Global Challenges | OCR GCSE Combined Science Chemistry, Higher tier (J250)

Global Challenges

175 questions
11 subtopics
Paper 10 (Chemistry)
Paper 10 (Chemistry)

Global Challenges is examined in Paper 10 (Chemistry).

It covers extracting metals by reduction and by electrolysis, biological methods of metal extraction, life cycle assessment, recycling, and the decisions behind it, fractional distillation of crude oil, the fractions, and crude oil as a feedstock, dependence on hydrocarbons, and cracking, the Earth's early atmosphere, and how oxygen developed, the greenhouse effect, and evidence for climate change, effects of carbon dioxide and methane, and air pollutants and producing potable water.

Sample questions from Global Challenges

Answer each one closed book first, then open the answer.

Extracting metals by reduction and by electrolysis

Give a word equation for the reduction of iron(III) oxide by carbon.

Show the answer
Iron(III) oxide + carbon → iron + carbon dioxide.
Biological methods of metal extraction

Describe the stages of phytoextraction.

Show the answer
Plants are grown on soil containing the metal, they absorb metal compounds as they grow, and the harvested plants are burned to leave an ash rich in the metal.
Life cycle assessment

A plastic bag is buried in landfill and does not break down. Which stage of a life cycle assessment does this belong to?

Show the answer
Disposal at the end of the product's life.
Recycling, and the decisions behind it

Give one reason a recycled material is often used for a lower-grade purpose than the original product.

Show the answer
Reprocessing can shorten polymer chains or leave impurities, so the material is weaker or less pure.
Fractional distillation of crude oil

What is kerosene used for?

Show the answer
It is used as fuel for aircraft.
The fractions, and crude oil as a feedstock

Name the alkane with the formula C₅H₁₂.

Show the answer
Pentane.
Dependence on hydrocarbons, and cracking

Why does dependence on hydrocarbons cause concern beyond the risk of supplies running out?

Show the answer
Burning them releases carbon dioxide and other pollutants into the atmosphere.
The Earth's early atmosphere, and how oxygen developed

Why are ideas about the early atmosphere less certain than measurements of the atmosphere today?

Show the answer
No one was there to measure it and the evidence from billions of years ago is fragmentary and indirect, so the models may change as new evidence is found.

The 11 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Extracting metals by reduction and by electrolysis	Reduction by carbon for metals below carbon, the blast furnace and its raw materials, why aluminium needs electrolysis, bauxite and cryolite, the electrodes in aluminium extraction, and the cost of each method.	16
Biological methods of metal extraction	Bioleaching with bacteria and phytoextraction with plants as ways to obtain metals from low-grade ores, how each works, recovering copper from a leachate by displacement with scrap iron or by electrolysis, why the ash is useful, their environmental and economic advantages, their slow rate and toxic leachate as drawbacks, and why these methods are becoming more important.	10
Life cycle assessment	The four stages of a life cycle assessment, placing quarrying, product use, transport and landfill in them, what is measured at each stage, and why comparing two products fairly is difficult.	16
Recycling, and the decisions behind it	How plastic bottles become fleece and why that is easier than making new bottles, recycling glass and car tyres for other uses, why recycling aluminium saves so much energy, why sorting comes first and recycled material is often lower grade, the resources saved, and the costs, contamination, scarcity, market prices and social factors that decide whether recycling is worthwhile.	16
Fractional distillation of crude oil	What crude oil is and how fractional distillation separates it in a column that is hot at the bottom and cool at the top, the fractions from refinery gases to bitumen and their uses, why larger hydrocarbons have higher boiling points and condense lower down, which forces are overcome on boiling, and why a fraction boils over a range of temperatures.	16
The fractions, and crude oil as a feedstock	The alkanes and their general formula CₙH₂ₙ₊₂, what a hydrocarbon is, working out and naming alkane formulae such as C₈H₁₈ and pentane, what saturated means, why a fraction is not pure, crude oil as the feedstock of the petrochemical industry for polymers, solvents, lubricants and medicines, how crude oil formed, and ethene for poly(ethene).	16
Dependence on hydrocarbons, and cracking	Why crude oil is finite and how modern life and transport depend on hydrocarbons, rising costs, arguments against burning oil, uses that do not burn it, pollution concerns, why replacing hydrocarbon fuels is hard, what cracking is and why refineries do it, the conditions for catalytic and steam cracking, its products and equation, and the ethical case against burning oil so quickly.	18
The Earth's early atmosphere, and how oxygen developed	The composition of today's atmosphere, the volcanic gases of the early atmosphere, why ideas about it are uncertain and the evidence used, how carbon dioxide dissolved in the oceans and was locked into carbonate rocks and fossil fuels, how photosynthesis slowly produced oxygen and its equation, why oxygen mattered for life, and why nitrogen became the most abundant gas.	16
The greenhouse effect, and evidence for climate change	Three greenhouse gases, how the greenhouse effect traps infrared radiation and why life needs it, the effect of more greenhouse gas, the correlation between fossil fuel use and carbon dioxide, ice core evidence, the uncertainty in the evidence and in climate predictions, judging climate claims, peer review, communicating findings, and the ethics of cutting emissions.	20
Effects of carbon dioxide and methane, and air pollutants	The effects of rising carbon dioxide and methane such as warming and rising sea levels, sources of methane, ways to cut emissions including carbon capture and storage and planting trees, why global action is hard, the sources and harm of carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates, and presenting and sampling pollution data with bar charts, histograms and frequency tables.	20
Producing potable water	What potable water is and how it differs from pure water, treating fresh water by screening, filtering and sterilising with chlorine, ozone or ultraviolet light, desalination by distillation or reverse osmosis and its cost, how hard ground, salt and waste water are to treat, and taking random, representative samples from a reservoir.	11

Global Challenges is 175 of the 916 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.