Earth's resources, potable water, waste water & bioextraction (RP8) – GCSE Chemistry exam questions with mark scheme

1Grade 1–3Foundation3 marks

In the UK, potable water is produced from fresh water, such as water from lakes, rivers and the ground.

(a) What is potable water? [1]

(b) Give the two main stages used to treat fresh water to make it potable. [2]

(a) water that is safe to drink; allow drinking water; ignore 'pure' / 'clean'(1)
(b) filtration / passing through filter beds (to remove solids)(1)
(b) sterilising / killing microbes (using chlorine, ozone or ultraviolet light); allow chlorination(1)
Answer: (a) water safe to drink (b) filtration and sterilisation
Examiner tip: Potable water is not pure water — it still contains dissolved substances.

2Grade 4–54 marks

A student analyses a sample of sea water.

The student evaporates 50.0 cm³ of sea water to dryness. The mass of solid left is 1.75 g.

(a) Calculate the mass of dissolved solids in g per dm³ of sea water. [2]

(b) The student wants to purify sea water by distillation. Explain how distillation produces pure water. [2]

(a) 50.0 cm³ = 0.0500 dm³ / 1.75 × (1000 ÷ 50.0)(1)
(a) 35 (g/dm³); allow 35.0(1)
(b) water is heated and evaporates / boils (leaving dissolved salts behind)(1)
(b) the steam / water vapour is cooled and condensed (and collected)(1)
Answer: (a) 35 g/dm³ (b) water evaporates leaving salts behind, then the vapour is condensed
Examiner tip: Convert cm³ to dm³ by dividing by 1000 before dividing the mass.

3Grade 4–54 marks

Sewage is treated before the water is released into rivers.

Describe the stages of sewage treatment. [4]

screening and grit removal; allow removing large objects / solids / grit (by screens / sieving / filtering)(1)
sedimentation (in settlement tanks) to produce sewage sludge and effluent; allow solids settle out (as sludge), leaving liquid (effluent)(1)
anaerobic digestion of the sewage sludge; allow sludge broken down / digested by bacteria without oxygen; must link anaerobic to sludge(1)
aerobic biological treatment of the effluent; allow air / oxygen bubbled through effluent so bacteria / microorganisms break down organic matter; must link aerobic to effluent(1)
Answer: Screening and grit removal; sedimentation into sludge and effluent; anaerobic digestion of sludge; aerobic treatment of effluent
Examiner tip: Learn which part gets which treatment: sludge is anaerobic, effluent is aerobic.

4Grade 6–7Higher4 marks

The Earth's supply of high-grade copper ores is running out, so methods that extract copper from low-grade ores are being used.

(a) Describe how phytomining is used to extract copper. [3]

(b) Give one advantage of phytomining compared with traditional mining. [1]

(a) plants are grown on land / soil containing (low-grade) copper ore / copper compounds and absorb the copper compounds (through their roots)(1)
(a) plants are harvested and burned to produce ash (containing copper compounds)(1)
(a) copper is obtained from the ash (compounds) by displacement using scrap iron, or by electrolysis; allow 'a more reactive metal' for scrap iron; allow either method; the ash may first be dissolved (e.g. in acid) but this is not required for the mark(1)
(b) any one from: can extract copper from low-grade ores / land with a low copper content (that would be uneconomic to mine); less damage to the landscape / no large open pits / less waste rock / less noise or dust than mining; conserves (limited) high-grade ores; can clean up contaminated land / old mine waste; uses less energy (no digging, transporting or crushing large amounts of rock); ignore 'cheaper' / 'better for the environment' unless qualified(1)
Answer: (a) plants absorb copper compounds, are burned to ash, ash dissolved and copper obtained by displacement (iron) or electrolysis (b) e.g. extracts metal from low-grade ores
Examiner tip: Phytomining doesn't produce pure copper directly — the ash still needs displacement or electrolysis.

5Grade 1–3Foundation4 marks

Humans use the Earth's resources for warmth, shelter, food and transport.

(a) What is meant by sustainable development? [1]

(b) The table lists four resources.

Resource	Finite	Renewable
coal
timber from managed forests
cotton
iron ore

Complete the table. Tick (✓) one box in each row. [2]

Give one example of a natural material and the synthetic product that can replace it. [1]

(a) development that meets the needs of people today (current generations) without stopping / compromising future generations meeting their own needs; must refer to both present and future; do not accept 'using renewable resources' alone(1)
(b) any two or three rows correct (coal – finite; timber – renewable; cotton – renewable; iron ore – finite)(1)
(b) all four rows correct; a row with two ticks scores no credit for that row(1)
(c) a correct natural / synthetic pair, e.g. natural rubber → synthetic rubber; wool or cotton or silk → polyester / nylon / acrylic; wood → plastic (e.g. PVC window frames); leather → synthetic leather / plastic; allow natural fibres → synthetic fibres; both parts needed(1)
Answer: (a) meeting today's needs without compromising the ability of future generations to meet theirs (b) coal finite; timber renewable; cotton renewable; iron ore finite (c) e.g. wool replaced by polyester
Examiner tip: In (a) you must mention future generations: 'using resources that won't run out' describes renewable resources, not sustainable development.

6Grade 4–54 marks

The table gives estimates for two minerals that are mined from the Earth's crust.

Mineral	Estimated reserves in tonnes	Mass extracted each year in tonnes
X	$6.0 \times 10^{8}$	$2.4 \times 10^{7}$
Y	$6.0 \times 10^{10}$	$4.0 \times 10^{8}$

(a) Calculate how many years the reserves of mineral X will last if the mass extracted each year stays the same. [2]

(b) How many orders of magnitude larger are the reserves of mineral Y than the reserves of mineral X? [1]

(a) $\dfrac{6.0 \times 10^{8}}{2.4 \times 10^{7}}$ (correct division shown)(1)
(a) 25 (years); correct answer with no working scores 2 marks(1)
(b) 2 (orders of magnitude); allow 'one hundred times / $10^{2}$ times'(1)
(c) any one from: new reserves are discovered; more of the mineral is recycled (so less needs mining); demand / the rate of extraction falls; a synthetic or alternative material replaces it; better technology extracts more from low-grade ores; ignore 'it is renewable'(1)
Answer: (a) 25 years (b) 2 orders of magnitude (100 times) (c) e.g. new reserves are found / more is recycled
Examiner tip: Orders of magnitude means powers of ten: compare $10^{10}$ with $10^{8}$, a difference of 2 — not the ratio of the extraction rates.

7Grade 4–56 marks

A student analysed three water samples, A, B and C.

One sample is rain water, one is ground water from a well and one is sea water.

Sample	pH	Mass of dissolved solids in g per dm³
A	7.6	0.35
B	8.1	35
C	5.6	0.02

(a) Name a piece of apparatus the student could use to measure the pH of each sample more accurately than universal indicator. [1]

(b) Which sample, A, B or C, is sea water? Give a reason for your answer using the data. [2]

The student then purified sample B using the apparatus in the diagram.

(d) Describe how the student could show that the distillate is pure water. [2]

(a) pH probe / pH meter; allow data logger with pH sensor; do not accept 'pH paper' or 'litmus'(1)
(b) B(1)
(b) (because it has) the highest / far higher mass of dissolved solids (salts) – 35 g/dm³ (about 100 times more than A); a reason about pH alone scores no credit(1)
(c) to cool the steam / water vapour so it condenses (to liquid water); must refer to condensing or turning back to liquid(1)
(d) measure the boiling point (of the distillate) / evaporate a sample to dryness; allow 'measure the melting / freezing point'(1)
(d) boiling point is exactly 100 °C / no solid residue left; allow 'melting point is 0 °C'; second mark must match the method given(1)
Answer: (a) pH probe / meter (b) B – it has by far the most dissolved solids (35 g/dm³) (c) to cool and condense the water vapour (d) measure its boiling point: pure water boils at exactly 100 °C (or evaporate it: no residue)
Examiner tip: In (b) you must quote the data: dissolved solids decide it, because sea water's pH (about 8) is not very different from ground water's.

8Grade 6–7Higher5 marks

In some dry countries, potable water is produced from sea water by desalination.

(a) Distillation is one method of desalination. Name the other method of desalination. [1]

(b) The table shows the energy needed to produce 1 m³ of potable water by the two methods at one plant.

Method	Energy needed in kWh per m³
distillation	15
the other method	3.5

The plant produces $2.0 \times 10^{5}$ m³ of potable water each day.

Calculate the energy saved each day by using the other method instead of distillation.

Give your answer in standard form. [2]

(c) In the UK, potable water is made from fresh water (from lakes, rivers, reservoirs and the ground) rather than from sea water.

Explain why. [2]

(a) reverse osmosis; allow 'using membranes'(1)
(b) $(15 - 3.5) \times 2.0 \times 10^{5}$ or $11.5 \times 2.0 \times 10^{5}$; allow $3.0 \times 10^{6} - 7.0 \times 10^{5}$(1)
(b) $2.3 \times 10^{6}$ (kWh); allow 2 300 000 kWh for 1 mark only (not in standard form)(1)
(c) the UK has plenty of fresh water / high rainfall fills lakes, rivers, reservoirs and aquifers; allow 'fresh water only needs filtering and sterilising'(1)
(c) desalination needs large amounts of energy so it is (much more) expensive; allow 'sea water must have dissolved salts removed, which uses a lot of energy'(1)
Answer: (a) reverse osmosis (b) $2.3 \times 10^{6}$ kWh (c) The UK has plenty of fresh water (high rainfall), which needs only filtering and sterilising; desalination needs a lot of energy so is expensive.
Examiner tip: In (b) find the difference per m³ first, then multiply by the daily volume; leaving the answer as 2 300 000 loses the standard-form mark.

9Grade 6–7Higher6 marks

Copper can be extracted from low-grade ores by bioleaching.

(a) Describe how bioleaching is used to obtain copper compounds from a low-grade ore. [2]

(b) The solution produced contains copper(II) sulfate. Copper metal is obtained by adding scrap iron:

$$\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}$$

Explain why iron can be used to obtain copper from this solution. [1]

Relative atomic masses ($A_r$): Fe = 56 Cu = 63.5

Give your answer to 3 significant figures. [3]

(a) bacteria are used / grown on (or mixed with) the low-grade ore(1)
(a) (the bacteria) produce a leachate / solution containing copper compounds / ions; allow 'bacteria break down the ore to release copper ions into solution'(1)
(b) iron is more reactive than copper (so displaces it); allow 'iron is higher in the reactivity series'; ignore 'iron is cheap' alone(1)
(c) moles of Fe $= \dfrac{140}{56} = 2.5$ (mol)(1)
(c) moles of Cu = 2.5 (mol) (1 : 1 ratio) and mass $= 2.5 \times 63.5$; allow ratio of masses $\dfrac{140}{56} \times 63.5$(1)
(c) 159 (g); 158.75 scores 2 marks only (not 3 s.f.); correct answer with no working scores 3 marks(1)
Answer: (a) Bacteria act on the low-grade ore, producing a leachate solution containing copper compounds/ions. (b) Iron is more reactive than copper, so it displaces it. (c) 159 g
Examiner tip: In (c) the 1 : 1 ratio means moles of copper = moles of iron. Then multiply by 63.5, not 56, and round to 3 s.f.

10Grade 6–76 marks

A coastal town in a dry region needs a new supply of potable water. The council is considering three sources:

Ground water pumped from a well. The level of the water underground has fallen each year for the last ten years.
Sea water from the coast next to the town.
Waste water from the town's sewage works. The works also receives waste water from a factory that uses heavy metal compounds.

Evaluate the use of each source to produce potable water for the town.

You should use the information given and your own knowledge. [6]

Ground water: only needs filtering (filter beds) and sterilising (chlorine / ozone / UV), so it is the easiest / cheapest to treat(1)
Ground water: supply is limited / the falling level suggests it is being used faster than it is replaced, so it is not sustainable(1)
Sea water: plentiful / unlimited supply next to the town(1)
Sea water: dissolved salts must be removed by distillation or reverse osmosis (desalination), which needs large amounts of energy so is expensive(1)
Waste water: the supply is reliable (produced continuously) and reusing it conserves water(1)
Waste water: needs most treatment – screening and grit removal, sedimentation, anaerobic digestion of sludge, aerobic treatment of effluent – then further treatment to make it potable(1)
Waste water: organic matter and harmful microbes must be removed; the factory waste means harmful chemicals (heavy metal compounds) must also be removed(1)
Waste water: people may not accept drinking treated sewage(1)
Conclusion, e.g. sea water best long term if energy (e.g. renewable) is available; or combine sources; any justified conclusion accepted(1)

Level	Marks	What it looks like
Level 3	5–6	Considers all three sources, giving both an advantage and a disadvantage for most of them, linked to the information given (e.g. falling water table, energy cost, contamination of waste water), and reaches a justified conclusion about the best option for the town.
Level 2	3–4	Gives relevant advantages and/or disadvantages for at least two sources, with some reasons, and states a conclusion that may be only partly justified.
Level 1	1–2	Makes simple, separate statements about one or more sources, e.g. 'sea water needs a lot of energy', with little linking to the town's situation and no clear conclusion.
No relevant content	0	No relevant content.

Answer: Indicative: ground water easiest to treat (filter + sterilise) but supply falling/unsustainable; sea water unlimited but desalination energy-intensive and costly; waste water reliable but needs most treatment including removal of heavy metals and microbes, plus public acceptance; justified conclusion.
Examiner tip: 'Evaluate' needs a judgement: after the pros and cons of each source, finish with which one you would choose and why.

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