GCSE Chemistry Resources revision

Finite resources — crude oil, metal ores, minerals — form over millions of years, so every tonne used is effectively gone. Renewable resources (timber from managed forests, wind, solar, crops) replace themselves within a human lifetime. Humans supplement natural supplies with agriculture (crops, livestock, timber plantations) to provide food, wood and fibres at the scale needed. Sustainable development is the guiding idea: meeting the needs of people today without preventing future generations meeting theirs — using resources no faster than they renew, and recycling what does not.

Potable water is water safe to drink: levels of dissolved salts and microbes low enough not to harm you. It is not chemically pure — pure water contains nothing but H₂O molecules, and almost no natural water is like that. UK supplies start from rain (fresh, low in dissolved solids) collected in reservoirs, rivers or groundwater aquifers, then pass through three steps: sedimentation (solids sink), filtration (a sand/gravel bed removes finer solids) and sterilisation — chlorine gas, ozone or ultraviolet light — to kill microbes.

Where fresh water is scarce, seawater is desalinated: distillation boils the water and condenses the vapour, leaving salts behind; reverse osmosis pushes water through a membrane that blocks ions. Both work — both are expensive in energy (pumps, heat), so they suit wealthy or water-poor regions such as the Gulf states.

1. 1Required practical: analyse a water sample

   Step 1 — measure the pH with universal indicator or a pH meter to check acidity (pure water sits at pH 7).

2. 2Dissolved solids

   Step 2 — evaporate a measured volume (e.g. 50 cm³) in an evaporating basin and weigh the residue; the mass of solid per volume reveals the dissolved-salt load.

3. 3Purify

   Step 3 — distil the sample: water boils first, cools in the condenser, and collects as pure liquid; salts (non-volatile) stay in the flask. Confirm purity by re-checking pH and re-evaporating a fresh sample to show no residue.

Sewage (plus agricultural and industrial waste water) must be cleaned before it re-enters rivers, or pathogens and organic matter would spread disease and strip oxygen from the water. Treatment runs: screening (remove large objects) → sedimentation (settle into sludge at the bottom and effluent above) → the sludge is digested anaerobically by microbes, producing biogas (fuel) and a digested residue usable as fertiliser → the effluent is aerobically treated (oxygen-loving microbes consume the organic matter) and then released safely. Water from sewage treatment is not usually drinking-water standard, but it spares the environment the raw load.

A life cycle assessment (LCA) totals a product’s environmental impacts across four stages: extracting raw materials (mining damage, energy), manufacturing (energy, emissions, waste), use (fuel, electricity, emissions — or little for a passive product) and disposal (landfill, incineration, recycling). Carrying an LCA honestly is hard: choosing which impacts to count and how to weight them involves judgement, so LCAs can be selective — a famous pitfall when companies advertise only the flattering numbers.

Recycling cuts the demand side: metals are separated, melted and recast (saving the enormous energy of electrolysis or reduction from ore); glass is crushed and remelted; plastics are sorted by type. Reuse (refillable bottles) beats recycling because the product skips reprocessing entirely. The order of preference: reduce, then reuse, then recycle — and energy saved is the headline number: recycling aluminium uses a small fraction of the energy of extracting new aluminium from bauxite.

Corrosion destroys surfaces by reaction with oxygen and moisture. Rusting specifically needs iron + oxygen + water, producing hydrated iron(III) oxide — the flaky brown coating that flakes off and lets rust eat deeper. (Aluminium corrodes too, but its oxide layer is thin and sticks, protecting the metal — that is why aluminium "resists".) Prevention: barriers (paint, oil, grease, plastic coating) keep oxygen and water off; sacrificial protection attaches a more reactive metal — zinc on galvanised steel or magnesium blocks — which oxidises in the iron’s place, keeping the iron safe even if the coating scratches.

Alloys tune properties by mixing in other atoms that disturb the regular lattice. Gold is alloyed for durability: 24 carat is pure, 18 carat is 75% gold (18/24). Bronze (copper + tin) is harder than copper; brass (copper + zinc) is harder and works for taps and instruments. Steels range from low-carbon (softish, easily shaped, for car bodies) to high-carbon (hard but brittle, for tools), with stainless steel (chromium and nickel added) resisting corrosion for cutlery.

Ceramics (pottery, glass, brick) are hard, strong in compression, stiff, brittle and good insulators — kiln-fired clay or melted sand. Polymers split by behaviour on heating: thermosoftening plastics (no cross-links) soften and remould, while thermosetting plastics (cross-linked chains) char instead of melting — pan handles. Chain packing matters too: HDPE (less branched, chains pack closely, strong intermolecular forces) is stiffer and stronger than LDPE (branched, flexible). Composites marry a matrix (binder) with a reinforcement: glass fibres in polymer make fibreglass (light, strong, flexible); carbon fibre in resin builds aircraft parts; concrete is a matrix of cement binding sand and gravel aggregates; MDF is wood fibres in resin.