OCR GCSE Chemistry: Global Challenges Guide (C6)
OCR GCSE Chemistry: Global Challenges Guide (C6)
Topic C6 — Global challenges is the final topic of the OCR Gateway Science A GCSE Chemistry specification (J248), and it is where the chemistry you have learned meets the planet. It covers how we extract and recycle metals sustainably, how we judge the environmental cost of a product, how crude oil is refined into the fuels and plastics modern life depends on, how the Earth's atmosphere evolved over billions of years, why human activity is changing the climate, which pollutants come out of our chimneys and exhausts, and how we make water safe to drink. Because so much of C6 connects to the news and to everyday life, students often find it the most engaging topic on the course — and a topic where clear, well-organised, balanced answers score very reliably.
This guide covers every major idea in C6 at GCSE depth: sustainable metal extraction and recycling, life cycle assessment, crude oil and hydrocarbons, fractional distillation, cracking and alkenes, the evolution of the atmosphere, greenhouse gases and climate change, atmospheric pollutants, and potable and waste water. Higher-tier material is flagged with [H] throughout. For structured practice alongside this guide, work through the LearningBro OCR GCSE Chemistry: Global Challenges course, which covers every section below with exam-style questions in the OCR format.
How C6 Fits the J248 Specification
OCR Gateway Science A GCSE Chemistry (J248) is assessed by two papers. Paper 1 covers topics C1–C3, and Paper 2 covers topics C4–C6, so C6 sits firmly on Paper 2. Each paper is worth 90 marks, lasts 1 hour 45 minutes, and counts for 50% of the qualification. The same content is examined on Foundation and Higher tiers, with Higher reaching further into the detail of sustainable extraction methods such as phytomining and bioleaching; the [H] flags below help you target your revision. The periodic table is provided in the exam.
C6 questions lean heavily on the OCR command words. "Describe" asks what happens; "Explain" asks why or by what mechanism; "Suggest" invites a sensible answer applied to an unfamiliar context; and "Evaluate" asks you to weigh advantages against disadvantages and reach a conclusion. The "explain" and "evaluate" questions — on climate change, life cycle assessment and sustainability — carry the most marks in this topic, so they deserve the most practice. For climate change in particular, you are expected to present the mainstream peer-reviewed scientific consensus accurately, without inventing a false sense of balance between it and fringe views.
Sustainable Metal Extraction
Most metals are found in the Earth as ores — rocks containing enough of a metal compound to make extraction worthwhile. Ores are a finite resource: they are being used up far faster than they form, so extracting metals sustainably is a genuine global challenge. Traditional extraction (reduction with carbon, or electrolysis) uses large amounts of energy and disturbs the landscape, and the richest ores are running out.
Two newer, lower-impact methods — examined at Higher tier [H] — extract metals from low-grade ores that would once have been waste:
- Phytomining uses plants to absorb metal compounds from the soil as they grow. The plants are harvested and burned, and the metal is extracted from the ash, which contains a relatively high concentration of the metal compound.
- Bioleaching uses bacteria to break down low-grade ores, producing a solution (a leachate) containing the metal compound, from which the metal can be extracted.
These methods have a much lower environmental impact than digging, moving and smelting huge quantities of rock, and they make use of ores too poor for traditional methods — though they are generally slower. The metal obtained, for example copper, is then extracted from the solution or ash by displacement with a more reactive metal such as scrap iron, or by electrolysis.
Recycling Metals
Because ores are finite and extraction is energy-intensive, recycling metals is one of the most important sustainability measures in chemistry. Recycling a metal such as aluminium uses far less energy than extracting it from its ore, conserves the finite ore, reduces the rock that must be quarried, and cuts the waste sent to landfill. The trade-off is the cost and energy of collecting, sorting and melting the scrap — but for most metals recycling is decisively better for the environment than extracting more. An "evaluate" question on recycling expects you to weigh these benefits against the collection and sorting costs and conclude.
Life Cycle Assessment
A life cycle assessment (LCA) is a way of judging the total environmental impact of a product across its whole life — "cradle to grave". It considers four main stages, and you should be able to discuss each:
- Extracting and processing the raw materials — which can use a lot of energy and damage habitats.
- Manufacturing the product — the energy and resources used in making it, and any pollution or waste produced.
- Using the product — its impact during use, including energy consumed and any emissions over its lifetime.
- Disposing of the product — what happens at the end of its life: landfill, incineration or recycling, and any pollution from disposal.
LCAs let us compare the impact of, say, a plastic bag against a paper or cotton one across all four stages, rather than judging by a single eye-catching factor. But they have real limitations, which the exam rewards you for recognising. Some impacts — particularly the effect of pollutants — are hard to measure objectively and rely on value judgements, so an LCA is not a purely objective process. Assessments can also be biased, for example if they are produced to support the marketing of a product. A good answer treats an LCA as a useful but imperfect guide whose conclusions depend on the assumptions made.
Common misconception: an LCA is not a single objective number. It involves value judgements (especially about pollution) and its scope can be selective, so two honest assessments of the same product can reach different conclusions.
Crude Oil and Hydrocarbons
Crude oil is a finite resource formed over millions of years from the remains of ancient organisms (mainly plankton) buried in mud. It is a mixture of a very large number of compounds, most of which are hydrocarbons — molecules made of hydrogen and carbon only.
Most of the hydrocarbons in crude oil are alkanes, which form a homologous series: a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties. The general formula of the alkanes is:
CnH2n+2
The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). You can check each against the formula: for ethane, n=2 gives 2n+2=6 hydrogens, so C2H6. Alkanes are saturated, meaning every carbon–carbon bond is a single bond.
As the chain length increases, the physical properties change in a predictable way that is heavily examined:
| Property | Short chains (few carbons) | Long chains (many carbons) |
|---|---|---|
| Boiling point | Low | High |
| Viscosity (how thick/runny) | Low (runny) | High (thick) |
| Flammability | High (ignite easily) | Low |
| State at room temperature | Often gases | Often liquids or solids |
The reason the boiling point rises with chain length is that longer molecules have stronger intermolecular forces between them, so more energy is needed to separate them. This single idea explains the whole trend — and it is the key to the next section.
Combustion of Hydrocarbons
Hydrocarbons make excellent fuels because they release energy when they burn. In complete combustion, with plenty of oxygen, a hydrocarbon burns to form only carbon dioxide and water, releasing energy. For methane:
CH4+2O2→CO2+2H2O
Check the balance: 1 carbon each side; 4 hydrogens each side (CH4 gives 4, and 2H2O gives 4); and 4 oxygens each side (2O2 gives 4, and CO2+2H2O gives 2+2=4). During combustion the carbon and hydrogen are oxidised, which is why complete combustion is an oxidation reaction.
Fractional Distillation
The mixture of hydrocarbons in crude oil is not useful as it is; it must be separated into fractions — groups of hydrocarbons with similar boiling points and chain lengths. This is done by fractional distillation, which separates the components according to their boiling points.
The process works like this. The crude oil is heated until most of it vaporises, and the vapour is fed into the bottom of a tall fractionating column that is hot at the bottom and cooler at the top. As the vapours rise, they cool; each fraction condenses at the level where the temperature matches its boiling point. Hydrocarbons with high boiling points (long chains) condense low down, near the hot bottom; those with low boiling points (short chains) rise higher and condense near the cool top; the smallest molecules leave the very top as gases. The fractions are tapped off at different heights.
The link to intermolecular forces is the reasoning examiners want: long-chain hydrocarbons have stronger intermolecular forces, so they have higher boiling points and condense lower in the column. The main fractions and their everyday uses are worth knowing:
| Fraction (shortest → longest chains) | Typical use |
|---|---|
| Petroleum gases | Bottled gas for heating and cooking |
| Petrol (gasoline) | Fuel for cars |
| Kerosene | Fuel for aircraft |
| Diesel oil | Fuel for some cars, lorries and trains |
| Heavy fuel oil | Fuel for ships and some power stations |
| Bitumen | Surfacing roads and roofs |
These fractions are valuable as fuels and as a feedstock for the petrochemical industry, which uses them to make polymers, solvents, lubricants, detergents and much more — a huge range of useful materials all derived from crude oil.
Cracking and Alkenes
Fractional distillation produces more of the long-chain fractions than there is demand for, and less of the short-chain fractions (such as petrol) than people want to buy. Cracking solves this mismatch between supply and demand by breaking long-chain hydrocarbons into smaller, more useful ones.
Cracking is a form of thermal decomposition: the long-chain molecules are heated until they break apart. There are two industrial methods — catalytic cracking, using a hot catalyst, and steam cracking, mixing with steam at a high temperature. Cracking a long alkane produces a shorter alkane plus an alkene. For example:
C10H22→C8H18+C2H4
Check the balance: 10 carbons on the left, and 8+2=10 on the right; 22 hydrogens on the left, and 18+4=22 on the right. The products are the useful short-chain alkane octane (a petrol component) and the alkene ethene.
Alkenes are a second homologous series. Their general formula is:
CnH2n
They are unsaturated because they contain a carbon–carbon double bond, C=C. The first alkenes are ethene (C2H4) and propene (C3H6). The double bond makes alkenes more reactive than alkanes, which makes them extremely useful as starting materials for other chemicals — and gives a simple way to tell them apart.
The Bromine-Water Test
You can distinguish an alkene from an alkane with bromine water. When orange bromine water is added to an alkene and shaken, the bromine adds across the double bond and the solution turns from orange to colourless — a positive test. With a saturated alkane, which has no double bond to react with, the bromine water stays orange. This colour change (orange → colourless) is one of the most reliable single-mark facts in C6.
Addition Polymerisation
The most important use of alkenes is making polymers. In addition polymerisation, many small alkene molecules (monomers) join together to form one very large molecule (a polymer). The double bonds "open up" and the monomers link in a long chain, with no other product formed. For example, many ethene molecules join to make poly(ethene) — the plastic used in bags and bottles — and propene makes poly(propene). The repeating unit of the polymer has the same atoms as the monomer, just with the double bond now used to join the units together.
The Evolution of the Atmosphere
The Earth's atmosphere has changed dramatically over its 4.6 billion years, and C6 expects you to tell the story in order. The evidence for the early atmosphere is limited, so theories are uncertain — but the broad sequence is well supported.
- The early atmosphere. In the first billion years, intense volcanic activity released gases that formed the early atmosphere. It is thought to have been rich in carbon dioxide, with little or no oxygen — rather like the atmospheres of Mars and Venus today. Volcanoes also released water vapour and small amounts of other gases such as nitrogen, ammonia and methane.
- The oceans form. As the Earth cooled, the water vapour condensed to form the oceans. Large amounts of carbon dioxide then dissolved into the new oceans, and over time formed carbonate precipitates and, with the shells of marine organisms, sedimentary rocks — so the level of CO2 in the air fell.
- Oxygen rises. When algae and then plants evolved, they began photosynthesis, which removes carbon dioxide and releases oxygen. Over about two billion years the oxygen level rose to the point where more complex life could evolve, while the carbon dioxide level continued to fall (also locked away in fossil fuels and sedimentary rocks).
- Today. The atmosphere has been roughly stable for about 200 million years. Its composition today is approximately 78% nitrogen (N2), 21% oxygen (O2), about 0.9% argon (Ar) and about 0.04% carbon dioxide (CO2), with small and varying amounts of water vapour.
The two headline drivers to remember are that photosynthesis is what added oxygen and removed carbon dioxide, and that dissolving in the oceans (and the formation of carbonates and fossil fuels) is what locked away most of the original carbon dioxide.
Greenhouse Gases and Climate Change
Some gases in the atmosphere are greenhouse gases, which keep the Earth warm enough to support life. The most important are carbon dioxide (CO2), methane (CH4) and water vapour. The mechanism of the greenhouse effect is examined precisely, so learn it in steps:
- Short-wavelength radiation (mostly visible light) from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it.
- The warmed Earth re-emits energy as longer-wavelength infrared radiation.
- Greenhouse gases absorb this outgoing infrared radiation and re-radiate it in all directions, including back towards the Earth — trapping heat and keeping the surface warmer than it would otherwise be.
Human activities are increasing the amounts of greenhouse gases. Burning fossil fuels and deforestation raise CO2 levels; farming (livestock and rice paddies) and waste in landfill raise methane. Based on peer-reviewed evidence, the scientific consensus is that these rising greenhouse-gas concentrations are causing the average global temperature to rise — human-caused (anthropogenic) global climate change. The likely consequences include rising sea levels (from melting ice and the expansion of warming water), more frequent and severe extreme weather, and changes to the distribution of species and to where crops can be grown.
It is worth being honest about the uncertainty in detail without overstating it: the climate is a hugely complex system, so precise predictions are difficult, and information in the media is sometimes oversimplified, biased or based on opinion rather than evidence. But the core conclusion — that human activity is warming the planet — rests on a large body of peer-reviewed science, and a strong answer reflects that consensus rather than inventing a false balance.
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. It can be reduced by using renewable or low-carbon energy, by improving energy efficiency, by carbon capture and storage, and through government taxes or limits (and "carbon offsetting" schemes). Each measure has limitations — cost, technology, and the difficulty of international agreement — which an "evaluate" question expects you to weigh.
Common misconception: the greenhouse effect is not "bad" in itself — without it the Earth would be frozen and lifeless. The problem is the enhanced greenhouse effect caused by the extra greenhouse gases that human activity adds.
Atmospheric Pollutants
When fuels burn, the combustion is not always complete and the fuel is not always pure, so a range of pollutants is released. You must know each pollutant, where it comes from and the harm it does.
| Pollutant | How it forms | Why it is harmful |
|---|---|---|
| Carbon dioxide (CO2) | Complete combustion of a hydrocarbon | A greenhouse gas — contributes to climate change |
| Carbon monoxide (CO) | Incomplete combustion (too little oxygen) | A toxic gas; it is colourless and odourless, and reduces the blood's ability to carry oxygen |
| Particulates (soot, carbon particles) | Incomplete combustion | Cause respiratory problems, and contribute to global dimming and dirtying of buildings |
| Sulfur dioxide (SO2) | Burning fuels that contain sulfur impurities | Dissolves in rain to cause acid rain, which damages plants, lakes and buildings; also causes breathing problems |
| Oxides of nitrogen (NO and NO2, "NOx") | The high temperature in an engine makes nitrogen and oxygen from the air react | Cause acid rain and respiratory problems |
The key distinction is between complete and incomplete combustion. With plenty of oxygen, complete combustion produces only carbon dioxide and water. With a limited oxygen supply, incomplete combustion occurs, producing carbon monoxide and/or carbon (particulates/soot) as well as water — releasing less energy in the process. Sulfur dioxide comes from impurities in the fuel, while oxides of nitrogen come from the air reacting at high temperatures, not from the fuel itself — a distinction examiners test often.
Potable and Waste Water
Potable water is water that is safe to drink — it has low levels of dissolved salts and microbes. The crucial point examiners probe is that potable water is not the same as pure water: chemically pure water contains nothing but H2O, whereas potable water still contains small, safe amounts of dissolved substances. The methods used to produce potable water depend on what is available.
For most fresh water, the process has two parts:
- Filtration to remove insoluble solids such as grit and larger particles (often by passing water through filter beds).
- Sterilisation to kill harmful microbes. This is done by adding chlorine, by bubbling ozone through the water, or by passing ultraviolet (UV) light through it.
Where fresh water is scarce, salty water (seawater) must be made potable by desalination, which removes the dissolved salts. The two methods are distillation — boiling the water and condensing the pure vapour — and reverse osmosis, which forces water through a partially permeable membrane that holds back the salts. Both require large amounts of energy, which makes desalination expensive and explains why it is used mainly in countries short of fresh water.
Finally, waste water (from homes, agriculture and industry) and sewage must be treated before it is returned to the environment, to remove organic matter and harmful microbes. Sewage treatment involves screening to remove large solids and grit, sedimentation to let solids settle out as sludge while the lighter effluent remains on top, and biological treatment in which aerobic bacteria break down organic matter and harmful microbes in the effluent. The settled sludge is digested anaerobically by bacteria, which also produces biogas, before the treated water is released.
Common misconception: "potable" does not mean "pure". Potable water is safe to drink but still contains small amounts of dissolved minerals; only distilled (or deionised) water is chemically pure.
Common Mistakes in C6
The same slips recur every year. Knowing them is half the battle.
- Confusing the alkane and alkene formulae. Alkanes are CnH2n+2 (saturated, single bonds); alkenes are CnH2n (unsaturated, a C=C double bond).
- Getting the bromine-water colour change backwards. It goes orange → colourless with an alkene; it stays orange with an alkane.
- Saying sulfur dioxide and oxides of nitrogen come from the same place. SO2 comes from sulfur in the fuel; NOx comes from nitrogen in the air reacting at high temperature.
- Treating "potable" as "pure". Potable water is safe to drink but still contains dissolved substances.
- Forgetting that photosynthesis drove the atmosphere's change. Algae and plants removed CO2 and added O2; oceans locked away much of the original carbon dioxide.
- Calling the greenhouse effect itself harmful. The natural effect keeps Earth warm; the problem is its enhancement by extra greenhouse gases.
- Treating a life cycle assessment as fully objective. It involves value judgements and can be biased.
Exam Technique for C6 on J248
- Sequence the atmosphere's evolution in order. Volcanic CO2-rich start → oceans form and CO2 dissolves → photosynthesis raises O2 → today's ∼78% N₂ / 21% O₂ composition earns marks for the correct order.
- Balance every equation and verify it. For combustion and cracking, count each element on both sides before you move on.
- Pair each pollutant with its source and its harm. Name the gas, where it comes from, and the damage it does.
- Give balanced "evaluate" answers. For recycling, life cycle assessment, desalination and reducing carbon footprints, state advantages and disadvantages, then conclude.
- Present the climate consensus accurately. Reflect the peer-reviewed mainstream view; do not manufacture a false balance, but do acknowledge genuine uncertainty in the detail.
Prepare with LearningBro
The LearningBro OCR GCSE Chemistry: Global Challenges course covers every part of C6 — sustainable metal extraction and recycling, life cycle assessment, crude oil and hydrocarbons, fractional distillation, cracking and alkenes, the evolution of the atmosphere, greenhouse gases and climate change, atmospheric pollutants, and potable and waste water — with worked examples and practice questions that match the OCR J248 format, plus immediate feedback on your answers.
For broader preparation across the whole specification and both papers, the OCR GCSE Chemistry Exam Prep course walks you through the paper structure, command words and answering technique. For the calculation and equilibrium chemistry that underpins industrial processes like the Haber process, see our Rates, Calculations and Equilibrium guide. And for the wider picture of the entire subject, start with our OCR GCSE Chemistry complete revision guide.
C6 connects chemistry to energy, the environment and everyday life, which makes it one of the most interesting topics to revise — and one where well-organised, balanced answers score very reliably. Work through the mechanisms and the storylines until you can explain each in order, and the global-challenges questions become a dependable source of marks.
Good luck with your revision.