Revision Library

revisionlibrary.co.uk

GCSE · Chemistry

C10a Using the Earth's Resources and Potable Water

AQA 8464 · Calculator allowed · about 80 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Table 1: Approximate energy requirements for producing potable water

Data compiled for Revision Library from typical published ranges for the energy use of water treatment processes; figures are illustrative for use in this question.

Method of producing potable water | Approximate energy required per 1000 dm3 of potable water produced

Standard fresh water treatment (screening, sedimentation, filtration and sterilisation) | 0.6 kWh

Desalination by thermal distillation | 12.0 kWh

Desalination by reverse osmosis | 3.5 kWh

Humans rely on natural resources for almost everything they use, from fuel to building materials. Some of these resources are finite and some are renewable.

(a)State whether coal is a finite resource or a renewable resource.(1)

(b)State whether wind is a finite resource or a renewable resource.(1)

(c)State whether metal ores, such as copper ore, are a finite resource or a renewable resource.(1)

(d)State whether timber from a sustainably managed forest is a finite resource or a renewable resource.(1)

(Total for Question 1 is 4 marks)

Governments and industries are increasingly expected to plan for sustainable development when using the Earth's resources.

(a)Define what is meant by the term 'sustainable development'.(2)

(b)Give one reason why using sustainable development is important when extracting and processing the Earth's resources.(1)

(Total for Question 2 is 3 marks)

A student says: 'tap water is pure water, because it is safe to drink.' This statement contains a common misunderstanding.

(a)Define the term 'potable water'.(1)

(b)Explain why the student's statement is incorrect, using the chemical meaning of the word 'pure'.(2)

(Total for Question 3 is 3 marks)

In the UK, potable water is usually produced by treating fresh water rather than sea water.

(a)State three different sources of fresh water that can be treated to produce potable water in the UK.(3)

(Total for Question 4 is 3 marks)

A reservoir supplies fresh water to a local water treatment works.

(a)Name the process used to allow larger suspended solid particles to settle out at the bottom of a tank.(1)

(b)Name the process used to remove any remaining small suspended particles by passing the water through a bed of sand (or a similar material).(1)

(Total for Question 5 is 2 marks)

After sedimentation and filtration, water must still be sterilised before it is safe to drink.

(a)State three methods that can be used to sterilise water during the production of potable water.(3)

(Total for Question 6 is 3 marks)

Chlorine is the most widely used method of sterilising water in the UK.

(a)Explain why chlorine is added to water during the production of potable water.(2)

(b)Give one disadvantage of using chlorine to sterilise water.(1)

(c)Complete and balance the symbol equation for the reaction between chlorine and water, including state symbols: Cl2 ( ) + H2O ( ) -> ___ ( ) + ___ ( )(2)

(Total for Question 7 is 5 marks)

In countries with very little fresh water, potable water is sometimes produced from sea water instead.

(a)Define the term 'desalination'.(1)

(b)Describe how thermal distillation is used to produce potable water from sea water.(3)

(Total for Question 8 is 4 marks)

Reverse osmosis is an alternative method of desalinating sea water.

(a)Describe how reverse osmosis is used to produce potable water from sea water.(2)

(Total for Question 9 is 2 marks)

Higher tier only. Use the information in Table 1 above. A water company is comparing thermal distillation with reverse osmosis to produce potable water from sea water for a coastal town. Electricity costs the company £0.28 per kWh.

(a)Calculate the cost of producing 5000 dm3 of potable water by thermal distillation.(2)

(b)Calculate the cost of producing the same volume of potable water by reverse osmosis.(2)

(c)Using your answers to parts (a) and (b), explain which method is more energy efficient for producing potable water from sea water.(2)

(Total for Question 10 is 6 marks)

Use the information in Table 1 above.

(a)Explain why producing potable water from sea water is generally more expensive than producing potable water from fresh water.(3)

(Total for Question 11 is 3 marks)

Waste water from homes and industry (sewage) must be treated before it can be safely released into rivers.

(a)State the purpose of the screening stage in the treatment of waste water.(1)

(b)After screening, the waste water is left to stand in a settlement tank. Name the two products formed during this sedimentation stage.(2)

(Total for Question 12 is 3 marks)

The sludge and effluent produced during waste water treatment are treated in different ways.

(a)Describe how sewage sludge is treated after it has been separated by sedimentation.(2)

(b)Describe how effluent is treated before it is released into rivers.(2)

(Total for Question 13 is 4 marks)

As part of the required practical on the analysis and purification of water samples, a student named Freya investigates the concentration of dissolved solids in a water sample collected from a local canal. She measures 25.0 cm3 of the water sample into a clean, dry evaporating basin. The mass of the empty evaporating basin is 42.15 g. She heats the basin gently until all the water has evaporated, then allows it to cool and reweighs it. The mass of the basin plus the dry residue is 42.34 g.

(a)Calculate the mass of dissolved solids left in the evaporating basin.(1)

(b)Calculate the concentration of dissolved solids in the water sample, in g/dm3.(2)

(c)Name one piece of apparatus Freya should use to accurately measure out the 25.0 cm3 of water sample.(1)

(d)State one variable that should be controlled when comparing the dissolved solids concentration of two different water samples.(1)

(e)Suggest why Freya should heat the basin, cool it and reweigh it repeatedly until the mass no longer changes, rather than weighing the residue after a single heating.(1)

(Total for Question 14 is 6 marks)

As part of the same required practical, another student, Ibrahim, sets up apparatus to distil a sample of water collected from a local stream, in order to test how pure it is.

Figure (to be drawn): Simple labelled distillation set-up (no image asset, described in the stem): flask of water heated over a Bunsen burner, thermometer positioned at the side arm, condenser cooled by running water, and a collecting beaker for the distillate.

(a)Name two pieces of apparatus, other than a Bunsen burner, tripod and thermometer, that Ibrahim needs to distil the water sample.(2)

(b)State the boiling point of pure water at standard atmospheric pressure.(1)

(c)Ibrahim's distilled sample was found to boil at exactly 100°C. Explain what this result suggests about the purity of the original stream water sample.(2)

(d)Suggest why anti-bumping granules are added to the flask before it is heated.(1)

(Total for Question 15 is 6 marks)

A water company tests a different sample. 500 cm3 of the water sample is evaporated to dryness, leaving 0.15 g of solid residue.

(a)Calculate the concentration of dissolved solids in this water sample, in g/dm3.(2)

(b)Convert your answer to part (a) into mg/dm3.(1)

(c)For this question, assume the guideline maximum level of dissolved solids in UK drinking water is 500 mg/dm3. State, with a reason, whether this water sample meets that guideline.(1)

(Total for Question 16 is 4 marks)

Higher tier only. A student distils 250 cm3 of sea water in the laboratory. After distillation, she collects 210 cm3 of potable water.

(a)Calculate the percentage of the original volume of sea water that was recovered as potable water.(2)

(b)Suggest one reason why the volume of potable water collected is less than the original volume of sea water used.(1)

(Total for Question 17 is 3 marks)

Water companies are increasingly encouraged to consider sustainable development when choosing how to produce potable water and treat waste water.

(a)Explain how using renewable resources, instead of finite resources, to produce potable water and treat waste water can support sustainable development.(3)

(Total for Question 18 is 3 marks)

Use the information in Table 1 above, and your own knowledge of how waste water and sea water are treated, to answer this question.

A region is deciding how best to increase its supply of potable water: by investing in further treatment of waste water for reuse, or by building a new desalination plant to treat sea water. Evaluate the processes, cost and environmental impact of producing potable water from waste water, compared with producing potable water from sea water by desalination.

(Total for Question 19 is 6 marks)

Higher tier only. A coastal town in the south-east of England has very low annual rainfall and no large rivers or reservoirs nearby, so it relies on desalination of sea water to produce enough potable water for its population. A town in the north-west of England, which has high annual rainfall and several large reservoirs, uses only standard fresh water treatment.

(a)Using ideas about resources and sustainable development, suggest why the coastal town continues to use desalination, despite its higher cost, while the town in the north-west does not need to.(4)

(Total for Question 20 is 4 marks)

Mark scheme · C10a Using the Earth's Resources and Potable Water

Question 1

(a) B1 finite cao
(a) Answer: Finite
(b) B1 renewable cao
(b) Answer: Renewable
(c) B1 finite cao
(c) Answer: Finite
(d) B1 renewable cao
(d) Answer: Renewable

Question 2

(a) B1 development that meets the needs of people today oe
(a) B1 without stopping/preventing future generations from being able to meet their own needs oe
(a) Answer: Development that meets the needs of people today without preventing future generations from meeting their own needs.
(b) B1 any suitable reason, e.g. to conserve finite resources for the future, or to reduce damage to the environment (oe)
(b) Answer: For example, to conserve finite resources so they remain available for future generations.

Question 3

(a) B1 water that is safe for humans to drink (oe)
(a) Answer: Water that is safe for humans to drink.
(b) B1 in chemistry, 'pure' means a substance that contains only one substance/compound, with nothing else mixed or dissolved in it (oe)
(b) B1 potable water contains low levels of dissolved substances, such as mineral ions, so it is a mixture and not chemically pure, even though it is safe to drink (oe)
(b) Answer: In chemistry, 'pure' means a single substance with nothing else mixed in. Potable water contains low levels of dissolved substances such as mineral ions, so although it is safe to drink, it is a mixture and not chemically pure.

Question 4

(a) B1 any three from: rivers, lakes, reservoirs, groundwater (aquifers), rainwater (oe)
(a) B1 second correct source from the list
(a) B1 third correct source from the list
(a) Answer: Any three from: rivers, lakes, reservoirs, groundwater (aquifers) and rainwater.

Question 5

(a) B1 sedimentation cao
(a) Answer: Sedimentation
(b) B1 filtration cao
(b) Answer: Filtration

Question 6

(a) B1 chlorine/chlorination
(a) B1 ozone
(a) B1 ultraviolet (UV) light
(a) Answer: Chlorination (using chlorine), ozone, and ultraviolet (UV) light.

Question 7

(a) B1 chlorine kills/removes harmful microorganisms/pathogens in the water (oe)
(a) B1 making the water safe to drink and preventing the spread of disease (oe)
(a) Answer: Chlorine kills harmful microorganisms in the water, making it safe to drink and preventing the spread of disease.
(b) B1 any suitable disadvantage, e.g. it can give the water an unpleasant taste/smell, or chlorine and its by-products can be harmful/toxic in high concentrations (oe)
(b) Answer: For example, chlorine can give the water an unpleasant taste, or it (and its by-products) can be harmful in high concentrations.
(c) M1 correct formulae for the products: HOCl and HCl (oe)
(c) A1 fully balanced equation with correct state symbols: Cl2(g) + H2O(l) -> HOCl(aq) + HCl(aq) cao
(c) Answer: Cl2(g) + H2O(l) -> HOCl(aq) + HCl(aq)

Question 8

(a) B1 the process of removing dissolved salts from salt water (sea water) to produce fresh/potable water (oe)
(a) Answer: The process of removing dissolved salts from sea water to produce fresh (potable) water.
(b) B1 the sea water is heated until it boils and evaporates (oe)
(b) B1 the water vapour produced is separated from the dissolved salts, which are left behind (oe)
(b) B1 the water vapour is then cooled/condensed to form pure liquid water (oe)
(b) Answer: The sea water is heated until it boils and evaporates, leaving the dissolved salts behind. The water vapour is then cooled and condensed to form pure liquid water.

Question 9

(a) B1 sea water is forced/pumped at high pressure through a partially permeable membrane (oe)
(a) B1 the membrane allows small water molecules through, but not the larger dissolved ions/salts, which are left behind (oe)
(a) Answer: Sea water is forced under high pressure through a partially permeable membrane. The membrane allows water molecules through but not the larger dissolved ions, which are left behind.

Question 10

(a) M1 5000 dm3 / 1000 = 5, so energy = 5 * 12.0 = 60 (kWh) oe
(a) A1 60 * 0.28 = 16.80, so £16.80 cao
(a) Answer: £16.80
(b) M1 energy = 5 * 3.5 = 17.5 (kWh) oe
(b) A1 17.5 * 0.28 = 4.90, so £4.90 cao
(b) Answer: £4.90
(c) B1 reverse osmosis is more energy efficient, as it costs less/uses less energy to produce the same volume of potable water (ft from parts a and b) oe
(c) B1 this is because reverse osmosis does not need to heat and evaporate the water like distillation; it only needs to pump the water under pressure through the membrane (oe)
(c) Answer: Reverse osmosis is more energy efficient (it costs less to produce the same volume of water), because it does not need to heat and evaporate the water like distillation; it only needs to pump the water under pressure through a membrane.

Question 11

(a) B1 desalination (by distillation or reverse osmosis) requires much more energy than standard fresh water treatment (e.g. 12.0 or 3.5 kWh per 1000 dm3, compared with 0.6 kWh per 1000 dm3 for standard treatment, ft Table 1) oe
(a) B1 sea water contains a much higher concentration of dissolved salts than fresh water, so more energy and more specialist equipment (e.g. membranes, distillation plant) are needed to remove them (oe)
(a) B1 this extra energy use and specialist equipment increases the overall cost of producing potable water from sea water (oe)
(a) Answer: Desalinating sea water uses much more energy than standard fresh water treatment (see Table 1), because sea water contains far more dissolved salts, which need more energy and specialist equipment (such as membranes or distillation plant) to remove. This extra energy use and equipment increases the cost.

Question 12

(a) B1 to remove large solid objects/debris (e.g. rags, plastic, grit) from the waste water (oe)
(a) Answer: To remove large solid objects and debris from the waste water.
(b) B1 sludge (the solid material that settles at the bottom) (oe)
(b) B1 effluent (the liquid that remains) (oe)
(b) Answer: Sludge (the settled solids) and effluent (the remaining liquid).

Question 13

(a) B1 the sludge is broken down by anaerobic digestion, in which bacteria/microorganisms break down the organic matter in the absence of oxygen (oe)
(a) B1 this produces biogas (mainly methane), which can be used as a fuel, and the remaining digested sludge can be used as a fertiliser (oe)
(a) Answer: Sludge is broken down by anaerobic digestion, in which bacteria break down the organic matter without oxygen. This produces biogas (mainly methane), which can be used as a fuel, and the remaining digested sludge can be used as a fertiliser.
(b) B1 effluent undergoes aerobic biological treatment, in which bacteria/microorganisms break down any remaining organic matter and harmful substances (oe)
(b) B1 this process requires oxygen, which is added to/pumped through the effluent (e.g. by aeration) (oe)
(b) Answer: Effluent undergoes aerobic biological treatment, in which bacteria break down any remaining organic matter and harmful substances. This process requires oxygen, which is pumped through the effluent.

Question 14

(a) B1 0.19 (g) cao
(a) Answer: 0.19 g
(b) M1 converts 25.0 cm3 to 0.0250 dm3 (oe) and divides the mass of residue by the volume in dm3
(b) A1 7.6 (g/dm3) cao ft from (a)
(b) Answer: 7.6 g/dm3
(c) B1 a measuring cylinder (accept pipette or burette) cao
(c) Answer: A measuring cylinder (a pipette or burette would also be accepted).
(d) B1 any suitable control variable, e.g. the volume of water sample evaporated, or the method/rate of heating (oe)
(d) Answer: For example, the volume of water sample evaporated should be kept the same for each sample.
(e) B1 to make sure that all of the water has evaporated (oe), so that the measured mass of dissolved solids is accurate and not an underestimate (oe)
(e) Answer: To make sure all the water has evaporated, so the mass of dissolved solids measured is accurate (not an underestimate).

Question 15

(a) B1 any one from: a (round-bottomed) flask, a condenser, a delivery tube, anti-bumping granules, a collecting beaker/vessel, a clamp stand, a heatproof mat
(a) B1 a second, different item from the same list
(a) Answer: Any two from: a (round-bottomed) flask, a condenser, a delivery tube, anti-bumping granules, a collecting beaker, a clamp stand, a heatproof mat.
(b) B1 100°C cao
(b) Answer: 100°C
(c) B1 dissolved substances raise the boiling point of water above 100°C (oe)
(c) B1 the distilled sample boiling at exactly 100°C suggests the distillation removed the dissolved solids successfully, i.e. it produced pure water (oe)
(c) Answer: Dissolved substances raise the boiling point of water above 100°C. Boiling at exactly 100°C suggests the distillation successfully removed the dissolved solids, producing pure water.
(d) B1 to promote smooth, even boiling and prevent the liquid suddenly bumping/spurting out of the flask (oe)
(d) Answer: To promote smooth, even boiling and prevent the liquid suddenly bumping or spurting out of the flask.

Question 16

(a) M1 500 cm3 / 1000 = 0.500 dm3 (oe)
(a) A1 0.3 (g/dm3) cao
(a) Answer: 0.3 g/dm3
(b) B1 300 (mg/dm3) cao ft from (a) * 1000
(b) Answer: 300 mg/dm3
(c) B1 yes, it meets the guideline, because 300 mg/dm3 is less than 500 mg/dm3 (ft from b) oe
(c) Answer: Yes, the sample meets the guideline, because 300 mg/dm3 is less than the 500 mg/dm3 limit.

Question 17

(a) M1 (210 / 250) * 100 oe
(a) A1 84% cao
(a) Answer: 84%
(b) B1 any suitable reason, e.g. some water vapour escapes/is lost before it condenses, or some water remains in the flask/apparatus with the salt residue (oe)
(b) Answer: For example, some water vapour may escape or be lost before it condenses, or some water remains in the flask with the salt residue.

Question 18

(a) B1 renewable resources (e.g. some sources of energy) can be replenished/replaced at the same rate they are used, so they will not run out (oe)
(a) B1 using them, rather than finite resources, helps to conserve finite resources so they remain available for future generations (oe)
(a) B1 this reduces the overall environmental impact and helps meet the needs of people today without compromising the ability of future generations to meet their own needs (oe)
(a) Answer: Renewable resources can be replaced at the same rate they are used, so they do not run out. Using them instead of finite resources helps conserve finite resources for future generations and reduces overall environmental impact, supporting sustainable development.

Question 19

Level 3 (5-6): A detailed and logically structured evaluation that accurately describes both the treatment of waste water (screening, sedimentation into sludge and effluent, anaerobic digestion of sludge and aerobic treatment of effluent) and desalination of sea water (distillation or reverse osmosis), compares their relative cost and energy use using the data in Table 1, and reaches a well-reasoned, justified conclusion about which approach is more appropriate, recognising that the answer may depend on local circumstances such as water availability.
Level 2 (3-4): Accurate description of at least some stages of waste water treatment and of desalination, with some comparison of cost, energy use or environmental impact, but the evaluation may be one-sided or only partly linked to a conclusion.
Level 1 (1-2): Basic, isolated statements about waste water treatment and/or desalination, with little or no comparison of cost or environmental impact and no supported conclusion.
Level 0 (0): No relevant content.
Indicative content:
- Waste water treatment: screening removes large solid objects; sedimentation separates the water into sludge and effluent; sludge is broken down by anaerobic digestion (producing biogas, mainly methane, which can be used as a fuel, and digested solids that can be used as fertiliser); effluent is treated aerobically by bacteria to remove remaining organic matter before release into rivers.
- Treating waste water for reuse requires comparatively little energy (Table 1 gives about 0.6 kWh per 1000 dm3 for standard treatment), because the water is already relatively low in dissolved salts, so it is generally the cheaper option.
- Desalination of sea water uses either thermal distillation (heating and evaporating the water, then condensing it) or reverse osmosis (forcing water through a partially permeable membrane under high pressure).
- Desalination requires far more energy than standard treatment (Table 1 gives 12.0 kWh per 1000 dm3 for distillation and 3.5 kWh per 1000 dm3 for reverse osmosis), making it considerably more expensive.
- Desalination may still be necessary in regions with very limited fresh water or waste water available for reuse, despite the higher cost, to guarantee a secure supply of potable water.
- Environmental considerations: disposal of the concentrated brine (waste salt solution) produced by desalination can harm marine ecosystems if not managed carefully; reusing treated waste water reduces the amount of fresh water that needs to be extracted from rivers, lakes and reservoirs, which can support sustainable development.
- A balanced conclusion should weigh the lower cost and energy use of treating waste water against the fact that desalination may be the only reliable option where fresh water and waste water resources are limited, for example in coastal areas with low rainfall.

Question 20

(a) B1 the coastal town has limited access to fresh water (low rainfall, no large rivers or reservoirs), so sea water/desalination may be the only reliable source of potable water available to it (oe)
(a) B1 the town in the north-west has plentiful fresh water resources (reservoirs and higher rainfall), so standard fresh water treatment is sufficient, and there is no need for the more expensive desalination process (oe)
(a) B1 desalination is more expensive and uses much more energy (ft Table 1), but may still be necessary where fresh water is a limited/finite local resource, to ensure a secure and sustainable supply (oe)
(a) B1 for sustainable development, each town uses the method of treatment appropriate to the resources locally available, avoiding unnecessary energy use and cost where standard treatment is sufficient (oe)
(a) Answer: The coastal town has little fresh water available locally (low rainfall, no large rivers/reservoirs), so desalination may be its only reliable source of potable water, even though it is expensive and energy-intensive. The north-west town has plentiful fresh water (higher rainfall, reservoirs), so cheaper standard treatment is sufficient. For sustainable development, each town should use the treatment method appropriate to the resources locally available.

Using the Earth's Resources and Potable Water - Worksheets, Questions and Revision