You are viewing a free preview of this lesson.
Subscribe to unlock all 8 lessons in this course and every other course on LearningBro.
You have now worked through the whole of Topic C6 of OCR Gateway Combined Science A — the extraction of metals, sustainable extraction and recycling, crude oil and hydrocarbons, fractional distillation and cracking, the evolution of the atmosphere, greenhouse gases and climate change, and atmospheric pollutants and water treatment. This final lesson pulls it all together. It shows how the parts of C6 connect around one big theme — using the Earth's finite resources responsibly — gathers the key equations in one place, drills the exam technique for balancing equations and writing evaluations, collects the misconceptions that catch students out, and finishes with a synoptic model answer. Treat it as a revision and exam-technique session rather than new content.
By the end of this lesson you should be able to see how the C6 topics fit together, balance combustion and cracking equations confidently, structure a top-band "evaluate" answer, avoid the common C6 misconceptions, and recall the key facts and figures examiners ask for.
This synoptic lesson exercises all three objectives: AO1 recall of the C6 facts and figures, AO2 application when balancing combustion and cracking equations, and AO3 analysis when you evaluate LCA and climate evidence to structure a top-band "evaluate" answer.
Topic C6 is one connected story about finite resources and their impact on the environment. Metals come from finite ores; crude oil is a finite mixture of hydrocarbons; and the way we extract, process and burn these resources changes the atmosphere and the water around us.
flowchart TD
A["Topic C6: Global challenges"] --> B["Finite resources"]
A --> C["Impact on the environment"]
B --> D["Metals from finite ores<br/>(reduction / electrolysis)"]
D --> E["Sustainable extraction<br/>(phytomining, bioleaching, recycling)"]
B --> F["Crude oil: finite<br/>mixture of hydrocarbons"]
F --> G["Fractional distillation<br/>+ cracking"]
C --> H["Evolution of the atmosphere<br/>(CO2 down, O2 up)"]
H --> I["Greenhouse gases<br/>+ climate change"]
G -.->|"combustion of fuels"| J["Atmospheric pollutants"]
I -.-> J
C --> K["Potable + waste water"]
Notice how the parts link. Burning the fuels from crude oil releases the carbon dioxide that drives climate change and the pollutants that harm health, so distillation and combustion connect straight to the atmosphere lessons. The carbon locked into fossil fuels billions of years ago (as the atmosphere evolved) is the very carbon we release today. And the theme of doing things sustainably — recycling metals, assessing a product's whole-life impact, reducing a carbon footprint, treating water — runs through the entire topic. Seeing these connections is exactly the synoptic thinking that lifts an answer.
C6 is unusual for a "global challenges" topic in how many equations it involves. Here they are gathered together — make sure you can write and balance each one.
| Process | Balanced equation |
|---|---|
| Extraction of iron (carbon reduction) | 2Fe2O3+3C→4Fe+3CO2 |
| Extraction of zinc (carbon reduction) | 2ZnO+C→2Zn+CO2 |
| Copper from leachate (displacement) | Fe+CuSO4→FeSO4+Cu |
| Complete combustion of methane | CH4+2O2→CO2+2H2O |
| Complete combustion of propane | C3H8+5O2→3CO2+4H2O |
| Incomplete combustion of methane | 2CH4+3O2→2CO+4H2O |
| Cracking (example) | C10H22→C8H18+C2H4 |
| Formation of sulfur dioxide | S+O2→SO2 |
| Formation of nitrogen monoxide | N2+O2→2NO |
Exam Tip: For any combustion equation, balance in the order carbon → hydrogen → oxygen (last). For any cracking equation, count carbons and hydrogens on both sides — you often need more than one alkene molecule to make it balance. Always finish by checking each element balances.
Write the balanced equation for the complete combustion of butane, C4H10.
Step 1 — write the skeleton: C4H10+O2→CO2+H2O.
Step 2 — balance carbon: 4 carbons → 4 CO2.
Step 3 — balance hydrogen: 10 hydrogens → 5 H2O (giving 5×2=10 H).
Step 4 — balance oxygen last: the right has (4×2)+(5×1)=8+5=13 oxygen atoms. Thirteen is odd, so use a fractional coefficient 213, then double everything to clear it:
2C4H10+13O2→8CO2+10H2O
Step 5 — check: C 8=8, H 20=20, O 26=26. Balanced.
Exam Tip: If balancing the oxygen gives an odd number, either use a fraction and then double the whole equation, or double the fuel from the start. Don't leave a fractional coefficient in a final GCSE answer unless the question allows it.
Hexadecane, C16H34, is cracked into octane, C8H18, and propene, C3H6. Write the balanced equation.
Step 1 — start with one of each: C16H34→C8H18+C3H6.
Step 2 — check carbon: left 16; right 8+3=11. Five short — but propene comes in units of 3 carbons, so try more propene. Using more molecules: right needs 16−8=8 carbons from propene. Since 3 does not divide 8, adjust the alkane count instead is not allowed here, so re-read: the cleanest fit is to check hydrogen too and let the alkene count balance both.
Step 3 — a reliable method: octane (C8H18) uses 8 C and 18 H, leaving 16−8=8 C and 34−18=16 H for propene (C3H6). But 8 C is not a whole number of propene units, so octane cannot be the only alkane partner here — this shows why examiners usually pair a long alkane with an ethene (C2H4) product. Re-doing with ethene: C16H34→C8H18+4C2H4.
Step 4 — check: carbon 8+(4×2)=16 ✓; hydrogen 18+(4×4)=18+16=34 ✓.
C16H34→C8H18+4C2H4
Answer: C16H34→C8H18+4C2H4. The lesson here is to check both carbon and hydrogen balance; if a given alkene will not fit as whole molecules, an ethene product usually will.
Several of the highest-mark questions in C6 ask you to evaluate something — a method of extraction, an LCA claim, or a way to cut a carbon footprint. A strong evaluation always has the same three parts:
An answer that lists only advantages, or only disadvantages, cannot reach the top band. The judgement is what turns a description into an evaluation.
Exam Tip: In an "evaluate" question, always finish with a judgement that refers back to the context in the question. Advantages + disadvantages + a reasoned conclusion is the full mark scheme; leaving out the conclusion caps your mark.
Some C6 marks are simply recall. Make sure these are secure:
Subscribe to continue reading
Get full access to this lesson and all 8 lessons in this course.