Acid Rain – IGCSE Chemistry Definition

Acid rain connects combustion chemistry to environmental damage. Examiners test the causes (which gases, where they come from), the effects (on buildings, aquatic life, vegetation), and the solutions (desulfurisation, catalytic converters). Each angle can carry 2–3 marks.

The 0620 definition

Acid rain is rain (or any form of precipitation) that is more acidic than normal, with a pH below about 5.5, caused by pollutant gases dissolving in rainwater.

Causes

Sulfur dioxide (SO₂)

Source: burning fossil fuels (coal, oil) that contain sulfur impurities.

S + O₂ → SO₂

SO₂ dissolves in rainwater: SO₂ + H₂O → H₂SO₃ (sulfurous acid)

Further oxidation in the atmosphere produces sulfuric acid: 2SO₂ + O₂ + 2H₂O → 2H₂SO₄

Nitrogen oxides (NOₓ)

Source: formed in car engines and power stations where high temperatures cause nitrogen and oxygen in the air to react.

N₂ + O₂ → 2NO (then further oxidation to NO₂)

NO₂ dissolves in rainwater to form nitric acid (HNO₃).

Effects of acid rain

Effect	Detail
Buildings and statues	Reacts with limestone/marble (CaCO₃), causing erosion
Lakes and rivers	Lowers pH, killing fish and aquatic organisms
Forests	Damages leaves, leaches nutrients from soil
Metal structures	Accelerates corrosion

Reaction with limestone: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Prevention and reduction

• Desulfurisation: removing SO₂ from power station exhaust gases (e.g. using CaO)
• Catalytic converters in cars: convert NOₓ back to N₂ and O₂
• Using low-sulfur fuels or alternative energy sources
• Liming: adding CaCO₃ or Ca(OH)₂ to acidified lakes (neutralisation)

Worked exam question

Power stations that burn coal produce sulfur dioxide. (a) Explain how sulfur dioxide causes acid rain. [2] (b) State one effect of acid rain on the environment. [1] (c) Describe how the problem of acid rain from power stations can be reduced. [1]

Mark scheme

(a) SO₂ dissolves in rainwater [1]; forms sulfurous acid / sulfuric acid / H₂SO₃ / H₂SO₄ [1]

(b) Damages limestone buildings / kills fish in lakes / damages trees and forests / corrodes metals [1] (any one)

(c) Remove sulfur from fuels before burning / desulfurisation of exhaust gases / use alternative energy sources [1]

Common exam mistakes

• Saying CO₂ causes acid rain — CO₂ makes normal rain slightly acidic (pH 5.5), but acid rain is specifically caused by SO₂ and NOₓ.
• Confusing acid rain with the greenhouse effect — they are different problems caused by different gases.
• Forgetting that nitrogen oxides come from high-temperature combustion in engines, not from the fuel itself.