GCSE Combined Science Atmosphere & resources revision

Phase 1 — volcanoes. The early atmosphere was probably dominated by carbon dioxide, with water vapour and nitrogen, released by intense volcanic activity. (There may also have been small amounts of methane and ammonia.) Almost no oxygen — nothing could breathe it yet.

Phase 2 — the oceans. As Earth cooled, the water vapour condensed and rained down to form the oceans. Carbon dioxide dissolved into all that water, and some also locked away in carbonate rocks formed through reactions — so the atmosphere's CO₂ share fell sharply. Nitrogen, which barely dissolves, built up instead.

Phase 3 — life goes green. From about 2.7 billion years ago, algae and green plants photosynthesised, absorbing carbon dioxide and releasing oxygen. Over billions of years the oxygen share climbed to today's level — enabling animals. Today's atmosphere: about 78% nitrogen, 21% oxygen, roughly 1% argon, and around 0.04% carbon dioxide, plus a changing amount of water vapour.

Some gases in the atmosphere — water vapour, carbon dioxide and methane — let sunlight through but absorb the long-wave (infrared) radiation the warm Earth radiates back out, then re-radiate it in all directions, including downwards. This natural greenhouse effect keeps Earth about 33 °C warmer than an airless world — without it the surface would freeze.

Human activity — burning fossil fuels, deforestation, farming and landfill — has raised CO₂ and methane levels. More greenhouse gas means more absorbed radiation, so more energy is trapped: the enhanced greenhouse effect, driving global warming and the wider climate change that follows (rising seas, changing rainfall, more extreme weather).

How do we know the warming is ours? The evidence is peer-reviewed — checked by independent experts — built on systematic measurements repeated over decades, and agreed by many independent sources: surface temperature records, satellite data, and natural archives such as ice cores (ancient air bubbles) and tree rings. No single dataset carries the argument; their agreement does.

A carbon footprint is the total greenhouse gas emitted over the full life cycle of a product, service or event — from raw materials to disposal. Footprints shrink by using less energy, switching to renewables, or changing processes — but reductions are hard because they need international cooperation (one country's cut is undone elsewhere) and carry economic costs: jobs, prices, competitiveness.

Fossil-fuel combustion releases several pollutants besides carbon dioxide. Carbon monoxide forms when oxygen is short — it is toxic because it binds to haemoglobin in place of oxygen. Soot (carbon particulates) also forms in poor combustion; particles worsen breathing problems and cause global dimming by reflecting sunlight. Sulfur dioxide forms when fuels containing sulfur impurities burn, and oxides of nitrogen form when nitrogen and oxygen react at the high temperatures inside engines. Both sulfur dioxide and nitrogen oxides dissolve in rain to make acid rain, which damages buildings and ecosystems, and both irritate lungs.

Earth's resources split into finite ones — fossil fuels, metal ores, minerals, used far faster than they form — and renewable ones that regenerate (wind, solar, timber from managed forests). Sustainable development means meeting today's needs without wrecking the ability of future generations to meet theirs. Chemistry's contribution: finding alternatives, cleaning what we have, and recycling what we use.

Potable water is water safe to drink — importantly, it is not chemically pure: safe levels of dissolved substances may remain, and microbes must be absent. UK treatment: choose a fresh source (reservoir, river, aquifer), let solids settle (sedimentation), filter out finer particles, then sterilise to kill microbes with chlorine, ozone or ultraviolet light. In dry regions seawater is desalinated — by distillation (boil, condense) or reverse osmosis (push water through a membrane) — both reliable but energy-expensive.

Urban waste water is cleaned in stages: screening removes large objects; sedimentation splits the flow into sludge (solids) and effluent (liquid); the sludge is digested anaerobically by bacteria, producing biogas (a fuel) and a safer residue; the effluent is treated aerobically — bacteria break down the remaining organic matter with oxygen — before the cleaned water is released.

(Higher) Low-grade copper ores — too poor for normal smelting — can still be mined with living helpers. Phytomining: plants are grown on the ore; they absorb copper ions as they grow; they are harvested and burned, leaving ash rich in copper. Bioleaching: bacteria are used to produce a leachate — a solution containing copper compounds — from the ore. In both routes the copper is then recovered from the ash or leachate by displacement with scrap iron or by electrolysis. These methods squeeze value from ores we would otherwise waste, though they are slow.

A life cycle assessment (LCA) tallies a product's environmental impacts across every stage: extracting raw materials → manufacturing → packaging and transport → use → disposal. Each stage has impacts — energy used, pollutants released, water consumed. But LCAs are not fully objective: the assessor chooses what to count and how to weight it, so they can be selective — a company may showcase one green stage and quietly skip another, or assign numbers that flatter its product.

The reduce–reuse–recycle ladder cuts resource use: reduce (less material per product), reuse (same item, again), recycle (process into new stock — metals, glass, plastics). Recycling metals saves enormous energy because melting scrap avoids both the extraction of ore and the processing of new metal — recycling aluminium, for instance, needs only a small fraction of the energy of electrolysis from ore.