Sec 4 Combined Science (Chemistry) Maintaining Air Quality notes

Clean, dry air — no water vapour, no pollution — is a mixture with a remarkably steady recipe: about 78% nitrogen (N₂), 21% oxygen (O₂), and a 1% remainder of noble gases (mostly argon) plus a small amount of carbon dioxide (CO₂). It is a mixture, not a compound: the gases keep their own properties and the percentages can wobble slightly from place to place.

Because the composition is quoted by volume, you can compute actual volumes: in a 500 cm³ sample of clean dry air there is 500 × 78/100 = 390 cm³ of nitrogen and 500 × 21/100 = 105 cm³ of oxygen. Pollution is what changes this recipe: cities carry measurable extras — carbon monoxide, sulfur dioxide, nitrogen oxides, ozone, methane and unburnt hydrocarbons — each with its own source.

Two source stories need precision. Carbon monoxide is a child of oxygen starvation: when a flame burns fuel with too little O₂, carbon ends as CO (or as soot) instead of CO₂ — the same incomplete combustion met in organic chemistry. Oxides of nitrogen do not come from impurities: nitrogen is already 78% of the air, and at the very high temperatures inside an engine cylinder (or a lightning bolt) even the unreactive N₂ joins with O₂ to make NO and NO₂.

Carbon monoxide is the quiet killer. Red blood cells carry oxygen on haemoglobin — and CO binds to haemoglobin far more strongly than oxygen does. Breathe enough CO and your blood carries less and less oxygen; the sufferer grows dizzy and drowsy (it gives no warning smell or irritation) and can die from what is essentially internal suffocation. This is why fuel-burning heaters and boilers must vent properly.

Rain is naturally slightly acidic — dissolved CO₂ makes it a weak carbonic acid solution — but acid rain is far more acidic than that. When sulfur dioxide and nitrogen dioxide from combustion dissolve in cloud droplets they form acids (sulfur dioxide dissolving gives sulfurous acid, SO₂ + H₂O → H₂SO₃), and the droplets fall as rain with a lowered pH. The two gases share the blame: SO₂ from dirty fuels and volcanoes, NO₂ from hot engines and lightning.

Its effects reach two targets named in every mark scheme. People: the dissolved acids and the pollutant gases themselves irritate the respiratory system, worsening breathing difficulties and attacks of asthma — the respiration link. Buildings: acid rain erodes building materials, fastest on limestone and marble, whose calcium carbonate reacts with the acid — the same marble-and-acid fizz of the acids chapter, now outdoors: statues blur, stone facades crumble, metal corrodes faster. Lakes and soils acidify too, damaging aquatic life.

Carbon moves through the atmosphere in a loop. Combustion (burning fuels, wildfires) releases CO₂. Respiration — in animals, plants and microbes — also releases CO₂ as organisms oxidise their food. Photosynthesis runs the other way: green plants absorb CO₂ and use light energy to build carbohydrates, releasing O₂. With oceans also dissolving CO₂, these processes have long regulated the atmosphere: carbon in ≈ carbon out, and the CO₂ level stays roughly steady. That regulation is the carbon cycle.

Human activity has overloaded it. Burning fossil fuels injects millions of years of buried carbon within centuries, while deforestation removes the very photosynthesis that would pull CO₂ back out. The cycle's sinks cannot absorb the surplus, so atmospheric CO₂ rises year after year — measurable, and accelerating.

Rising CO₂ matters because it is a greenhouse gas: like methane (CH₄, from livestock, decaying waste and natural-gas leaks), it traps heat in the atmosphere that would otherwise escape to space. More trapping means a warmer climate, and the consequences are already listed in every syllabus: more extreme weather — fiercer storms, longer droughts, harsher heatwaves — and melting of polar ice, which raises sea levels and shrinks the habitat of ice-dependent life.