What Is Acid Rain?
Normal rainwater is already slightly acidic (pH ~5.5) because atmospheric carbon dioxide dissolves in it to form carbonic acid. Acid rain is much more acidic (pH 4 or lower) because it also contains stronger acids formed from air pollution.
How Acid Rain Forms
When coal and oil are burned in power stations, factories, and vehicles, they release:
- Sulfur dioxide (SO₂) — from sulfur impurities in fossil fuels
- Nitrogen oxides (NOₓ) — formed when nitrogen and oxygen in air react at high combustion temperatures
These gases dissolve in clouds and react with water and oxygen:
The result is rain, snow, or fog with a pH as low as 4 — ten times more acidic than normal rain.
Acid rain is a transboundary pollutant — winds can carry it hundreds or thousands of kilometres from its source. Pollution produced in the UK or Germany has damaged Scandinavian forests and lakes.
Effects of Acid Rain
Forests: acid rain leaches essential minerals from soil and directly damages leaves and roots, weakening trees and leaving them vulnerable to disease.
Lakes and rivers: pH drops, killing fish, amphibians, and aquatic insects. At pH below 6, many species cannot survive.
Buildings and statues: sulfuric acid attacks limestone and marble (calcium carbonate):
The calcium sulfate formed is soluble and washes away, eroding statues and historic buildings.
Solutions
- Catalytic converters in vehicles convert NOₓ to harmless nitrogen
- Flue gas desulfurisation in power stations removes SO₂ before release
- Switching to renewable energy reduces SO₂ and NOₓ emissions
- Liming lakes: adding calcium oxide or calcium carbonate neutralises acidic lakes
The drop from pH 6 to pH 5 is not just "one unit" — it means ten times more acidic. Because pH is a logarithmic scale, a change of just one unit represents a tenfold change in acidity.
Real world: Acid rain eroding the Parthenon and Cologne Cathedral
The Parthenon in Athens and Cologne Cathedral in Germany are both built from limestone and marble (calcium carbonate). Acid rain — produced by SO₂ from burning coal — reacts with the stone: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The calcium sulfate formed is soluble and washes away, taking the carved stone details with it. Thousands of years of weathering did less damage to these buildings than a few decades of industrial acid rain in the 20th century.
Real world: Dead lakes in Scandinavia
In the 1970s–80s, thousands of lakes in Norway and Sweden became too acidic to support fish. The acid rain (mostly from UK and German coal power stations) lowered lake pH below 5, killing fish eggs, amphibians, and aquatic insects. The entire food chain collapsed. This environmental disaster led to the Clean Air Act regulations requiring flue gas desulfurisation on power stations. Many Scandinavian lakes have since recovered after liming (adding calcium carbonate to neutralise the acid).
Real world: Catalytic converters reducing NOₓ
Vehicle exhaust gases include nitrogen oxides (NOₓ), which contribute to acid rain and smog. Catalytic converters (containing platinum and rhodium) in car exhausts convert NOₓ back to harmless nitrogen gas: 2NO → N₂ + O₂. Since catalytic converters became mandatory in Europe (1993) and the USA, NOₓ emissions from vehicles have dropped dramatically, reducing their contribution to acid rain. This is transition metal catalysis solving an environmental acid problem.
Where do you see this in real life? Eroding stone gargoyles and historic statues in cities (acid rain dissolving limestone), "sick" forests and dead lakes in downwind countries (transboundary pollution), catalytic converters on every modern car (reducing NOₓ), flue gas desulfurisation chimneys at power stations (removing SO₂ before release) — acid rain is chemistry meeting environmental policy.