmosphere at the surface, causing global climate change. The climate is a complex system, so models are simplified. Media reports may use only parts of the evidence and may be biased.

When you evaluate a report, ask:

• Is it based on peer-reviewed work? Peer review means other scientists check the methods and conclusions before publication.
• How large is the data set, and over how long?
• Who funded or wrote it? Could they be biased?
• Does correlation (temperature and carbon dioxide rising together) prove cause on its own?
• What uncertainties are there? Measurements from the distant past rely on indirect evidence, and models make assumptions.

Worked example 2. A monitoring station records carbon dioxide levels (illustrative values): 320 parts per million (ppm) at the start of a period and 400 ppm at the end. Calculate the percentage increase.

increase = 400 − 320 = 80 ppm
percentage increase = 80 / 320 × 100 = 25%

Always divide by the starting value. Dividing by 400 gives 20%, which is wrong.

Global climate change (4.9.2.3)

An increase in average global temperature is a major cause of climate change. Learn four potential effects:

• sea level rise (melting ice and water expanding), causing flooding and coastal erosion
• more frequent and severe storms and other extreme weather
• changes in the amount, timing and distribution of rainfall, causing droughts in some places and floods in others
• temperature and water stress for humans and wildlife
• changes in food-producing capacity of some regions
• changes to the distribution of wildlife species, some becoming extinct.

When asked to discuss scale and risk, think about how many people or places are affected, how likely each effect is, and whether the damage can be reversed.

The carbon footprint (4.9.2.4)

The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.

It can be reduced by cutting emissions of carbon dioxide and methane:

Action	Why it may be limited
More renewable energy (wind, solar) and nuclear power	Cost of building; public opposition; supply varies with weather
Energy conservation (insulation, efficient appliances)	Up-front cost; people’s habits are slow to change
Carbon capture and storage	Technology still developing; expensive
Carbon taxes and emission limits	Unpopular; countries may not agree, fearing harm to their economies
Carbon off-setting, including planting trees	Needs land; takes years to have an effect
Using plant-based fuels	Land used for fuel crops cannot grow food

Common reasons for limits: lack of scientific agreement in public debate, cost, lack of international cooperation, and people not wanting to change their lifestyle.

4.9.3 Atmospheric pollutants

Pollutants from burning fuels (4.9.3.1)

The combustion of fuels is a major source of pollutants. Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur. The gases released can include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles and unburned hydrocarbons form particulates.

How each pollutant forms:

Pollutant	How it forms
Carbon monoxide, CO	Incomplete combustion of carbon in the fuel when oxygen is limited
Soot (carbon particles)	Incomplete combustion with even less oxygen
Sulfur dioxide, SO₂	Sulfur impurities in the fuel react with oxygen
Oxides of nitrogen (NO, NO₂)	Nitrogen and oxygen from the air react at the high temperatures in engines and furnaces

The nitrogen in oxides of nitrogen comes from the air, not from the fuel. That point is often tested.

Worked example 3. Butane, C₄H₁₀, is burned. Predict the products and write balanced equations for (a) plenty of oxygen and (b) limited oxygen, forming carbon monoxide.

(a) 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
(b) 2C₄H₁₀ + 9O₂  → 8CO  + 10H₂O

Method: balance carbon first (8 on each side), then hydrogen (20 H, so 10H₂O), then count oxygen atoms on the right and halve for O₂. In (a) the right has 16 + 10 = 26 oxygen atoms, so 13O₂. In (b) it has 8 + 10 = 18, so 9O₂.

To predict products for any fuel, list its elements. Carbon gives CO₂ (or CO or C if oxygen is limited). Hydrogen gives H₂O. Sulfur gives SO₂. At high temperature, oxides of nitrogen also form from air.

Effects of the pollutants (4.9.3.2)

Pollutant	Problem
Carbon monoxide	Toxic. It is colourless and odourless, so it is not easily detected. It reduces the ability of blood to carry oxygen
Sulfur dioxide	Respiratory problems; dissolves in rain to form acid rain
Oxides of nitrogen	Respiratory problems; acid rain
Particulates	Global dimming (less sunlight reaches the ground); health problems, as fine particles are breathed deep into the lungs

Acid rain damages buildings and statues made of limestone, harms trees and plants, and lowers the pH of lakes, which can kill aquatic life.

Common errors

• Saying the early atmosphere was mainly nitrogen. It was mainly carbon dioxide; nitrogen built up gradually.
• Writing that oxygen came from volcanoes. It came from photosynthesis by algae and plants.
• Confusing the greenhouse effect with the ozone layer or with acid rain. They are separate.
• Saying greenhouse gases “trap heat” with no mention of long wavelength radiation being absorbed. Use the wavelength language.
• Giving “pollution” as a human activity that increases methane. Name a real source: livestock, rice paddies, landfill.
• Saying carbon monoxide causes acid rain. It does not; sulfur dioxide and oxides of nitrogen do.
• Saying nitrogen oxides come from nitrogen in the fuel. At GCSE, the nitrogen comes from the air.
• Unbalanced combustion equations. Check each element before moving on.

Next steps

Go to the revision notes for a condensed version and a quick self-test, then try the practice questions with full mark schemes. To find gaps across the whole course, take one of the free 10-minute diagnostics.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.9 Chemistry of the atmosphere.