Acid Rain

Summary: Rainwater with a pH below ~5.6, caused by dissolved SO₂ and NOx forming sulfuric and nitric acids. Tags: igcse chemistry definition environmental-chem Created: 2026-07-14 Last Updated: 2026-07-14

Acid rain is precipitation with a pH below approximately 5.6, caused when acidic gases dissolve in atmospheric water droplets to form stronger acids than the naturally present carbonic acid. The primary pollutants are sulfur dioxide (SO₂) and nitrogen oxides (NOx), released from the combustion of fossil fuels in power stations and vehicle engines; SO₂ forms sulfurous acid (H₂SO₃) and eventually sulfuric acid (H₂SO₄), while NO₂ forms a mixture of nitrous acid (HNO₂) and nitric acid (HNO₃). The environmental consequences are significant and wide-ranging: acid rain acidifies lakes and rivers, killing aquatic life; it leaches essential nutrients like calcium and magnesium from soils, harming forests; and it chemically erodes limestone and marble buildings through the reaction CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Mitigation strategies include flue gas desulfurisation in power stations (scrubbing SO₂ with calcium oxide or limestone slurry), catalytic converters in vehicles that reduce NOx emissions, and the use of low-sulfur fuels. In the IGCSE exam, students should remember that natural rain is already slightly acidic (pH ~5.6) due to dissolved CO₂ forming weak carbonic acid, and that acid rain is defined as rain with a pH significantly below this baseline, typically below pH 5.2.

Key term — IGCSE Chemistry (0620/0971) glossary.