Using Resources — AQA Combined Science: Trilogy

This unit is about how humans use the Earth's resources sustainably: water treatment, life cycle assessment, recycling and alternative ways of extracting metals.

Finite and renewable resources

The Earth's resources provide everything we need: warmth, shelter, food and transport.

• Finite resources will run out (e.g. fossil fuels, metal ores). They form much more slowly than we use them.
• Renewable resources can be replaced at the rate they are used (e.g. timber from replanted forests).

Chemistry improves agricultural and industrial processes to provide new products and to make resources go further. Sustainable development meets the needs of today without compromising the ability of future generations to meet their own needs.

Potable water

Potable water is water that is safe to drink — it is not pure (it contains dissolved substances) but has low levels of dissolved salts and microbes.

Producing potable water in the UK (from fresh water):

1. Choose an appropriate source (e.g. rivers, reservoirs, aquifers).
2. Filter to remove solid particles (e.g. through sand/gravel).
3. Sterilise to kill microbes, using chlorine, ozone or ultraviolet light.

From sea water (desalination)

Where fresh water is scarce, sea water is desalinated by distillation or by reverse osmosis. These methods need large amounts of energy, making them expensive.

Required practical (separate science): analysis and purification of water samples, including distillation.

Waste water treatment

Sewage and agricultural/industrial waste water must be treated before being released. Treatment involves:

1. Screening and grit removal — removing large solids and grit.
2. Sedimentation — solids settle out to form sludge; liquid effluent stays on top.
3. Aerobic biological treatment — bacteria break down organic matter and harmful microbes in the effluent (with air bubbled through).
4. The sludge is treated by anaerobic digestion (producing biogas and fertiliser).

Treating sewage requires more steps than treating fresh ground water, but uses less energy than desalinating sea water.

Alternative methods of extracting metals

Most copper now comes from low-grade ores because high-grade ores are running out. New biological methods avoid traditional mining:

• Phytomining — plants absorb metal compounds from the soil as they grow. The plants are burned, and the ash contains the metal compounds, from which the metal is extracted.
• Bioleaching — bacteria produce solutions (leachates) containing metal compounds from low-grade ores, without the need for high temperatures.

These methods have a lower environmental impact than traditional mining and let us use ores that would otherwise be uneconomic, but they are slow. The metal can be obtained from the solutions by displacement with scrap iron or by electrolysis.

Life cycle assessment (LCA)

A life cycle assessment evaluates the environmental impact of a product over its whole life:

1. Extracting and processing raw materials.
2. Manufacturing and packaging.
3. Using and operating the product.
4. Disposal at the end of life (including transport at every stage).

LCAs consider use of resources, energy, water, and the release of pollutants. Some impacts (like CO₂ produced) are easy to quantify; others (like the effect of pollutants) require value judgements, so LCAs can be biased — for example, when used in advertising. Be ready to interpret and critically evaluate LCA data.

Reduce, reuse and recycle

Using less of a resource (reducing), reusing items, and recycling all conserve finite resources and reduce energy use and waste.

• Recycling metals saves the energy and ore needed to extract new metal. Metals are melted and recast; some need sorting by type first.
• Glass can be reused or crushed, melted and reshaped.

Recycling reduces the use of limited resources, the use of energy, waste, and environmental impact.

Exam tips

• Distinguish potable water (safe to drink, not pure) from pure water (no dissolved substances).
• Learn the steps for producing potable water (filter then sterilise) and treating waste water.
• Be able to describe phytomining and bioleaching and give their advantages and disadvantages.
• For LCA questions, work through the four stages and recognise where value judgements make them subjective.
• Explain why recycling metals saves both energy and finite resources.