What you'll learn
Using resources: potable water and water treatment is the topic in AQA GCSE Chemistry that asks how chemistry meets human needs without exhausting the planet. It covers the distinction between finite and renewable resources, the production of drinking water from fresh and from salt sources, the treatment of waste water before it returns to the environment, the assessment of environmental impact through life cycle assessment, and the reuse and recycling of materials. It also contains one of the course's most precise vocabulary points: potable water is safe to drink but is not pure water, and confusing the two costs marks every year. By the end of this guide you should be able to classify resources, describe both routes to potable water, describe sewage treatment in order, explain the four stages of a life cycle assessment and why it is not fully objective, and evaluate recycling.
Key terms and definitions
Finite resource — a resource not being replaced, or replaced far more slowly than it is used, such as crude oil and metal ores
Renewable resource — a resource replaced at the same rate as, or faster than, it is used
Sustainable development — development that meets present needs without compromising the ability of future generations to meet their own
Potable water — water that is safe to drink; it is not chemically pure, since it contains dissolved substances
Desalination — removal of dissolved salts from salty water, by distillation or reverse osmosis
Sterilising agent — a treatment used to kill microbes in water, such as chlorine, ozone or ultraviolet light
Sedimentation — the settling of solids out of a liquid under gravity
Effluent — the liquid remaining after sewage has been separated from sludge
Sludge — the semi-solid material separated from sewage during settlement
Life cycle assessment — an assessment of a product's environmental impact from raw materials to disposal
Aerobic digestion — breakdown of organic matter by microorganisms using oxygen
Anaerobic digestion — breakdown of organic matter by microorganisms without oxygen, producing methane
Core concepts
Finite and renewable resources
The Earth's resources provide warmth, shelter, food and transport. Natural resources supplemented by agriculture provide food, timber, clothing and fuels, and chemistry improves on them in two ways: agricultural processes increase the yield of natural products, and synthetic products can replace natural ones, as synthetic rubber replaces natural rubber.
Finite resources, including fossil fuels and metal ores, are consumed far faster than natural processes replace them. Renewable resources are replenished at least as fast as they are used, so they can continue indefinitely provided replacement continues.
Sustainable development means meeting present needs without compromising future generations. In chemistry this usually means using less of a finite resource, using it more efficiently, or substituting a renewable alternative.
Potable water is not pure water
This distinction is the single most examined vocabulary point in the topic.
Potable water is water that is safe to drink. It contains dissolved substances, including mineral salts, so chemically it is a mixture, not a pure substance. It simply has sufficiently low levels of dissolved salts and microbes to be safe.
Pure water, in the chemical sense, is water and nothing else. It is produced in the laboratory by distillation and would leave no residue on evaporation.
Any question asking whether drinking water is pure is testing this point, and the answer is that it is potable but not pure.
Producing potable water from fresh water
In the United Kingdom, rain provides water with low levels of dissolved substances, known as fresh water, which collects in the ground and in lakes and rivers.
Producing potable water from it involves three stages. First, an appropriate source is chosen, one with low levels of dissolved substances and contamination. Second, the water is passed through filter beds to remove solid particles and any remaining insoluble material. Third, it is sterilised to kill microbes.
The sterilising agents used are chlorine, ozone or ultraviolet light. Naming all three is worth doing when the question asks for methods.
Desalination
Where supplies of fresh water are limited, potable water is produced from salty water or sea water by desalination.
Two methods are used. Distillation boils the water and condenses the vapour, leaving the dissolved salts behind. Membrane processes such as reverse osmosis force water through a membrane that the dissolved ions cannot pass.
Both require large amounts of energy, which makes desalination expensive. This is the reason it is used only where there is no fresher alternative, and it is the specific point examiners want rather than a general statement that it is difficult.
The required practical on water
The practical analyses water samples and then purifies one by distillation.
Two tests assess purity. Measuring the pH with universal indicator or a pH probe shows whether the water is acidic, neutral or alkaline. Evaporating a known volume to dryness and weighing the residue shows how much dissolved solid was present — pure water leaves no residue at all.
The sample is then purified by simple distillation: the water is boiled, the vapour passes into a condenser where it cools and condenses, and the distillate is collected. Testing the distillate again should show a neutral pH and no solid residue, confirming that the dissolved substances were left behind in the flask.
Waste water treatment
Urban lifestyles and industrial processes produce large volumes of waste water that must be treated before release. Sewage and agricultural waste water require the removal of organic matter and harmful microbes, while industrial waste water may additionally require the removal of harmful chemicals.
The stages follow a logical order that should be learned as a sequence.
Screening and grit removal takes out large solids and grit.
Sedimentation in settlement tanks separates the waste into a liquid effluent and a semi-solid sludge.
The effluent is treated by aerobic biological digestion, in which air is bubbled through so that microorganisms break down the remaining organic matter using oxygen.
The sludge is treated separately by anaerobic digestion, carried out by microorganisms in the absence of oxygen. This produces methane gas, which can be burned as a fuel, and digested sludge, which can be used as a fertiliser.
The distinction between the aerobic treatment of effluent and the anaerobic digestion of sludge is examined regularly, and reversing them is among the commonest errors in the topic.
Waste water treatment requires less energy than desalination but more than treating fresh water, which is the comparison questions usually seek.
Life cycle assessments
A life cycle assessment assesses a product's environmental impact across its whole life, in four stages.
Extracting and processing raw materials, including damage from mining or quarrying and the energy consumed.
Manufacturing and packaging.
Use and operation during the product's lifetime, including energy or materials consumed and any maintenance required.
Disposal at the end of life, including whether the product goes to landfill, and the energy used in transport and in any disposal process.
Transport applies at every stage and is the element candidates most often omit.
Some parts of an assessment can be quantified reasonably objectively: the use of water, resources and energy, and the production of some wastes. Other parts cannot. Allocating numerical values to the effects of pollutants requires value judgements, which makes life cycle assessment open to bias. Selective or abbreviated assessments can be devised to support a desired conclusion, for example in advertising, and being able to say this is frequently worth a mark of its own.
Reducing, reusing and recycling
The principle is to reduce the use of limited resources, to reuse products where possible, and to recycle where reuse is not.
Metals are recycled by melting and recasting or reforming into different products. The degree of separation required depends on the material and its intended use: some scrap steel can be added directly to iron from a blast furnace, reducing the quantity of iron ore needed.
Glass bottles can be reused directly. Alternatively glass is recycled by crushing, then melting and reshaping into new products.
The benefits of recycling are specific and should be named rather than described generally: it reduces the use of finite raw materials, it usually requires far less energy than extracting a metal from its ore, it reduces waste sent to landfill, and it reduces the environmental damage caused by mining and quarrying.
The costs are that collection, transport and sorting themselves consume energy, and that recycled material may be of lower quality than newly produced material.
Worked examples
Example 1: Explaining the purity distinction (3 marks)
A bottle of spring water is labelled pure. Explain why a chemist would disagree.
In chemistry a pure substance is a single element or compound with nothing else present. Spring water contains dissolved substances, including mineral salts, so it is a mixture rather than a single compound. It is potable, meaning the levels of dissolved salts and microbes are low enough for it to be safe to drink, but it is not chemically pure. Evaporating it would leave a solid residue, whereas pure water would leave none.
Example 2: Comparing two sources (4 marks)
A country with limited rainfall is deciding whether to build a desalination plant. Compare producing potable water by desalination with producing it from a river.
Treating river water requires only filtration to remove solid particles and sterilisation with chlorine, ozone or ultraviolet light. This uses relatively little energy and is therefore cheap.
Desalination requires either distillation, which means boiling the water and condensing the vapour, or a membrane process such as reverse osmosis. Both consume large quantities of energy, making the process considerably more expensive.
Desalination would be chosen only where fresh water sources cannot meet demand, since sea water is effectively unlimited whereas a river supply may be insufficient, particularly where rainfall is low.
Example 3: Ordering sewage treatment (4 marks)
Describe the stages of sewage treatment in order, identifying which stages are aerobic and which anaerobic.
First, screening removes large solids and grit from the incoming sewage. Second, the sewage passes into settlement tanks where sedimentation separates it into liquid effluent and semi-solid sludge.
Third, the effluent is treated by aerobic biological digestion, with air bubbled through so that microorganisms break down the remaining organic matter using oxygen.
Fourth, the sludge is treated separately by anaerobic digestion, in which microorganisms break it down without oxygen, producing methane that can be burned as a fuel and digested sludge that can be used as a fertiliser.
Common mistakes and how to avoid them
The most frequent error in this topic is describing potable water as pure. Potable means safe to drink; it contains dissolved substances and is a mixture.
Students regularly reverse the two digestion stages. The effluent is treated aerobically with air bubbled through; the sludge is digested anaerobically and produces methane.
Another common slip is explaining that desalination is not used because it is difficult. The specific reason is the large quantity of energy required, which makes it expensive.
In recycling questions, many answers state only that recycling helps the environment. Name the benefit: less use of finite resources, less energy than extraction, less landfill, or less quarrying.
Finally, life cycle assessment answers often list only the four stages. Questions frequently also ask about objectivity, and the point that quantifying pollutant effects requires value judgements is a separate mark.
Exam technique for "Using resources: potable water and water treatment"
Identify the starting source before describing any treatment. Fresh water needs filtering and sterilising; salt water needs desalination. The treatment follows from the source.
Learn the four life cycle assessment stages as a sequence and add transport, which applies throughout and is the mark most often missed.
When comparing two processes or materials, use the same headings for both. Comparing energy for one and cost for the other is not a comparison.
For six-mark evaluation questions on recycling or life cycle assessment, plan two advantages, two disadvantages and a conclusion, and refer to the specific material named in the question rather than to materials in general.
Quick revision summary
Finite resources such as fossil fuels and metal ores are used faster than they are replaced; renewable resources are replaced at least as fast as used, and sustainable development meets present needs without compromising future ones. Potable water is safe to drink but not chemically pure, since it contains dissolved substances. Fresh water is treated by choosing a suitable source, filtering to remove solids, and sterilising with chlorine, ozone or ultraviolet light. Salt water requires desalination by distillation or reverse osmosis, both energy-intensive and therefore expensive. Sewage treatment screens and removes grit, separates effluent from sludge by sedimentation, treats the effluent by aerobic digestion with air bubbled through, and digests the sludge anaerobically to give methane and fertiliser. Life cycle assessments cover raw materials, manufacture, use and disposal, with transport throughout, and are not fully objective because quantifying pollutant effects requires value judgements, so they can be selective or biased. Reducing, reusing and recycling cut the use of finite resources, the energy needed compared with extraction, the waste sent to landfill and the damage from quarrying.