Acid Rain: Chemistry, Causes, and Impact Explained

Acid rain is rain, snow or fog that is more acidic than normal because it contains dissolved sulphuric acid and nitric acid. Normal rainwater is already slightly acidic (pH about 5.6) because dissolved carbon dioxide forms weak carbonic acid. Rain is called acid rain when its pH drops below 5.6, and in badly polluted regions it can fall as low as 4.0.

This topic matters because acid rain damages stone monuments, kills fish in lakes, harms forests and corrodes buildings and metal structures — problems that cost economies billions and erase cultural heritage. Understanding its chemistry is the first step to understanding why controlling sulphur dioxide and nitrogen oxide emissions matters.

Key Concepts

• pH scale: a logarithmic measure of hydrogen ion concentration, pH = −log₁₀[H⁺]. Each one-unit drop means ten times more hydrogen ions.
• Sulphur dioxide (SO₂): from burning coal and oil and from smelters; it oxidises to SO₃, which reacts with water to form sulphuric acid (H₂SO₄).
• Nitrogen oxides (NOₓ): from vehicle engines and power plants; nitrogen dioxide reacts with water to form nitric acid (HNO₃).
• Wet vs dry deposition: acids reach the ground dissolved in rain and snow (wet deposition) or as acidic gases and particles settling directly (dry deposition).

Formulas

• Sulphuric acid formation: 2SO₂ + O₂ → 2SO₃ (catalysed by particles and sunlight), then SO₃ + H₂O → H₂SO₄..
• Nitric acid formation: 2NO₂ + H₂O → HNO₃ + HNO₂, and 3NO₂ + H₂O → 2HNO₃ + NO. Assumes high-temperature combustion as the NOₓ source.
• pH of rain: pH = −log₁₀[H⁺], with [H⁺] in moles per litre. Limit: this gives the acidity of the water sample, not the total acid load deposited over time.
• Marble corrosion: CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O. The calcium sulphate crust that forms is more soluble than marble, so the stone surface crumbles away.

Worked Example

A rainwater sample is found to have a hydrogen ion concentration of 6.3 × 10⁻⁵ mol/L. Calculate its pH and compare it with normal rain.

1. Use pH = −log₁₀[H⁺] = −log₁₀(6.3 × 10⁻⁵).
2. Split the logarithm: log₁₀(6.3 × 10⁻⁵) = log₁₀(6.3) + log₁₀(10⁻⁵) = 0.80 + (−5) = −4.20.
3. Therefore pH = −(−4.20) = 4.20.
4. Normal rain has [H⁺] ≈ 2.5 × 10⁻⁶ mol/L, giving pH = 5.6.
5. Since 4.20 < 5.6, this is acid rain — with about 25 times more hydrogen ions than normal rain.

Common Misconceptions

• "Acid rain burns your skin when you walk in it." Corrected: acid rain is far too dilute to harm skin. Its damage is cumulative and environmental — dissolving stone, acidifying lakes and leaching nutrients from soil over years.
• "All pollution damage near a factory is acid rain." Corrected: Soot causes local damage, but acid rain is regional — emissions can travel hundreds of kilometres before falling.
• "Pure water has pH 7, so rain should too." Corrected: rainwater in clean air equilibrates with atmospheric CO₂ to form carbonic acid, giving pH ≈ 5.6. Only values below this count as acid rain.

Real-World Applications

• Heritage conservation: monuments like the Taj Mahal have suffered marble discolouration and erosion from acidic air pollution, driving emission-control zones around heritage sites.
• Emission policy: sulphur scrubbers in power plants and low-sulphur fuels were adopted worldwide specifically to cut the SO₂ behind acid rain.
• Limnology: lake scientists add limestone to acidified lakes (liming) to restore pH so fish populations can recover.

In short, acid rain forms when SO₂ and NOₓ from fuel burning convert in the atmosphere to sulphuric and nitric acids, dropping rain pH below the natural 5.6. Its chemistry explains the damage to marble, lakes and forests — and why scrubbers and emission limits exist.