HookThe experiment a woman did in 1856 that no one listened to
In 1856 an American scientist named Eunice Newton Foote filled two glass cylinders with different gases, slid a thermometer into each, and left them in the sun. The cylinder holding carbon dioxide climbed to the highest temperature and, tellingly, stayed hot longest once she moved it into the shade. She wrote that an atmosphere richer in that gas "would give to our earth a high temperature". It was, three years before John Tyndall's celebrated infrared experiments, the first recorded description of the greenhouse effect — and because a colleague read her paper aloud for her at the 1856 science meeting and the field then looked elsewhere, her result sat all but ignored for over a hundred years.
C9 runs on two clocks at once. The slow clock is the 4.6-billion-year story of how the air became roughly four-fifths nitrogen and one-fifth oxygen. The fast clock is the two-century experiment humanity is running by digging up the carbon that took those aeons to bury and burning it in a few generations. Both are chemistry with consequences you can measure, balance, and — exactly as Foote did — test on a bench.
ModelToday's air, and how we pin down the recipe
The modern atmosphere has been remarkably steady for about 200 million years: roughly 78% nitrogen (about four-fifths), 21% oxygen (about one-fifth), and a final one per cent that is mostly argon plus a small but pivotal 0.04% carbon dioxide, with variable water vapour and traces of the other noble gases. Those two headline fractions — four-fifths and one-fifth — are worth memorising as a pair, because examiners ask for them as fractions, as percentages and as ratios, and one memorised pair answers all three.
The oxygen fraction is not just quoted, it is measured, and the classic school demonstration is a small piece of exam gold. Pass a fixed volume of air back and forth over heated copper: the copper turns black as it grabs oxygen to form copper oxide, and the gas volume shrinks by about a fifth before it stops falling, because only the oxygen has been removed. Reading a real percentage off a real experiment, rather than reciting a figure, is exactly the kind of quantitative handling the specification rewards.
Suppose you push 100 cm³ of air slowly and repeatedly over heated copper until the volume stops changing, and it settles at 79 cm³. The gas that disappeared was oxygen, reacting to form solid copper oxide, so the oxygen used is \(100-79 = 21\ \text{cm}^3\). As a fraction of the starting air that is \(\frac{21}{100} = 0.21\), or 21% — bang on the accepted value. Check the logic the other way: nitrogen is about four-fifths, \(\frac{4}{5}=0.80\) or 80%, close to the measured 78%, and the last slice is argon plus that vital 0.04% of carbon dioxide. One experiment, and the whole composition falls out of it.
MechanismThe early atmosphere and where it came from
The Earth formed about 4.6 billion years ago, and for its first billion years its surface was a furnace of intense volcanic activity. The leading theory is that this volcanism belched out the gases of the first atmosphere, and that it was dominated by carbon dioxide, with water vapour, some nitrogen, and smaller amounts of methane and ammonia — but little or no oxygen. Scientists point to the atmospheres of Mars and Venus today, both overwhelmingly carbon dioxide, as a snapshot of what the young Earth may have resembled.
As the planet slowly cooled, the water vapour condensed to form the oceans, and large amounts of carbon dioxide dissolved straight into that new water. The honest scientific point, and one examiners specifically credit, is that no samples of four-billion-year-old air exist. Every claim about the early atmosphere is reconstructed from indirect evidence, so it carries genuine uncertainty and is stated as a theory to be evaluated, not a settled fact to be recited.
MechanismHow oxygen rose and carbon dioxide fell
Two of the biggest shifts in Earth's history — oxygen rising, carbon dioxide falling — both come back to life. About 2.7 billion years ago primitive algae and cyanobacteria evolved photosynthesis, taking in carbon dioxide and water and releasing oxygen. For a long time the new oxygen was mopped up by dissolved iron, laying down the striped rocks geologists call banded iron formations; only once those sinks were saturated did oxygen build up in the air, over roughly a billion years, to the levels that made animal life possible.
Carbon dioxide fell for four connected reasons. It dissolved into the newly formed oceans; it was consumed by that ever-growing crowd of photosynthesisers; marine organisms used the dissolved carbon to build carbonate shells and skeletons that sank and were compressed into sedimentary rocks such as limestone; and vast quantities of carbon were locked into fossil fuels — coal from buried plants, oil and gas from buried plankton. In short, the carbon that once filled the sky was slowly filed away into rock and fuel. That buried reserve is precisely what humans are now digging up and burning.
The single reaction that reshaped the planet is worth being able to write: \[\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}\] Read it as a planetary bookkeeping entry — every glucose molecule an alga builds removes six molecules of carbon dioxide from the air and adds six of oxygen. Run that ledger across uncountable organisms for billions of years and you have simultaneously drained the early atmosphere of its carbon dioxide and filled it with the oxygen you are breathing right now.
MechanismThe greenhouse effect and the gases that thicken the blanket
Three gases do most of the warming: carbon dioxide, methane and water vapour. The greenhouse effect works in three steps you should be able to describe in order. Short-wavelength radiation from the Sun passes easily through the atmosphere and is absorbed by the ground, warming it. The warmed surface re-emits energy as longer-wavelength infrared radiation. Greenhouse gases absorb that outgoing infrared and re-radiate it in all directions, including back down to the surface — trapping heat that would otherwise escape to space.
This natural effect is not the villain; it is what keeps the planet liveable. Without it the average surface temperature would be roughly 30°C colder, well below freezing, and the Earth would be a frozen rock. The trouble is one of degree: add more greenhouse gas and you thicken the blanket, so more infrared is trapped and the surface warms further. Human activities driving that increase are the ones to name precisely — burning fossil fuels for electricity, transport and industry (carbon dioxide); deforestation, which both releases carbon and removes the trees that would absorb it; and agriculture, where livestock and flooded rice paddies release methane, with landfill waste adding still more. Separating the natural effect (essential) from this enhanced, human-driven warming is the distinction a strong answer draws explicitly.
DataReading the evidence: climate change and carbon footprints
Since 1958, continuous measurements from the Mauna Loa observatory in Hawaii — the famous Keeling Curve — have tracked carbon dioxide climbing from about 280 parts per million before the Industrial Revolution to around 420 ppm today. Evaluating this evidence is an examinable skill in its own right. The measurements of rising greenhouse gases are firm and peer-reviewed, but climate is a hugely complex system, and the models that project the future are simplifications carrying real uncertainty — which is why careful scientists give ranges rather than single numbers, and why media reports, often simplified or one-sided, must be read critically.
The likely consequences of continued warming include rising sea levels from melting ice and expanding oceans, more frequent extreme weather, and shifting rainfall that stresses farming and habitats. A carbon footprint is the total amount of greenhouse gas emitted over the whole life cycle of a product, service or event, expressed as carbon dioxide equivalent. It can be cut by switching to renewable energy, improving efficiency, capturing and storing carbon, and taxing or capping emissions — but every route runs into real barriers of cost, technology, and the political and lifestyle change people will accept.
First, size the change. Carbon dioxide has risen from 280 ppm to about 420 ppm, a rise of \[\frac{420-280}{280}\times 100 = \frac{140}{280}\times 100 = 50\%.\] A 50% increase in barely two centuries is the number that makes the graph alarming. Now a footprint comparison, kept deliberately honest: suppose a home emits about 2.4 tonnes of carbon dioxide a year heating with a gas boiler, and a heat pump on today's grid would cut that to roughly 0.8 tonnes — a saving of 1.6 tonnes. The catch a good answer flags is that this counts only the gas replaced; a full footprint must also add the emissions of manufacturing and installing the heat pump, which is why lifetime figures, not headline figures, are the fair comparison.
CasePollutants from burning fuels
When a hydrocarbon fuel burns in plenty of oxygen you get clean complete combustion — carbon dioxide and water only. Real engines, boilers and fires often burn in a limited oxygen supply, giving incomplete combustion and a set of dangerous by-products. Too little oxygen leaves carbon only partly oxidised as carbon monoxide (CO), a toxic, colourless, odourless gas that binds to haemoglobin in place of oxygen and can kill without warning — the silent poison behind faulty boilers. Even less oxygen leaves unburnt carbon as particulates, or soot, which cause global dimming by reflecting sunlight, blacken buildings, and lodge in lungs to cause respiratory disease.
Two further pollutants ride along with the carbon rather than coming from it. Coal and diesel contain sulfur impurities that burn to sulfur dioxide (\(\mathrm{SO_2}\)); and in the fierce heat of an engine, nitrogen and oxygen from the air itself react to form oxides of nitrogen (\(\mathrm{NO_x}\)). Both sulfur dioxide and the nitrogen oxides dissolve in rain to make acid rain, which kills trees, acidifies lakes and eats away limestone buildings and statues, and both irritate the airways. Sorting these pollutants by their cause — not lumping them together — is what turns a vague answer into a full-mark one.
Take propane, the gas in a camping stove, and show with equations how the oxygen supply decides the fate of its carbon. In plenty of oxygen combustion is complete: \[\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}\] In a restricted supply the carbon is only partly oxidised, to carbon monoxide: \[\mathrm{2C_3H_8 + 7O_2 \rightarrow 6CO + 8H_2O}\] and starved of oxygen it is left as solid soot: \[\mathrm{C_3H_8 + 2O_2 \rightarrow 3C + 4H_2O}\] The hydrogen still burns to water in every case; it is the carbon — ending as \(\mathrm{CO_2}\), \(\mathrm{CO}\) or \(\mathrm{C}\) — whose destination depends entirely on how much oxygen is available. Check each: three carbons and eight hydrogens balance across all three.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Quote the numbers. The atmosphere is about 78% nitrogen and 21% oxygen with roughly 0.04% carbon dioxide, and that carbon dioxide has risen from around 280 ppm to about 420 ppm — a 50% increase you can calculate. AO1 recall marks reward the precise figure; a bald 'lots of nitrogen' does not.
Keep mechanisms in their own lanes. Oxygen came from photosynthesis, not volcanoes; volcanoes supplied the early carbon dioxide. When a question is about pollutants (an AO2 application task), sort them by cause: incomplete combustion gives carbon monoxide and soot; sulfur impurities give sulfur dioxide; engine heat gives oxides of nitrogen; sulfur dioxide and nitrogen oxides give acid rain. Never mix the greenhouse gases (carbon dioxide, methane, water vapour) into the toxic-pollutants list — that muddle is a classic dropped mark. Balance every combustion equation and let the amount of oxygen decide whether carbon ends as carbon dioxide, carbon monoxide or soot.
The 6-mark 'evaluate the evidence' or 'discuss climate change' questions are pure AO3, and the marks are for handling uncertainty like a scientist: distinguish peer-reviewed measurements from media opinion, acknowledge that climate models are simplifications, and give consequences (sea-level rise, extreme weather, shifting rainfall) alongside the possible actions and their real barriers of cost and politics — not a one-sided answer.