<aside> 📋
Your teacher is away for this lesson. First, duplicate this page and work on your own copy. Then work through the phases in order, typing your answers under each question and into the empty table cells. Where a phase has a "Check your numbers" toggle, try the questions first and only then open it to check.
</aside>
Last lesson you met the gases that come out of exhaust pipes and chimneys. Geneva keeps its most polluting cars out of the city centre on bad days because they give out the most nitrogen oxides, and West African countries have cut the sulfur in their fuel to 50 ppm. This lesson follows two of those gases, sulfur dioxide (SO2) and nitrogen oxides (NOx), after they leave the chimney. They do not stay where they were released. In the air they turn into acid, and they come back down, sometimes hundreds of kilometres away.
Ticino, Switzerland's southern canton, shows how far they travel. Warm, humid air from the Mediterranean Sea blows north over the Po Plain in northern Italy, past the big industrial cities of Milan and Turin, and picks up their pollution. When this air meets the Alps it is forced to rise, cools and drops its rain, and much of the pollution falls with it on Ticino [Source].
Map of Switzerland and northern Italy, with the canton of Ticino shaded and an arrow showing warm, humid air moving from the Mediterranean Sea north over the Po Plain, past Turin and Milan, to Ticino. Geneva is marked in the west.
Map: Natural Earth (public domain).
[24 November 2025]
Task 0 Watch the whole clip from Cognito, a chemistry revision channel. The first half recaps last lesson.
<aside> <img src="/icons/leaf_green.svg" alt="/icons/leaf_green.svg" width="40px" />
8.3.5 NOx and sulfur dioxide react with water and oxygen in the air to produce nitric and sulfuric acid, resulting in acid rain.
Include the chemistry of acid rain formation.
</aside>
The pH scale. pH measures how acidic or alkaline a solution is. A pH of 7 is neutral, and the lower the number, the more acidic the solution. The scale is logarithmic: each step down by one unit means ten times more acidic. So pH 4.6 is ten times more acidic than pH 5.6, and pH 3.6 is a hundred times more acidic than pH 5.6.
Clean rain is already slightly acidic. Carbon dioxide in the air dissolves in raindrops and forms a weak acid, carbonic acid, so normal rain has a pH of about 5.6. Acid rain is rain that is more acidic than this. In polluted areas it usually has a pH of about 4.2 to 4.4 [Source].
From gas to acid. Sulfur dioxide and nitrogen oxides are primary pollutants. In the air they react with oxygen and water to form two strong acids: sulfuric acid and nitric acid. Nobody releases these acids; they form in the air, so they are secondary pollutants.
Sulfuric acid, step by step.
Nitric acid. Engines release mostly nitric oxide (NO), which reacts with oxygen in the air to form nitrogen dioxide: 2NO + O₂ → 2NO₂. Nitrogen dioxide then reacts with oxygen and water to form nitric acid. You will balance that equation in Task 1.1.
Wet and dry deposition. Acid deposition is the general name for acids reaching the ground. It happens in two ways:
flowchart TD
A["Coal, oil and petrol<br>burned"] --> B["SO2 and NOx<br>released"]
B --> C["React with oxygen<br>and water in the air"]
C --> D["Sulfuric acid and<br>nitric acid"]
D --> E["Wet deposition<br>rain, snow, fog"]
B --> F["Dry deposition<br>gases, particles"]
D --> F
Task 1.1
Balance the symbol equation by filling in the three missing numbers. Start with nitrogen, then hydrogen, then check the oxygen.
☐NO₂ + O₂ + ☐H₂O → ☐HNO₃
Task 1.2
<aside> <img src="/icons/leaf_green.svg" alt="/icons/leaf_green.svg" width="40px" />
8.3.6 Acid rain has impacts on ecology, humans and buildings.
Effects are: on terrestrial habitats (leaching, toxification of the soil, direct impact on foliage); on freshwater habitats (toxicity due to aluminium solubilization, impacts on fish gills and invertebrate exoskeletons); the corrosion of marble, limestone, steel, paint and other construction materials; on breathing, from nitrate and sulfate particles (tissue damage and lung inflammation from components of PM2.5 and acid deposition).
</aside>
On land.
In freshwater.
On buildings. Marble and limestone are made of calcium carbonate, which reacts with acid and slowly dissolves. Acid deposition also makes steel corrode and paint break down faster [Source]. The photograph shows what this can do to carved stone.
On people. Acid rain is no more dangerous to walk in or swim in than normal rain [Source]. The danger is in the air. SO2 and NOx form tiny sulfate and nitrate particles, which are part of the PM2.5 you met last lesson. Breathed deep into the lungs, they damage lung tissue and cause inflammation, and they make breathing harder for people with asthma.
Close-up of a carved sandstone head in Dresden, Germany. The nose, mouth and chin have crumbled away and the stone surface is flaking, while some carved hair survives, after years of polluted air and acid rain.
Sandstone sculpture in Dresden, Germany, worn by polluted air and acid rain. Photo: Slick, Wikimedia Commons (public domain).
St Paul's Cathedral is built of limestone. For thirty years, scientists measured how fast the stone surface of its balustrade, the stone railing around the roof, was wearing away. Over that time, London's air changed: the daily average level of sulfur dioxide fell from about 80 ppb (parts per billion) in the early 1980s to less than 3 ppb by the late 2000s.
The stone wore away more slowly as the air got cleaner. The rate of erosion fell from 0.049 mm a year to 0.035 mm a year. The scientists found that the stone is now wearing away at close to the rate of natural limestone surfaces [Source].
Task 2.1
