AQA GCSE Chemistry: Organic Chemistry, The Atmosphere, and Using Resources Revision Guide

Topics 7, 9, and 10 on the AQA GCSE Chemistry specification -- Organic Chemistry, Chemistry of the Atmosphere, and Using Resources -- are sometimes treated as the "lesser" topics. They appear on Paper 2, they carry fewer marks individually than Bonding or Chemical Changes, and students often leave them until the final weeks of revision. That is a mistake. Together, these three topics account for a substantial portion of Paper 2, and they are among the most accessible on the entire specification. The content is concrete, the mark schemes are predictable, and a well-prepared student can pick up marks here that are harder to earn elsewhere.

This guide covers all three topics, following the AQA specification closely, highlighting what examiners test most frequently and the common errors that cost students marks.

Organic Chemistry

Organic chemistry is the study of carbon compounds. Carbon is unique because its atoms can form four covalent bonds and can bond to other carbon atoms in long chains and rings, producing an enormous variety of molecules. On the AQA GCSE specification, this topic covers hydrocarbons, homologous series, fractional distillation, cracking, and polymers.

Crude Oil and Hydrocarbons

Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms. It is a mixture of hydrocarbons -- compounds containing only hydrogen and carbon atoms -- that can be separated into useful fractions by fractional distillation.

The alkanes are a homologous series of saturated hydrocarbons (containing only single covalent bonds) with the general formula CnH2n+2. You need to know the first four members:

• Methane -- CH4
• Ethane -- C2H6
• Propane -- C3H8
• Butane -- C4H10

As the carbon chain length increases, the boiling point increases and the viscosity increases, while the flammability decreases. These trends are central to understanding why fractional distillation works and how different fractions are used.

Fractional Distillation

Fractional distillation separates crude oil into fractions based on their boiling points. The crude oil is heated until most of it vaporises, and the vapour rises up a fractionating column that is hotter at the bottom and cooler at the top. Hydrocarbons with high boiling points condense near the bottom as heavy fractions (such as bitumen), while those with low boiling points rise further before condensing as lighter fractions (such as petrol and liquefied petroleum gas).

Each fraction contains molecules with a similar number of carbon atoms. Shorter-chain fractions are more useful as fuels because they ignite more easily and burn more cleanly, while longer-chain fractions are less useful in their original form -- which is why cracking is so important.

Combustion of Hydrocarbons

Complete combustion of hydrocarbons occurs when there is a plentiful supply of oxygen:

hydrocarbon + oxygen --> carbon dioxide + water

Incomplete combustion occurs when the oxygen supply is limited, producing carbon monoxide (a toxic, colourless gas), carbon (soot), and water. It releases less energy than complete combustion.

Cracking

Supply and demand for different fractions do not match. There is greater demand for shorter-chain hydrocarbons than for longer-chain ones. Cracking breaks down long-chain hydrocarbons into shorter, more useful molecules. It requires a high temperature and either a catalyst (catalytic cracking) or steam (steam cracking). The products include alkanes (used as fuels) and alkenes (used to make polymers and other chemicals).

Alkenes are unsaturated hydrocarbons -- they contain at least one carbon-carbon double bond. The simplest alkene is ethene, C2H4, and the general formula is CnH2n. You can distinguish alkenes from alkanes using bromine water: alkenes decolourise it (turning it from orange to colourless), while alkanes have no effect.

Addition Polymers

In addition polymerisation, many small alkene molecules (monomers) join together to form a long chain molecule (a polymer) with no other substance produced. The double bond in each monomer opens up to allow the monomers to link together. For example, ethene polymerises to form poly(ethene) and propene forms poly(propene). You need to be able to draw the repeating unit given the monomer, and identify the monomer given the repeating unit.

Addition polymers are very useful but present environmental challenges. Most are non-biodegradable, persisting in the environment for hundreds of years, and they are derived from crude oil, a finite resource. Incineration releases carbon dioxide and potentially toxic gases. Recycling and developing biodegradable alternatives are important areas of ongoing research.

Chemistry of the Atmosphere

This topic traces the history of the Earth's atmosphere from its earliest formation to the present day, and then examines how human activity is changing its composition with potentially serious consequences.

The Early Atmosphere

During the first billion years of the Earth's existence, volcanic activity released large quantities of gases. Scientists believe the early atmosphere was predominantly carbon dioxide, with little or no oxygen, along with water vapour and small amounts of methane, ammonia, and nitrogen. This composition was broadly similar to the atmospheres of Mars and Venus today. We cannot be certain about the exact composition because there is limited direct evidence -- the AQA specification notes that theories about the early atmosphere may change as new evidence is found.

As the Earth cooled, water vapour condensed to form the oceans. Carbon dioxide dissolved in the oceans and eventually became locked up in sedimentary rocks (such as limestone) and fossil fuels. Marine organisms used dissolved carbon dioxide to make calcium carbonate for their shells, and when these organisms died, their remains formed sedimentary rocks -- removing carbon dioxide from the cycle for millions of years.

How Oxygen Increased

The development of photosynthetic organisms -- first algae, then plants -- was the key event that transformed the atmosphere:

carbon dioxide + water --> glucose + oxygen

Over hundreds of millions of years, photosynthesis increased the proportion of oxygen while decreasing carbon dioxide. Rising oxygen levels allowed more complex life to evolve and enabled the formation of the ozone layer, which protects the surface from ultraviolet radiation.

The Atmosphere Today

The atmosphere has been broadly stable for around 200 million years. Its current composition is approximately:

• Nitrogen -- about 78%
• Oxygen -- about 21%
• Argon -- about 0.9%
• Carbon dioxide -- about 0.04%
• Water vapour -- variable amounts

There are also trace amounts of other gases, including the other noble gases. You must know these approximate percentages -- they are frequently tested.

Greenhouse Gases and Climate Change

Greenhouse gases -- including carbon dioxide, methane, and water vapour -- absorb heat radiation from the Earth's surface and re-radiate it in all directions. This natural greenhouse effect keeps the Earth warm enough for life. The problem arises when human activities increase greenhouse gas concentrations beyond natural levels:

• Burning fossil fuels, which releases carbon dioxide
• Deforestation, which reduces the removal of carbon dioxide by photosynthesis
• Agriculture, particularly rice paddies and livestock farming, which releases methane
• Landfill sites, where decomposing waste produces methane

The scientific consensus, supported by data from ice cores, temperature records, and atmospheric measurements, is that increasing greenhouse gas concentrations are causing global temperatures to rise. The peer-reviewed evidence overwhelmingly supports the link between human activity and recent climate change, though uncertainties remain about the precise scale and timing of future impacts.

Consequences of Climate Change

The potential consequences include rising sea levels (from thermal expansion and melting ice caps), more frequent extreme weather events, changes to rainfall patterns affecting agriculture, loss of biodiversity, and ocean acidification as more carbon dioxide dissolves in seawater.

In exam answers, always distinguish between the greenhouse effect (natural and necessary) and the enhanced greenhouse effect (additional warming caused by human activity). The examiners specifically test whether you understand this distinction.

Carbon Footprint

A 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. Reducing carbon footprints is an important strategy for addressing climate change.

Methods of reducing carbon footprints include using renewable energy, improving energy efficiency, reducing waste, and carbon capture technologies. However, not all countries agree on emission reduction targets, changing infrastructure is expensive, and some low-carbon technologies are not yet economically competitive. You may be asked to evaluate these trade-offs in the exam.

Atmospheric Pollutants from Fuels

The combustion of fossil fuels produces several pollutants beyond carbon dioxide:

• Carbon monoxide -- produced by incomplete combustion, toxic because it binds to haemoglobin and reduces the blood's oxygen-carrying capacity
• Soot (particulate carbon) -- causes respiratory problems and contributes to global dimming
• Sulfur dioxide -- produced from sulfur impurities in fuels, dissolves in rainwater to form acid rain
• Nitrogen oxides -- formed at high temperatures in vehicle engines, contribute to acid rain and respiratory problems

Catalytic converters in vehicle exhaust systems reduce emissions of carbon monoxide and nitrogen oxides by converting them into less harmful gases (carbon dioxide and nitrogen). You need to know how each pollutant is formed and the problems it causes.

Using Resources

The final topic on the AQA GCSE Chemistry specification deals with how humans use the Earth's resources and the importance of doing so sustainably.

Natural Resources and Sustainability

Humans use the Earth's natural resources to provide warmth, shelter, food, and transport. Chemistry plays an essential role in sustainable development -- meeting the needs of the present generation without compromising the ability of future generations to meet their own needs. Finite resources (fossil fuels, metal ores) are being used up and cannot be replaced. Renewable resources (such as timber from managed forests) can be replenished.

Potable Water

Potable water is water that is safe to drink -- not chemically pure, but with dissolved salts and microbe levels low enough for consumption. In the United Kingdom, it is produced from fresh water sources by passing water through filter beds (sedimentation and filtration) and then sterilising it with chlorine, ozone, or ultraviolet light. Where fresh water is scarce, desalination by distillation or reverse osmosis can be used, though both require large amounts of energy.

The required practical involves analysing and purifying a water sample. You need to describe how to test for pH and dissolved solids, and how to produce pure water from an impure sample by distillation.

Waste Water Treatment

Urban lifestyles and industrial processes produce large volumes of waste water that must be treated before being released into the environment. Sewage treatment involves several stages:

1. Screening and grit removal to take out large solids
2. Sedimentation, where heavier particles settle out as sludge
3. Biological treatment, where aerobic bacteria break down organic matter in the liquid effluent
4. The sludge is treated separately by anaerobic digestion, producing methane gas (biogas) that can be used as an energy source

The treated effluent is then safe to release into rivers or the sea.

Life Cycle Assessments

A life cycle assessment (LCA) examines the environmental impact of a product at every stage: extracting raw materials, manufacturing, use during its lifetime, and disposal. LCAs allow more informed choices -- for example, comparing a paper bag versus a plastic bag. However, some impacts are difficult to quantify, and the relative importance assigned to different types of impact involves subjective value judgements. Be prepared to interpret LCA data and discuss its limitations in the exam.

Reducing the Use of Resources

The three main strategies are reduce (use fewer resources), reuse (use products again without reprocessing), and recycle (recover raw materials for new products). Metals are particularly suitable for recycling because they can be melted down and reformed using much less energy than extraction from ores. Glass, paper, and some plastics can also be recycled, though the economics depend on collection, sorting, and reprocessing costs compared to virgin materials.

Corrosion and Its Prevention

Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting of iron requires both oxygen and water -- if either is absent, iron will not rust. Prevention methods include barrier methods (painting, oiling, electroplating), galvanising (coating with zinc), and sacrificial protection (attaching a more reactive metal such as zinc or magnesium that corrodes preferentially). Sacrificial protection works because the more reactive metal oxidises in preference to iron, even if the coating is scratched.

Alloys, Ceramics, Polymers, and Composites

The specification also covers alloys, ceramics, polymers, and composites. Alloys are harder than pure metals because differently sized atoms disrupt the regular lattice. Common examples include steel, bronze, and brass. Ceramics are hard, brittle, and heat-resistant. Polymers divide into thermosoftening (soften on heating, can be remoulded) and thermosetting (strong cross-links, do not soften). Composites such as concrete, fibreglass, and carbon fibre reinforced plastic combine materials for improved properties.

The Haber Process

The Haber process manufactures ammonia from nitrogen and hydrogen:

N2 + 3H2 (reversible) 2NH3

The conditions are approximately 450 degrees Celsius, 200 atmospheres pressure, and an iron catalyst. These represent a compromise: a lower temperature would increase yield (the forward reaction is exothermic) but slow the rate; a higher pressure would increase yield but is expensive and poses safety risks. The iron catalyst speeds up the rate at which equilibrium is reached but does not change the yield.

Ammonia is used mainly to manufacture fertilisers and to produce nitric acid.

NPK Fertilisers

Fertilisers provide plants with the essential elements nitrogen (N), phosphorus (P), and potassium (K). Compounds used in NPK fertilisers include ammonium nitrate, potassium chloride, and ammonium phosphate. In the laboratory, fertilisers can be produced by neutralisation reactions -- for example, reacting ammonia solution with nitric acid produces ammonium nitrate:

ammonia + nitric acid --> ammonium nitrate

NH3 + HNO3 --> NH4NO3

Exam Tips for These Topics

Six-mark extended response questions. Organic Chemistry and Using Resources are popular choices for longer questions on Paper 2. Practise writing structured answers with correct scientific terminology. A common six-mark question might ask you to evaluate the environmental impact of a particular material or explain fractional distillation.

"Evaluate" and "discuss" questions. The Atmosphere and Using Resources topics frequently ask you to weigh up advantages and disadvantages. Always give both sides before stating your overall judgement.

Graph and data interpretation. Climate change questions often present graphical data. Read axes carefully, quote specific data, and distinguish between correlation and causation.

Required practical skills. The water purification practical is regularly tested. Be able to describe the method clearly, explain why each step is necessary, and discuss how to ensure reliable results.

General formula and polymer questions. Practise writing molecular formulae from names, drawing displayed formulae, and converting between monomer structures and polymer repeating units.

Prepare with LearningBro

LearningBro offers targeted courses for each of these topics, built around the AQA specification with practice questions that match real exam style and difficulty.

• AQA GCSE Chemistry: Organic Chemistry -- covers hydrocarbons, fractional distillation, cracking, alkanes, alkenes, combustion, and addition polymerisation
• AQA GCSE Chemistry: The Atmosphere -- takes you through the evolution of the atmosphere, greenhouse gases, climate change, carbon footprints, and atmospheric pollutants
• AQA GCSE Chemistry: Using Resources -- covers potable water, waste water treatment, life cycle assessments, corrosion, alloys, ceramics, polymers, composites, the Haber process, and NPK fertilisers

Work through these courses alongside past papers and mark schemes, and you will be well prepared for every aspect of these topics on exam day.

Good luck with your revision.