Using resources

Explanation

• Natural resources provide warmth, shelter, food and transport; farmed timber and cotton can supplement wild supplies, while synthetic fibres and rubber can replace natural materials.
• Classify a resource as finite if it is used much faster than it is replaced, and as renewable if natural processes replace it on a human timescale.
• Sustainable development meets present needs without reducing the ability of future generations to meet theirs, so comparisons should include material use, energy, waste and environmental effects.
• A common error is to call every natural resource renewable: crude oil and metal ores are natural but finite, whereas sustainably managed timber can be renewable.
• Whether a supply is renewable also depends on the rate of use and management, not simply whether the material originated in nature.

Explanation

• Potable water is safe to drink because it has sufficiently low levels of dissolved salts and microbes; it is not chemically pure because dissolved substances remain.
• In the UK, potable water is usually made by selecting a suitable fresh-water source, passing the water through filter beds and sterilising it with chlorine, ozone or ultraviolet light.
• In water analysis, measure pH and dissolved solids and use distillation for purification; where fresh water is scarce, distillation or reverse osmosis can desalinate salty water but requires substantial energy.
• A common error is to say filtration sterilises water: filter beds remove suspended solids, whereas sterilisation kills harmful microorganisms.
• A potable-water process must be selected for the source: fresh water generally needs less energy to treat than seawater.

Explanation

• Sewage and agricultural waste water need organic matter and harmful microbes removed; industrial waste water may also contain harmful chemicals requiring extra treatment. Sewage treatment begins with screening and grit removal, followed by sedimentation that separates sewage sludge from liquid effluent.
• The sludge is treated by anaerobic digestion, while the effluent receives aerobic biological treatment before it is released.
• Fresh ground water is generally easiest to make potable; waste water needs biological treatment and salt water needs energy-intensive desalination.
• Do not reverse the anaerobic sludge and aerobic effluent stages.
• The amount and type of contamination determine why industrial waste water may need additional chemical treatment.

Explanation

• Higher tier: Phytomining uses plants to absorb metal compounds from low-grade ore; the plants are harvested and burned, leaving ash rich in metal compounds.
• Bioleaching uses bacteria to produce a leachate solution containing metal compounds, avoiding the movement and disposal of large amounts of rock.
• Copper can be recovered from these solutions by electrolysis or by displacement with a metal more reactive than copper, such as scrap iron.
• A common error is to claim biological extraction gives pure metal directly: both methods first produce metal compounds that need further processing.
• Higher tier: these methods can exploit low-grade ores with less rock movement, but extraction is slow and further processing still uses energy.

Explanation

• A life cycle assessment considers extracting and processing raw materials, manufacture and packaging, use during the product's lifetime, and end-of-life disposal, including transport at every stage.
• Compare products using available data for energy, water, resources and waste, and keep the functional use the same before making numerical comparisons.
• For comparisons such as paper and plastic shopping bags, use the same function: divide a reusable product's lifetime impacts by its uses and add per-use impacts such as washing.
• A common error is to treat an LCA as fully objective: pollutant effects can require value judgements, and selective LCAs can omit stages to support a preferred conclusion.
• Transport can contribute at every stage, and a fair comparison must use consistent system boundaries for both products.

Explanation

• Reducing use, reusing products and recycling materials can lower demand for limited raw materials, energy use, waste and environmental damage.
• Reuse keeps a product in service without remaking it; recycling processes the material into a new product and usually requires collection, separation and energy.
• Metals can be melted and recast, glass can be crushed and remelted, and scrap steel can replace some newly extracted iron in steel production.
• A common error is to assume recycling has no impact: it often saves raw materials and energy but still needs transport, sorting and processing.
• The best option depends on the material and product because reduction usually avoids more processing than reuse or recycling.

Explanation

• Corrosion is the destruction of a material by chemical reactions with substances in its environment; rusting is the corrosion of iron and needs both oxygen and water.
• To test rusting conditions, change the presence of air or water while keeping identical iron samples, temperature and time the same.
• Barrier methods such as grease, paint and electroplating keep air and water away; aluminium protects itself with an adherent oxide layer.
• A common error is to say any metal coating protects a scratch: sacrificial protection works only when the coating metal is more reactive than iron, as zinc is in galvanising.
• An experiment establishing rusting conditions needs comparison samples that isolate oxygen and water while all other variables remain controlled.

Explanation

• Bronze is copper and tin and is used for statues; brass is copper and zinc and is used for fittings. Jewellery gold is alloyed with silver, copper or zinc: $24$ carat is pure gold and $18$ carat is $75\%$ gold.
• High-carbon steel is strong but brittle and suits cutting tools; low-carbon steel is softer and shapeable for car bodies; chromium-nickel stainless steel resists corrosion and suits sinks or cutlery.
• Choose an alloy by linking composition to measured properties and the intended use; low-density aluminium alloys are useful for aircraft parts.
• A common error is to describe an alloy as a compound: its proportions can vary, and different compositions can give different properties.
• Different steels are not interchangeable: carbon content and alloying elements change strength, brittleness, shapeability and corrosion resistance.

Explanation

• Soda-lime glass is made by heating sand, sodium carbonate and limestone; borosilicate glass contains sand and boron trioxide and melts at a higher temperature. Clay ceramics are shaped while wet and heated in a furnace; different reaction conditions allow the same ethene monomer to form both LDPE and HDPE.
• Thermosoftening polymers melt on heating because chains can move past one another, whereas cross-links between chains prevent thermosetting polymers from melting.
• A composite has a matrix or binder surrounding a reinforcement; examples include glass-fibre polymer, carbon-fibre polymer and concrete.
• Link the components' combined properties to the use.
• Material comparisons may require quantitative data, so values and units should support rather than replace the structure-property explanation.

Explanation

• The Haber process makes ammonia from nitrogen obtained from air and hydrogen commonly obtained from natural gas: $\mathrm{N_2+3H_2\rightleftharpoons2NH_3}$.
• Purified gases pass over an iron catalyst at about $450$ °C and $200$ atmospheres; cooling liquefies ammonia, and unreacted nitrogen and hydrogen are recycled.
• Higher tier: the equation gives a $1:3:2$ mole ratio and the same gas-volume ratio at equal temperature and pressure, so one amount of nitrogen reacts with three of hydrogen to form two of ammonia.
• Higher tier: lower temperature and higher pressure favour ammonia equilibrium yield, but industry compromises for a fast enough rate and acceptable energy and equipment costs; a catalyst changes rate, not equilibrium position.
• Removing liquid ammonia and recycling unreacted gases allow continuous production despite incomplete conversion in one pass.

Explanation

• NPK fertilisers are formulations containing salts that supply nitrogen, phosphorus and potassium in suitable percentages to improve agricultural productivity.
• Ammonia is used to make ammonium salts and nitric acid; potassium chloride, potassium sulfate and phosphate rock are obtained by mining.
• Phosphate rock is insoluble and is treated with acids to make useful salts: nitric acid gives calcium nitrate, sulfuric acid can give calcium sulfate or single superphosphate, and phosphoric acid gives calcium phosphate or triple superphosphate.
• A common error is to describe industrial fertiliser production as a scaled-up school crystallisation only: industry uses integrated, often continuous processes with different raw materials and controls.
• The required NPK percentages make the fertiliser a formulation rather than a single pure compound.