What you'll learn
Using the Earth's resources sustainably is a major theme of GCSE Chemistry. In this guide you will learn how potable (drinking) water is produced and treated, how waste water is treated, the difference between finite and renewable resources, the principles of sustainable development, and how reusing and recycling materials and using life cycle assessments help conserve resources and reduce environmental impact. This topic links chemistry to the environment and society.
Key terms and definitions
Potable water — water that is safe to drink (low in dissolved salts and microbes), though not chemically pure.
Finite resource — a resource that will run out because it is not replaced as fast as it is used (e.g. crude oil, metal ores).
Sustainable development — meeting the needs of today without harming the ability of future generations to meet their needs.
Desalination — removing salt from sea water to make it potable.
Potable — fit to drink.
Life cycle assessment (LCA) — an evaluation of the environmental impact of a product over its whole life.
Core concepts
Potable water
Potable water is water that is safe to drink — it has low levels of dissolved salts and microbes, but is not pure water (it still contains dissolved substances). In the UK, potable water is usually produced from fresh water (rivers, lakes, groundwater) by:
- Choosing an appropriate source of fresh water.
- Filtering to remove solids (passing through filter beds).
- Sterilising to kill microbes, using chlorine, ozone or ultraviolet light.
Desalination
In countries with little fresh water, potable water is made from sea water by desalination, using distillation (boiling and condensing) or reverse osmosis (forcing water through a membrane). These methods need large amounts of energy, making them expensive — a reason they are used mainly where fresh water is scarce.
Treating waste water
Waste water (from homes, industry and agriculture) must be treated before being returned to the environment. Sewage treatment involves: screening to remove large solids and grit; sedimentation to let solids settle out as sludge; anaerobic digestion of the sludge by microbes; and aerobic biological treatment of the liquid effluent (using microbes and air) to break down organic matter and harmful microorganisms. Treating sewage requires more steps than treating fresh water because it contains more contaminants.
Finite and renewable resources
The Earth's resources include finite resources (such as fossil fuels and metal ores), which are being used up faster than they form, and renewable resources, which can be replaced. Chemistry helps by improving processes, developing alternatives and using resources more efficiently. Sustainable development aims to meet today's needs without compromising future generations.
Reducing, reusing and recycling
We can conserve resources and reduce environmental impact by reducing how much we use, reusing items, and recycling materials. Recycling metals saves the limited ore and uses much less energy than extracting new metal; glass can be reused or melted and reshaped. Recycling reduces waste sent to landfill and lowers the use of finite resources, though sorting and processing have their own energy costs.
Life cycle assessments
A life cycle assessment (LCA) evaluates the environmental impact of a product through its whole life: extracting and processing raw materials, manufacturing, use, and disposal (including transport at each stage). LCAs help compare products, but some parts (like the effect of pollutants) involve judgements rather than precise measurements, so they are not entirely objective and can be misused in misleading advertising.
Worked examples
Example 1: Producing potable water
Outline how fresh water is made potable.
Choose a suitable fresh water source, filter it to remove solids, then sterilise it (with chlorine, ozone or UV) to kill microbes.
Example 2: Why desalination is costly
Why isn't sea water usually used for drinking water in the UK?
Desalination (distillation or reverse osmosis) needs large amounts of energy, making it expensive, so fresh water is used where available.
Example 3: Benefit of recycling metal
Give one advantage of recycling metals.
Recycling saves limited metal ore and uses much less energy than extracting the metal from its ore.
Common mistakes and how to avoid them
Saying potable water is pure. Potable water is safe to drink but still contains dissolved substances.
Confusing fresh water and sewage treatment. Sewage needs more steps (sedimentation, digestion, biological treatment).
Forgetting the energy cost of desalination. It is effective but energy-intensive and expensive.
Treating LCAs as fully objective. Some impacts involve judgement, so LCAs can be biased.
Ignoring the benefits of recycling. It saves finite resources and energy and reduces landfill.
Exam technique for Using Resources
Describe potable water treatment in order: source, filter, sterilise.
Explain desalination methods and why they are costly.
Outline sewage treatment and why it needs more steps.
Define sustainable development and link reduce/reuse/recycle to conserving finite resources.
Evaluate LCAs, noting which stages are measured and which involve judgement.
Quick revision summary
Potable water is safe to drink (low in salts and microbes) but not pure. In the UK it is made from fresh water by choosing a source, filtering out solids, and sterilising with chlorine, ozone or UV. Where fresh water is scarce, desalination of sea water by distillation or reverse osmosis is used, but it needs lots of energy and is expensive. Waste water/sewage treatment needs more steps: screening, sedimentation, anaerobic digestion of sludge, and aerobic biological treatment of effluent. The Earth's finite resources (fossil fuels, metal ores) are running out, so sustainable development — meeting today's needs without harming the future — matters. Reducing, reusing and recycling conserve resources: recycling metals saves ore and energy compared with extraction. A life cycle assessment (LCA) evaluates a product's environmental impact across raw materials, manufacture, use and disposal, but some stages involve judgement and can be misused. Describe water and sewage treatment in order, explain desalination's cost, and evaluate recycling and LCAs critically.