GCSE Chemistry Atmosphere revision

Phase 1 — volcanic beginnings. The first atmosphere is thought to have been belched out by intense volcanic activity: mostly carbon dioxide and water vapour, with nitrogen building up and — crucially — little or no oxygen. Phase 2 — the oceans form. As Earth cooled, the water vapour condensed and rained down to form the oceans, which dissolved much of the carbon dioxide; some reacted with dissolved minerals to form carbonate sediments, locking CO₂ into rock. Phase 3 — the green revolution. From about 2.7 billion years ago, algae and then plants photosynthesised: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Oxygen accumulated in the air while carbon dioxide was consumed — the two gases essentially swapped places over hundreds of millions of years.

These are theories built from indirect evidence: rocks and their chemistry, ancient sediments, and comparisons with other planets. No one was there to measure — so the phrasing "is thought to" matters in exam answers.

Compared with the early atmosphere, the story is stark: oxygen rose from nothing to a fifth of the air; carbon dioxide fell from the dominant gas to a trace; water vapour mostly left the sky by condensing into oceans; and nitrogen, unreactive and slowly accumulated, became the bulk gas. The trace 0.04% of CO₂ is tiny — but powerful, because it is a greenhouse gas, and it is the fraction humans are steadily enlarging.

Short-wave radiation from the Sun passes through the atmosphere and warms the surface. Earth re-radiates that energy as long-wave (infrared) radiation — but greenhouse gases (water vapour, carbon dioxide, methane) absorb it and re-radiate it back in all directions, including downwards. That keeps Earth about 33 °C warmer than it would otherwise be — the greenhouse effect, a natural and life-saving phenomenon. The enhanced greenhouse effect is the human addition: burning fossil fuels, deforestation and agriculture (cattle, rice paddies, fertilisers) add CO₂ and CH₄ faster than the natural sinks absorb them, so more energy is trapped and global temperature rises — global warming.

The evidence for human-caused climate change is broad and peer-reviewed: direct temperature records since the 1850s, ice cores whose trapped air bubbles archive CO₂ levels back hundreds of thousands of years, tree rings and historical records, plus modern satellite measurements. Many independent lines of evidence pointing the same way is what makes the conclusion robust — and what distinguishes climate (long-term averages) from weather (one day).

A carbon footprint is the total greenhouse gas emitted over the full life cycle of a product, service or event — from raw materials through manufacture, use and disposal. Footprints shrink through renewable energy, energy efficiency, less consumption, carbon offsets and carbon capture and storage. Why is progress slow? Cutting emissions meaningfully needs international cooperation (no single country can solve it), costs money in the short term, and lifestyle changes are unpopular — economics and politics, not physics, are the bottleneck.

Carbon monoxide (CO) forms when fuels burn in insufficient oxygen — the incomplete-combustion gas that binds to haemoglobin and silently poisons. Soot (particulates of carbon) forms the same way: unburnt carbon particles that damage lungs and, in the atmosphere, reflect sunlight causing global dimming. These two are fixed the same way: enough air for complete combustion.

Sulfur dioxide (SO₂) comes from the sulfur impurities in fossil fuels burning: S + O₂ → SO₂. Oxides of nitrogen (NOₓ) are made inside engines, where the high temperature lets nitrogen and oxygen from the air react: N₂ + O₂ → 2NO, which then oxidises further to NO₂. Both SO₂ and NOₓ dissolve in rainwater to form acid rain, which damages lakes, forests, buildings and statues, and both irritate airways. Their fix is different: remove the sulfur from fuels before or after burning, and cool or clean the combustion in engines and power stations.