Using the Earth's Resources and Potable Water
Using the Earth's resources and producing potable water are covered in more depth: the distinction between finite and renewable resources, what sustainable development means in practice, and the two routes used to produce potable water, treating an available fresh water source, or desalinating seawater where fresh water is scarce, including why each method is chosen and what makes desalination more costly in energy terms.
Method
- Classify a named resource as finite or renewable by asking whether natural processes can replace it as fast as it is used: metal ores, crude oil and coal are finite, since they took millions of years to form and are extracted far faster than they are replaced; timber from managed forestry and crops grown for biofuel are renewable, since new stock can be grown within a human timescale.
- Explain sustainable development as balancing present needs against future ones: using resources, including energy, water and raw materials, in a way that meets the needs of the current population without so depleting or damaging those resources that future generations cannot meet their own needs.
- Describe potable water production from a fresh water source in the correct order: filtration through a mesh and then sand or gravel beds removes solid particles and suspended matter, and sterilisation (adding chlorine gas or chlorine compounds, or using ozone gas, or exposing the water to ultraviolet light) then kills harmful microorganisms; the choice of sterilising agent balances effectiveness against cost and any taste or safety concerns.
- Describe the two desalination methods and how each works: in distillation, seawater is heated until water evaporates, and the water vapour is then cooled and condensed elsewhere, leaving the dissolved salts behind in the original container; in reverse osmosis, seawater is forced under pressure through a partially permeable membrane whose pores allow water molecules through but are too small for the larger, hydrated dissolved ions to pass.
- Explain why desalination is more expensive than treating fresh water: distillation requires enough energy to evaporate large volumes of water, and reverse osmosis requires enough energy to force water through the membrane against the natural direction of osmosis, so both processes use substantially more energy, and therefore cost more, than the filtration and sterilisation needed for an existing fresh water supply.
- Recognise that potable water is judged by safety and low levels of dissolved substances and microorganisms, not by chemical purity: it will still contain dissolved minerals and ions at levels considered safe, unlike laboratory pure water, which contains only H2O.
Worked example
A coastal town has no nearby river or lake and relies entirely on desalinating seawater by reverse osmosis for its drinking water. Explain how reverse osmosis produces potable water from seawater, and give one reason the town's water is likely to cost more to produce than water in a nearby town that uses a river as its source.
- State the setup: seawater, which contains dissolved salts (ions) as well as water molecules, is pumped at high pressure against a partially permeable membrane.
- Explain what the membrane does: its pores are small enough to let individual water molecules pass through, but too small for the larger, hydrated dissolved ions to pass through.
- State the result: water molecules pass through the membrane to the other side as fresh, potable water, while the dissolved salts are left behind in an increasingly concentrated seawater solution.
- Explain the cost difference: forcing water through the membrane against the natural flow requires pumping at high pressure, which uses a large amount of energy, whereas the river-fed town only needs filtration and sterilisation of an already fresh water supply, which use far less energy.
- Final answer: because reverse osmosis needs significant energy input to force water through the membrane, while treating river water only needs filtration and sterilisation, the coastal town's desalinated water costs more to produce.
Practice questions
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Q1Classify crude oil as finite or renewable, and justify your answer.Show answer
Answer: Finite, because it took millions of years to form and is being extracted and used far faster than natural processes can replace it.
Q2State what is meant by sustainable development.Show answer
Answer: Meeting the needs of the current population without compromising the ability of future generations to meet their own needs.
Q3Name one method used to sterilise fresh water during the production of potable water.Show answer
Answer: Adding chlorine (or using ozone or ultraviolet light).
Q4Explain, in terms of particle size, how reverse osmosis separates water from dissolved salt.Show answer
Answer: The membrane has pores small enough to let individual water molecules pass through but too small to let the larger dissolved ions pass, so pure water passes through while the salts are left behind.
Q5Give one reason distillation of seawater requires a large energy input.Show answer
Answer: It requires heating a large volume of seawater until the water evaporates, which needs substantial thermal energy.
Q6Explain why potable water is not required to be chemically pure.Show answer
Answer: Potable water only has to be safe to drink, with dissolved substances and microorganisms kept below safe limits; it can still contain dissolved minerals and ions, unlike chemically pure water, which is H2O alone.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Two towns need to produce potable water. Town A has a large freshwater lake nearby. Town B is on a dry coastline with no freshwater source and must desalinate seawater. Describe the process each town would use, and explain why Town B's water production uses significantly more energy.
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Explain why timber from a sustainably managed forest is described as a renewable resource, but the coal used to power the sawmill that processes it is not.
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Free printable worksheet
Want more practice on paper? Download the using the earth's resources and potable water worksheet pack - 14 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 30 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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