Acid rain: how it forms and what it damages

Rain that looks clean but eats stone

Acid rain looks, feels and tastes like clean rain, yet the Taj Mahal and the Parthenon are wearing away under it. How does smoke from a power station hundreds of kilometres away end up as acid on a marble dome?

Fossil fuels contain sulphur, so burning them gives sulphur dioxide. In the air, oxides of nitrogen help turn SO₂ into SO₃, which forms sulphuric acid with water. Oxides of nitrogen form when nitrogen and oxygen combine in hot furnaces, in car engines and during lightning; NO₂ with water gives nitric and nitrous acids.

These gases dissolve in the moisture always present in air to make acid droplets. Wind carries them far, and they fall as acid rain, hail, snow or fog, often well away from where the gases were made. In 1852 Robert Angus Smith first linked acid rain to air pollution in Manchester.

Acid rain makes soil acidic, washes out nutrients and releases toxic aluminium and mercury. It damages forests, so soil on hills washes away, and makes lakes too acidic for fish. Crops suffer less, because farmers can add crushed lime (CaO) to neutralise the soil. Stone and metal buildings, statues and cars corrode.

SO₂ from sulphur in fuels and NOx from hot combustion dissolve in moisture to make sulphuric and nitric acids.

Key terms

Acid rain

Naming polluted rain.

Sulphur dioxide

Main acid-rain gas.

Oxides of nitrogen

Nitric acid; help SO₂ become SO₃.

Crushed lime

Protecting crops.

Short questions with model answers

1. Q1. Show how sulphur in coal ends up as sulphuric acid in rain.

   • S + O₂ → SO₂
   • 2SO₂ + O₂ → 2SO₃
   • SO₃ + H₂O → H₂SO₄

   Sulphur → SO₂ → SO₃ → H₂SO₄ in rain droplets.

2. Q2. Which acids form when SO₃ and NO₂ dissolve in rain? (Exercise 1, p. 136)

   • SO₃ + H₂O → H₂SO₄
   • 2NO₂ + H₂O → HNO₃ + HNO₂

   Sulphuric acid, nitric acid and nitrous acid.

Common mistakes

• ✗ Acid rain only falls next to the factory that made the gases.

  ✓ Wind carries the droplets far; it often falls far away (Fig 10.1).

• ✗ You can tell acid rain by its colour or taste.

  ✓ It looks, feels and tastes like clean rain (p. 135).

• ✗ Carbon dioxide is the main cause of acid rain.

  ✓ Oxides of sulphur and nitrogen cause it (p. 134).

MCQs

1. 1. Where is acid rain most likely? (Exercise vi, p. 143)

   1. (a) Around villages
   2. (b) Around big cities
   3. (c) Around industrial areas
   4. (d) Around water bodies
   Show answer

   (c) Industry burns the most sulphur-containing fuel.

2. 2. Who first linked acid rain to air pollution, in 1852?

   1. (a) Robert Angus Smith
   2. (b) John Dalton
   3. (c) Antoine Lavoisier
   4. (d) Robert Boyle
   Show answer

   (a) p. 134.

3. 3. What helps change SO₂ into SO₃ in the atmosphere?

   1. (a) Argon
   2. (b) Oxides of nitrogen
   3. (c) Carbon monoxide
   4. (d) Methane
   Show answer

   (b) p. 134.

4. 4. What do farmers add to soil to neutralise acid rain?

   1. (a) Salt
   2. (b) Crushed lime (CaO)
   3. (c) Sugar
   4. (d) Sulphur
   Show answer

   (b) p. 136.

5. 5. Which two famous buildings does the book name as damaged by acid rain?

   1. (a) Parthenon and Taj Mahal
   2. (b) Eiffel Tower and Big Ben
   3. (c) Pyramids and Colosseum
   4. (d) Minar-e-Pakistan and Faisal Mosque
   Show answer

   (a) p. 136.

Quick revision

• Sulphur in fuels gives SO₂; NOx come from hot combustion, engines and lightning.
• With moisture they form sulphuric, nitric and nitrous acids that wind carries far.
• Acid rain harms soil, forests, lakes, health and buildings; lime protects farm soil.