AQA GCSE Chemistry (8462) · Paper 2
♻️ Using Resources
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Water & the Earth's resources
Humans use the Earth's resources for warmth, shelter, food and transport. Finite resources, such as fossil fuels and metal ores, cannot be replaced once they are used up, while renewable resources, such as timber, cotton and fresh water, regrow or replenish naturally. Sustainable development means meeting the needs of today without compromising the ability of future generations to meet their own needs, which is the exact phrase examiners want to see.
Drinking water
Potable water is water that is safe to drink; it is not chemically pure, since it still contains dissolved minerals. In the UK, this is achieved by choosing a good fresh water source, passing it through filter beds to remove solids, and then sterilising it with chlorine, ozone or UV to kill microbes.
Where fresh water is scarce, desalination of seawater is an alternative, using either distillation or reverse osmosis. Both methods need large amounts of energy, which makes them expensive.
Waste water treatment begins with screening, which removes grit, followed by sedimentation, which allows solids to settle out as sludge. The sludge then undergoes anaerobic digestion, which produces biogas, while the remaining liquid undergoes aerobic biological treatment before it is released.
At Higher tier: as metal ores run low, chemists are using living organisms to extract more metal. In phytomining, plants absorb metal compounds from the soil and are then burned, so the metal can be extracted from the ash. In bioleaching, bacteria produce soluble metal compounds, called a leachate, from low-grade ores. Both methods are slower than traditional mining, but they can be used on ore that is too poor in metal to mine conventionally.
LCAs & recycling
A Life Cycle Assessment audits the whole environmental impact of a product, in four stages:
- Extracting and processing the raw materials.
- Manufacturing the product and its packaging.
- Using the product over its lifetime.
- Disposing of it at the end of its life, whether by landfill, incineration or recycling.
Some of the data used, such as energy, water and waste, is objective, but assigning a value to pollution involves judgement, so LCAs can be biased, and a selectively chosen LCA can be used to support an advert's claims. A classic exam example compares plastic and paper bags: plastic often uses less energy to manufacture, while paper degrades more easily, so a good answer argues both sides.
Reduce, reuse, recycle cuts the energy use, resource use and waste involved in extracting new materials. Metals can be melted down and recast; for example, recycling aluminium saves around 95% of the energy needed to smelt new aluminium. Glass can be remelted too, avoiding the scars left by quarrying and mining.
Materials, Haber & NPK (Triple) Separate science only
Corrosion
Rusting needs both oxygen and water to happen. This can be shown experimentally: a nail kept in dry air stays fine, a nail kept in boiled, airless water also stays fine, but a nail exposed to both oxygen and water rusts. Rusting can be prevented with a barrier, such as paint, oil or plastic, with galvanising, where a zinc coating acts as both a barrier and a backup, or with sacrificial protection, where a more reactive metal such as zinc or magnesium is attached and corrodes first instead.
Alloys and materials
The Haber process
Ammonia for fertilisers is made at around 450°C and 200 atmospheres of pressure, over an iron catalyst.
Higher content: these conditions are a compromise. The forward reaction is exothermic, so a lower temperature would give more ammonia but far too slowly; a high pressure favours the side with fewer gas molecules, since four molecules of gas become two, but a very high pressure costs more in energy and safety. 450°C and 200 atmospheres give the best profit, and the catalyst only speeds up how quickly equilibrium is reached, without changing its position.
NPK fertilisers
NPK fertilisers are formulations of the three main plant nutrients: nitrogen, from ammonia converted into ammonium nitrate; phosphorus, from phosphate rock treated with acid; and potassium, mined directly as potassium chloride or potassium sulfate. Industrial production runs continuously and at a large scale, whereas the school-laboratory version, using titration and crystallisation, is small-batch and slow; this comparison is often needed in evaluation questions.