Acid rain and its effects

From exhaust to rain

Burning fossil fuels releases sulphur dioxide (SO₂) and nitrogen dioxide (NO₂) into the air. These gases mix with moisture to form acid droplets, which fall to the ground as acid rain. The book describes rain with a pH of about 4.2–4.4 as acid rain.

Sulphur dioxide first reacts with oxygen to form sulphur trioxide, which reacts with water to form sulphuric acid. Nitrogen dioxide reacts with water to form nitric acid and nitrous acid.

Acid rain makes soil more acidic. It dissolves and washes away the nutrients plants need, so many plants cannot live or grow, and vegetation is damaged.

Lakes, rivers, ponds and streams can become too acidic for animals, so many no longer have fish. Acid rain and dry acidic particles also eat away buildings, statues, cars and other stone and metal structures, because acids react with carbonates and metals.

Acid rain damages stone and metal for the same reasons you met in 7.4: acids attack carbonates and metals.

Key terms

Acid rain

An air pollution problem.

Fossil fuel

Source of SO₂ and NO₂ when burned.

Sulphur trioxide

Step to sulphuric acid.

Dry deposition

Damage to buildings.

Short questions with model answers

1. Q1. Trace how sulphur dioxide from a power station becomes sulphuric acid.

   • Add oxygen
   • Add water

   SO₂ is oxidised to SO₃, which dissolves in rain water as sulphuric acid.

2. Q2. Why does acid rain damage marble statues? Link it to the properties of acids.

   • Marble is a carbonate
   • Acid + carbonate

   The acid reacts with the calcium carbonate, so the stone slowly wears away.

Common mistakes

• ✗ Acid rain is mainly caused by carbon dioxide.

  ✓ The book names SO₂ and NO₂ from burning fossil fuels (p. 102).

• ✗ Acid rain only harms plants.

  ✓ It also harms soil, fish and stone and metal structures (p. 102).

• ✗ SO₂ turns straight into sulphuric acid.

  ✓ It first becomes SO₃, which then reacts with water (p. 102).

MCQs

1. 1. Which gases from burning fossil fuels cause acid rain?

   1. (a) O₂ and N₂
   2. (b) SO₂ and NO₂
   3. (c) H₂ and He
   4. (d) Ar and Ne
   Show answer

   (b) p. 102.

2. 2. Which compound forms when SO₂ dissolves in water? (Exercise vii, p. 103)

   1. (a) SO₃
   2. (b) H₂SO₃
   3. (c) H₂SO₄
   4. (d) H₂S₂O₇
   Show answer

   (b) SO₂ + H₂O gives sulphurous acid; H₂SO₄ needs SO₃.

3. 3. Nitrogen dioxide reacts with water to form:

   1. (a) HNO₃ and HNO₂
   2. (b) NH₃
   3. (c) H₂SO₄
   4. (d) N₂ and O₂
   Show answer

   (a) p. 102.

4. 4. Why do many lakes affected by acid rain no longer have fish?

   1. (a) The water is too cold
   2. (b) The water is too acidic
   3. (c) The water is too basic
   4. (d) There is too much salt
   Show answer

   (b) p. 102.

5. 5. Acid rain harms plants mainly by:

   1. (a) Washing nutrients out of the soil
   2. (b) Making the soil basic
   3. (c) Adding oxygen to roots
   4. (d) Cooling the leaves
   Show answer

   (a) p. 102.

Quick revision

• Burning fossil fuels releases SO₂ and NO₂, which form acid droplets with moisture.
• SO₂ becomes SO₃ then H₂SO₄; NO₂ gives HNO₃ and HNO₂.
• Acid rain harms soil, plants, fish, buildings, statues and cars.