What you'll learn
Using resources is the unit of AQA GCSE Combined Science: Trilogy that asks how chemistry can meet human needs without exhausting the planet. It divides into three connected parts: the distinction between finite and renewable resources and what sustainable development means; the treatment of water to make it safe to drink and the treatment of waste water before it is returned to the environment; and the assessment of environmental impact through life cycle assessments and through reusing and recycling materials. By the end of this unit you should be able to distinguish finite from renewable resources, explain what makes a process sustainable, describe how potable water is produced from fresh and from salt water, describe the stages of sewage treatment, explain the four stages of a life cycle assessment and why such assessments are not purely objective, and evaluate the reuse and recycling of materials. This unit is assessed on Chemistry Paper 2 and includes the required practical on analysing and purifying water samples.
Key terms and definitions
Finite resource — a resource that is not being replaced, or is being replaced far more slowly than it is used, such as crude oil and metal ores
Renewable resource — a resource that is replaced at the same rate as, or faster than, it is used, such as timber from replanted forests
Sustainable development — development that meets the needs of the present without compromising the ability of future generations to meet their own needs
Potable water — water that is safe to drink; it is not chemically pure, because it contains dissolved substances
Desalination — the removal of dissolved salts from salty water to produce potable water, by distillation or reverse osmosis
Sterilising agent — a substance or treatment used to kill microbes in water, such as chlorine, ozone or ultraviolet light
Sewage treatment — the process of removing organic matter and harmful microbes from waste water before it is returned to the environment
Life cycle assessment — an assessment of the environmental impact of a product across its whole life, from raw materials to disposal
Effluent — the liquid remaining after sewage has been treated, which can be discharged into rivers or the sea
Core concepts
Finite and renewable resources
The Earth's resources are used to provide warmth, shelter, food and transport. Natural resources supplemented by agriculture provide food, timber, clothing and fuels, and chemistry plays a part in improving on them: agricultural processes increase the yield of natural products, and synthetic products can replace natural ones, as synthetic rubber replaces natural rubber.
Resources are classed as finite or renewable. Finite resources, including fossil fuels and metal ores, are being used far faster than natural processes replace them. Renewable resources are replenished at least as fast as they are consumed, so they can be used indefinitely provided the replacement continues.
Sustainable development means meeting present needs without compromising the ability of future generations to meet theirs. In chemistry this usually means using less of a finite resource, using it more efficiently, or finding a renewable substitute.
Potable water
Potable water is water that is safe to drink. It is important to be precise here: potable water is not pure water in the chemical sense, because it contains dissolved substances. It simply has sufficiently low levels of dissolved salts and microbes to be safe.
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 choosing an appropriate source, passing the water through filter beds to remove solid particles, and then sterilising it to kill microbes. The sterilising agents used are chlorine, ozone or ultraviolet light.
Where supplies of fresh water are limited, potable water can be produced from salty water by desalination. This can be done by distillation, in which the water is boiled and the vapour condensed, or by processes that use membranes such as reverse osmosis. Both require large amounts of energy, which is the reason desalination is used only where there is no fresher alternative — a point worth stating whenever a question asks why a country does or does not desalinate.
The required practical on water
The practical involves analysing water samples and then purifying one by distillation. Two tests are used to judge purity. Measuring the pH with universal indicator or a pH probe shows whether the water is acidic, neutral or alkaline. Evaporating a known volume of the water to dryness and weighing the residue shows how much dissolved solid was present: pure water leaves no residue.
The sample is then purified by simple distillation, in which the water is boiled, the vapour passes into a condenser where it cools and condenses, and the distilled water is collected. Testing the distillate again should now show a neutral pH and no solid residue, confirming that the dissolved substances were left behind.
Waste water treatment
Urban lifestyles and industrial processes produce large amounts of waste water that must be treated before it is released into the environment. Sewage and agricultural waste water require the removal of organic matter and harmful microbes, while industrial waste water may also require the removal of harmful chemicals.
The stages of sewage treatment follow a logical order. First comes screening and grit removal, which takes out large solids and grit. The sewage then passes into settlement tanks, where sedimentation separates it into a liquid effluent and a semi-solid sludge. The effluent is then treated by aerobic biological digestion, in which air is bubbled through so that microbes break down the remaining organic matter using oxygen. The sludge is treated separately by anaerobic digestion, carried out by microbes in the absence of oxygen, which breaks down the organic matter and produces methane gas that can be burned as a fuel, along with digested sludge that can be used as a fertiliser.
The distinction between the aerobic treatment of effluent and the anaerobic digestion of sludge is examined regularly, and swapping them is a common error.
Life cycle assessments
A life cycle assessment is carried out to assess the environmental impact of a product across its whole life. It has four stages.
The first stage considers extracting and processing the raw materials, including the damage caused by mining or quarrying and the energy used. The second considers manufacturing and packaging. The third considers use and operation during the product's lifetime, including any energy or materials consumed and any maintenance. The fourth considers disposal at the end of life, including whether the product goes to landfill, and the energy used in transport and in any disposal process.
Some parts of an assessment are straightforward to quantify. The use of water, of resources, of energy and the production of some wastes can be measured reasonably objectively. Other parts, particularly the allocation of numerical values to pollutant effects, are not objective and require value judgements. This makes life cycle assessment open to bias, and selective or abbreviated assessments can be devised to support a claim — for example, in advertising. Being able to say this is frequently worth a mark in itself.
Reducing use, reuse and recycling
The general 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 amount of separation required depends on the material and the use: for example, some scrap steel can be added directly to iron from a blast furnace to reduce the amount of iron ore needed.
Glass bottles can be reused. Alternatively, glass can be recycled by crushing it and then melting and reshaping it into new glass products.
Recycling saves in several ways that questions expect you to name: it reduces the use of finite raw materials, it usually requires much less energy than extracting a metal from its ore, it reduces the amount of waste sent to landfill, and it reduces the environmental damage of mining and quarrying. Against this, recycling requires collection, transport and sorting, which themselves use energy, and the recycled material may be of lower quality than newly produced material.
Worked examples
Example 1: Explaining why potable water is not pure (3 marks)
A bottle of drinking water is labelled pure. Explain why a chemist would disagree.
In chemistry a pure substance is a single element or compound with nothing else in it. Drinking water contains dissolved substances, including mineral salts, so it is a mixture rather than a single compound. It is potable, meaning safe to drink because levels of dissolved salts and microbes are low enough, but it is not chemically pure.
Example 2: Comparing two sources of potable water (4 marks)
A country with limited fresh water is considering desalination. Compare producing potable water by desalination with producing it from a river.
Producing potable water from a river requires only filtration to remove solids and sterilisation with chlorine, ozone or ultraviolet light, so it uses relatively little energy and is cheap. Desalination requires either distillation, which means boiling the water, or reverse osmosis through membranes, and both use large amounts of energy, making the process expensive. Desalination would only be chosen where fresh water sources are insufficient, since sea water is effectively unlimited whereas the river may not meet demand.
Example 3: Evaluating a life cycle assessment claim (3 marks)
A company claims its plastic bag has a lower environmental impact than a paper bag, based on a life cycle assessment. Suggest why this claim should be treated with caution.
Not all parts of a life cycle assessment can be measured objectively; assigning numerical values to the effects of pollutants requires value judgements. An assessment can also be shortened or made selective by leaving out stages that would be unfavourable. Because the company has an interest in the outcome, the assessment may be biased, so it would be more reliable if carried out independently.
Common mistakes and how to avoid them
The most common error in this unit is describing potable water as pure water. Potable means safe to drink; it contains dissolved substances and is therefore a mixture.
Students frequently swap the aerobic and anaerobic stages of sewage treatment. The effluent is treated aerobically with air bubbled through it; the sludge is digested anaerobically, without oxygen, producing methane.
Another regular slip is stating that desalination is not used because it is difficult. The specific reason is the large amount of energy required, which makes it expensive.
In recycling questions, many answers say only that recycling is good for the environment. Name the specific benefit: less use of finite resources, less energy than extraction from ore, less landfill, or less quarrying.
Finally, life cycle assessment answers often list only the four stages. Questions frequently ask about objectivity as well, so be ready to explain that quantifying pollutant effects requires value judgements.
Exam technique for "Chemistry: Using Resources"
Learn the four life cycle assessment stages as a sequence — raw materials, manufacture, use, disposal — and add transport, which applies at every stage and is often the missing mark.
Where a question asks you to compare two materials or two processes, use the same headings for both. Comparing energy for one and cost for the other does not constitute a comparison.
For potable water questions, always identify the starting source first, because the treatment required follows from it. Fresh water needs filtering and sterilising; salt water needs desalination.
Six-mark questions in this unit usually ask for an evaluation of recycling or of a life cycle assessment. Plan two advantages, two disadvantages and a conclusion before writing, 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 they are used, and sustainable development meets present needs without compromising future ones. Potable water is safe to drink but not chemically pure; fresh water is filtered and then sterilised with chlorine, ozone or ultraviolet light, while salt water requires energy-intensive desalination by distillation or reverse osmosis. Sewage treatment screens and removes grit, separates effluent from sludge by sedimentation, treats the effluent by aerobic biological digestion and the sludge by anaerobic digestion, which yields methane and fertiliser. Life cycle assessments cover raw materials, manufacture, use and disposal, and are not fully objective because pollutant effects require value judgements, so they can be biased or selective. Reducing, reusing and recycling all cut the use of finite resources, the energy needed, the waste sent to landfill and the damage from quarrying.