Acid Rain Chemistry

Acid rain is examined in the Cambridge 0620 environmental chemistry section. Examiners test the chemical causes, the reactions that form acidic gases, the effects on the environment, and methods of reducing emissions.

What is acid rain?

Normal rainwater is slightly acidic (pH about 5.5) because atmospheric CO₂ dissolves in it to form weak carbonic acid:

CO₂ + H₂O → H₂CO₃

Acid rain has a pH significantly lower than 5.5, caused by higher concentrations of stronger acids — sulfuric acid (H₂SO₄) and nitric acid (HNO₃) — dissolved in the rainwater.

Causes

Sulfur dioxide (SO₂)

Sources:

• Burning fossil fuels (coal and oil) that contain sulfur impurities
• Volcanic eruptions (natural source)

Formation: S + O₂ → SO₂ (sulfur in the fuel burns along with the hydrocarbon)

In the atmosphere, SO₂ reacts with water and oxygen to form sulfuric acid:

SO₂ + H₂O → H₂SO₃ (sulfurous acid)

2SO₂ + O₂ + 2H₂O → 2H₂SO₄ (sulfuric acid — the main acid in acid rain)

Nitrogen oxides (NOₓ)

Sources:

• Car engines and power stations where temperatures are high enough for N₂ and O₂ in the air to react

Formation: N₂ + O₂ → 2NO (at high temperatures inside engines)

The NO is further oxidised in the atmosphere:

2NO + O₂ → 2NO₂

NO₂ dissolves in rainwater:

4NO₂ + O₂ + 2H₂O → 4HNO₃ (nitric acid)

Effects of acid rain

On buildings and statues

Acid rain attacks limestone and marble, which are forms of calcium carbonate:

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

The stone dissolves, causing erosion. Historic buildings and statues lose detail and structural integrity over time. This is visible in limestone buildings across Europe and Southeast Asia.

On lakes and rivers

Acid rain lowers the pH of lake water. When the pH drops below about 5:

• Fish and aquatic organisms die (enzymes and gills are damaged)
• Aluminium ions are leached from the soil into the water, which is toxic to fish
• Biodiversity decreases dramatically
• Lakes become “dead” — clear but lifeless

On vegetation and soil

• Acid rain damages leaves by destroying the waxy coating
• It leaches essential minerals (calcium, magnesium) from the soil
• Aluminium ions released from acidified soil damage tree roots
• Forests at high altitude are especially vulnerable because they are often in cloud-level fog

On metals

Acid rain accelerates the corrosion of iron and steel structures, increasing maintenance costs for bridges, railways, and buildings.

Reducing acid rain

Method	How it works
Flue gas desulfurisation (FGD)	Smoke from power stations is passed through a spray of calcium carbonate or calcium oxide, which reacts with SO₂ to form calcium sulfate (used in plaster)
Catalytic converters in cars	Convert NOₓ to harmless N₂ and O₂ (2NO → N₂ + O₂)
Using low-sulfur fuels	Reduces the amount of SO₂ produced
Using alternative energy sources	Wind, solar, and hydroelectric power produce no SO₂ or NOₓ
Burning natural gas instead of coal	Natural gas contains very little sulfur

The FGD reaction: CaCO₃ + SO₂ → CaSO₃ + CO₂ (or with oxidation: CaSO₄)

Acid rain in Malaysia

Malaysia’s rapid industrialisation and the use of coal-fired power stations have made acid rain a concern in some areas. The transboundary haze from forest fires in the region also contributes acidic particles. Environmental regulations and the shift towards cleaner fuels aim to reduce these impacts.

Worked exam question

Sulfur dioxide is produced when coal is burned. (a) Explain how sulfur dioxide leads to acid rain. [2] (b) State one effect of acid rain on the environment. [1] (c) Describe one method of reducing SO₂ emissions from power stations. [2]

(a) SO₂ dissolves in rainwater [1] and reacts with water and oxygen to form sulfuric acid (H₂SO₄), making the rain acidic [1]

(b) Damages limestone buildings/statues by reacting with CaCO₃ / kills aquatic life by lowering pH of lakes / damages vegetation [1]

(c) Flue gas desulfurisation — smoke is passed through calcium carbonate/calcium oxide [1], which reacts with the SO₂ to form calcium sulfate, removing the SO₂ before it enters the atmosphere [1]

Common exam mistakes

1. Saying CO₂ causes acid rain — CO₂ makes rain slightly acidic (pH 5.5), but this is normal rain. Acid rain is caused by SO₂ and NOₓ forming strong acids.
2. Confusing the sources: SO₂ comes from sulfur impurities in fuel; NOₓ comes from N₂ and O₂ reacting at high temperatures.
3. Writing “acid rain melts buildings” — the correct term is corrodes or erodes. It is a chemical reaction, not melting.
4. Forgetting to name specific effects when asked — “damages the environment” is too vague. State the specific effect (e.g. kills fish, erodes limestone).