Finite, renewable, natural, synthetic

We use the Earth's resources for warmth, shelter, food and transport. Some come straight from nature; others we make or grow on purpose.

• Natural resources — provided by the Earth, sea and atmosphere (e.g. wood, cotton, crude oil, ores).
• Synthetic / agricultural products — made or farmed to supplement or replace natural ones (e.g. synthetic rubber replacing latex, fertilisers boosting crops).
• Finite resources are being used faster than they re-form — once gone, they're gone (crude oil, metal ores, natural gas).
• Renewable resources re-form, or are replaced, as fast as we use them (timber from replanted forests, fresh water from the water cycle).

Sustainable development = meeting the needs of today without stopping future generations meeting their own needs. Chemistry helps by improving processes, finding substitutes and reducing waste.

Finite or renewable?

Natural, synthetic, finite, renewable

Pick the right description for each example. Three in a row to clear it.

The whole life of a product

An LCA weighs up a product's environmental impact across its entire life — not just when you use it. It has four stages:

Watch out: an LCA covers the whole life of a product. A bag that's "greener" to use might be far worse to make or dispose of. Use of water, energy and waste can be measured fairly easily, but putting numbers on pollution needs value judgements — so an LCA is not purely objective, and shortened "selective" LCAs can be misused in advertising.

Which LCA stage?

Reduce ▸ Reuse ▸ Recycle

Cutting how much we take from the Earth saves raw materials, energy, waste and environmental damage. The three R's run in order of impact:

• Reduce — use less in the first place. Best of all: avoids extraction and processing energy.
• Reuse — use a product again as it is (e.g. glass bottles washed and refilled).
• Recycle — reprocess into new products. Glass bottles can be crushed and melted; metals are recycled by melting and recasting. Some scrap steel is added to iron from the blast furnace, so less iron must be extracted from ore.

Watch out: the order matters — reduce > reuse > recycle. Recycling still uses energy (melting, separating), so it's the last resort, not the first. The amount of separation needed depends on the material and the final product's properties.

Potable water — not the same as pure

Potable water is water that is safe to drink: low levels of dissolved salts and microbes. It is not chemically pure — it still contains dissolved substances.

Watch out: potable ≠ pure. Pure water has nothing dissolved in it; potable water just has to be safe. Filtering removes solids — it does not make water sterile; sterilising (chlorine, ozone or UV) kills the microbes.

Required practical 8: analysis & purification of water samples — measure pH, evaporate to find dissolved solids, and use distillation to produce pure water from a sample.

Potable vs pure water

Treating waste water

Sewage and agricultural waste water must have organic matter and harmful microbes removed before it returns to the environment. Sewage treatment runs in order:

• 1 · Screening & grit removal — large solids and grit are filtered out.
• 2 · Sedimentation — solids settle to form sludge; the liquid on top is the effluent.
• 3 · Anaerobic digestion of the sludge (microbes work without oxygen).
• 4 · Aerobic biological treatment of the effluent (microbes work with air/oxygen pumped through).

Compare the sources: it's easiest to make potable water from fresh ground/surface water (just filter + sterilise), harder from waste water, and hardest (most energy) from salt water (desalination).

Order the treatment steps

Tap a step, then tap the box it belongs to. Sort the four sewage treatment stages into the right order.

Corrosion & how to stop it

Corrosion is the destruction of materials by reaction with substances in the environment. For iron this is rusting, which forms hydrated iron(III) oxide.

Rusting needs BOTH things present together:

We prevent corrosion in two ways:

• Barrier methods — exclude oxygen and/or water: paint, oil, grease, plastic coating, or electroplating.
• Sacrificial protection — attach a more reactive metal (e.g. zinc or magnesium). It corrodes instead of the iron. Galvanising coats iron with zinc and works as both a barrier and sacrificial protection.

Watch out: rusting needs BOTH air (oxygen) AND water — remove either and it stops. The sacrificial metal must be MORE reactive than iron (so it is the one that reacts), not less.

What makes iron rust?

Alloys — useful mixtures

Pure metals are often too soft because their layers of atoms slide over each other. An alloy mixes in atoms of a different size, which distort the layers so they can't slide as easily — making the metal harder and more useful.

• Bronze = copper + tin.
• Brass = copper + zinc.
• Gold for jewellery is alloyed with silver, copper and zinc; purity is measured in carats (24-carat = pure; 18-carat = 18/24 = 75% gold).
• Steels are iron alloys: high-carbon steel is strong but brittle; low-carbon steel is softer and easily shaped; stainless steel (chromium + nickel) resists corrosion.
• Aluminium alloys are low density yet strong — used for aircraft.

Watch out: an alloy is a mixture, not a compound. The different-sized atoms are what stop the layers sliding, so an alloy is harder than the pure metal.

Ceramics, polymers & composites

• Ceramics — soda-lime glass (heat sand, sodium carbonate & limestone, then cool) vs borosilicate glass (made from sand + boron trioxide, melts at higher temperatures). Clay ceramics (pottery, bricks) are made by shaping wet clay then firing in a kiln.
• Polymers — properties depend on the monomer and the conditions. Low-density poly(ethene) (LDPE) is made at high pressure with a small amount of oxygen; high-density poly(ethene) (HDPE) is made at lower temperature/pressure with a catalyst.
• Thermosoftening polymers have separate, tangled chains with weak forces between them — they melt and can be remoulded. Thermosetting polymers have cross-links (strong covalent bonds) between chains — they do not melt when heated.
• Composites = a matrix/binder surrounding reinforcement (fibres or fragments), e.g. fibreglass, carbon-fibre, concrete. You choose a material by matching its properties to the job.

Watch out: thermosetting polymers cannot be re-melted and reshaped — their cross-links lock the chains in place. Only thermosoftening polymers soften and remould on heating.

Materials check

Phytomining & bioleaching

Copper-rich ores are running out, so we extract copper from low-grade ores (too little metal for normal smelting) using biology — avoiding the digging, moving and disposing of huge amounts of rock.

Watch out: phytomining and bioleaching are for low-grade ores — they're slow but avoid the huge environmental cost of traditional mining. Iron is more reactive than copper, so scrap iron displaces copper from solution.

Which method, and why?

The Haber process

The Haber process makes ammonia (used for fertilisers) from nitrogen (from the air) and hydrogen (from natural gas). It's reversible:

Watch out: the Haber conditions are a compromise. Because it's reversible, you can never convert everything in one pass — the recycle loop is what makes it efficient overall.

Why those conditions? (equilibrium)

Use Le Chatelier's principle — the position of equilibrium shifts to oppose a change — to explain the compromise:

• Pressure ~200 atm: high pressure shifts equilibrium to the side with fewer gas molecules (4 on the left → 2 on the right), giving more ammonia. We don't go higher because very high pressure is dangerous and expensive.
• Temperature ~450°C: the forward reaction is exothermic, so a lower temperature would give more ammonia at equilibrium — but the rate would be far too slow. 450°C is a compromise between yield and rate.
• Iron catalyst: speeds up the rate (reaching equilibrium faster) but does not change the position of equilibrium or the yield.

Trade-off: low temperature = higher yield but slow; high temperature = fast but lower yield. 450°C balances rate vs equilibrium position, alongside the cost of raw materials and energy.

Le Chatelier reasoning

NPK fertilisers

Plants need Nitrogen, Phosphorus and K (potassium). An NPK fertiliser is a formulation — a mixture of salts giving the right percentage of all three.

• Ammonia (from Haber) → ammonium salts and nitric acid (a nitrogen source).
• Potassium chloride, potassium sulfate and phosphate rock are mined.
• Phosphate rock can't be used directly — it's treated with acid: with nitric acid → phosphoric acid + calcium nitrate; with sulfuric acid → "single superphosphate"; with phosphoric acid → "triple superphosphate".

Industry vs lab: industrial NPK production uses several integrated processes at huge scale to make a formulation. A laboratory preparation makes a single pure salt in small amounts (e.g. AT4 — preparing an ammonium salt by titration/crystallisation). Same chemistry, very different scale and purpose.

Lock it in

One last challenge — then you're done. 💪

Haber conditions