11.5 Acid Rain and Its Effects

This topic explains acid rain — what it is, how it forms, and its harmful effects on the environment, buildings, and living things.

Definition

Acid rain is defined as rain having pH less than 5.6.

Simple explanation: Acid rain is rain that is more acidic than normal.

Main Idea

Normal rainwater is saturated with carbon dioxide. It has a pH of 5.6.

However, the acidity of rain increases during thunderstorms in polluted areas.

Fossil fuels contain compounds of sulphur and nitrogen. - Sulphur dioxide from fossil fuels used in power plants and nitrogen oxides from car exhausts dissolve in rainwater, creating acids such as sulphuric acid, nitrous acid, and nitric acid.

Therefore, the pH of rainwater can be much lower due to sulphuric acid and nitric acid from precipitation during thunderstorms. The pH of this rain can be as low as 2.1. This value is lower than the pH of vinegar or lemon juice.

Simple explanation: Normal rain is slightly acidic (pH 5.6) because of CO₂. Acid rain is much more acidic (pH < 5.6) because of pollutants like SO₂ and NOₓ.

Core Concept

How Acid Rain Forms

Step 1: Fossil fuels containing sulphur and nitrogen compounds are burned.

Step 2: This releases sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere.

Step 3: These gases react with oxygen and water vapour in the atmosphere.

Step 4: They form sulphuric acid (H₂SO₄) and nitric acid (HNO₃).

Step 5: These acids fall to the ground with rain — this is acid rain.

Simple explanation: Pollutants from burning fossil fuels mix with water in clouds and form acids, which fall as acid rain.

Chemical Equations from the Book

1. Formation of sulphur trioxide:

2SO_2(g) + O_2(g) → 2SO_3(g)

2. Formation of sulphuric acid:

SO_3(g) + H₂O_(l) → H₂SO_4(aq)

3. Formation of nitric acid:

2NO_2(g) + 3O_2(g) + 2H₂O_(l) → 4HNO_3(aq)

Simple explanation: SO₂ and NO₂ react with oxygen and water to form sulphuric acid and nitric acid.

Role of Nitrogen Dioxide

Nitrogen dioxide also plays a significant role in the catalytic oxidation of sulphur dioxide into sulphur trioxide, which forms sulphuric acid in the atmosphere.

These reactions are key contributors to the acidity of rainwater.

Simple explanation: NO₂ helps convert SO₂ into SO₃, which then becomes sulphuric acid.

The pH of Acid Rain

Simple explanation: Acid rain can be as acidic as vinegar or lemon juice — or even more.

Detailed Explanation

What Makes Rain Acidic?

• Rainwater is saturated with carbon dioxide (CO₂) from the air.
• CO₂ dissolves in water to form carbonic acid (H₂CO₃):

CO₂ + H₂O → H₂CO₃

- This makes normal rain slightly acidic (pH ~5.6).

• Sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels dissolve in rainwater.
• They form strong acids (sulphuric acid, nitric acid).
• This makes rain much more acidic (pH < 5.6, sometimes as low as 2.1).

Simple explanation: Normal rain is slightly acidic from CO₂; acid rain is much more acidic from SO₂ and NOₓ.

Effects of Acid Rain

The book lists several harmful effects:

1. Corrodes metals, stone structures, and statues.

Sulphuric acid eats metals and forms water-soluble salts and hydrogen:

Fe_(s) + H₂SO_4(aq) → FeSO_4(aq) + H_2(g)

2. Disintegrates marble buildings and statues.

Marble (calcium carbonate) is attacked by acids:

CaCO_3(s) + H₂SO_4(aq) → CaSO_4(aq) + H₂O_(g) + CO_2(g)

CaCO_3(s) + 2HNO_3(aq) → Ca(NO₃)_2(aq) + H₂O_(g) + CO_2(g)

3. Kills fish and destroys trees.

4. Makes lakes and rivers too acidic for living things to survive.

Simple explanation: Acid rain damages buildings, statues, metals, fish, trees, and entire ecosystems.

Where Acid Rain Falls

Acid rain can often fall hundreds of kilometres from its source.

This means that the pollutants released in one country can cause acid rain in another country — acid rain is a transboundary problem.

Simple explanation: Acid rain doesn't stay where it's created — it can travel far and affect other regions.

Concept Assessment Exercise 11.6 (from the Book)

1. Define acid rain.

Answer: Acid rain is defined as rain having pH less than 5.6.

2. Write names of gases that cause acid rain.

Answer: Sulphur dioxide (SO₂) and nitrogen oxides (NOₓ).

3. What is the effect of acid rain on iron and marble? Give balanced chemical equation.

Answer:

On iron:

Fe_(s) + H₂SO_4(aq) → FeSO_4(aq) + H_2(g)

On marble:

CaCO_3(s) + H₂SO_4(aq) → CaSO_4(aq) + H₂O_(g) + CO_2(g)

4. List some effects caused by acid rain.

• Corrodes metals, stone structures, and statues
• Disintegrates marble buildings and statues
• Kills fish and destroys trees
• Makes lakes and rivers too acidic for living things

5. Justify that acid rain is dangerous than normal rain.

Answer: Acid rain has a pH lower than 5.6 (sometimes as low as 2.1), making it much more acidic than normal rain (pH 5.6). This increased acidity makes it more corrosive and harmful to buildings, statues, metals, aquatic life, and forests.

Intermediate Level

The Chemistry of Acid Rain

Step 1: Release of pollutants

S + O₂ → SO₂

N₂ + O₂ → 2NO

2NO + O₂ → 2NO₂

Step 2: Formation of acids

2SO₂ + O₂ → 2SO₃

SO₃ + H₂O → H₂SO₄

2NO₂ + 3O₂ + 2H₂O → 4HNO₃

Step 3: Acid rain falls

The acids mix with rainwater and fall to the ground as acid rain.

Effects in Detail

Why Acid Rain Is Dangerous

• It has a very low pH (as low as 2.1).
• It is more corrosive than normal rain.
• It can travel long distances.
• It damages both natural and man-made structures.
• It harms aquatic life, forests, and soil.

Normal Rain vs Acid Rain

Important Points

• - Acid rain: rain having pH less than 5.6.
• - Normal rain: pH 5.6 due to dissolved CO₂.
• - Cause: SO₂ and NOₓ from burning fossil fuels.
• - Acids formed: sulphuric acid (H₂SO₄), nitric acid (HNO₃).
• - pH of acid rain: can be as low as 2.1 (lower than vinegar or lemon juice).
• - Effects:
• - Corrodes metals
• - Disintegrates marble buildings and statues
• - Kills fish
• - Destroys trees
• - Makes lakes and rivers too acidic for life
• - Transboundary: acid rain can fall hundreds of kilometres from its source.
• - Equations:
• - Fe + H₂SO₄ → FeSO₄ + H₂
• - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
• - CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂

Remember

• - Acid rain = rain with pH < 5.6.
• - Normal rain = pH 5.6 (from CO₂).
• - Causes: SO₂ and NOₓ from burning fossil fuels.
• - Acids formed: H₂SO₄ and HNO₃.
• - Effects: corrodes metals, damages marble, kills fish, destroys trees.
• - Equations:
• - Fe + H₂SO₄ → FeSO₄ + H₂
• - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

Common Mistakes

• 1. Thinking all rain is acid rain. Normal rain has pH 5.6; acid rain has pH < 5.6.
• 2. Thinking normal rain is neutral. Normal rain is slightly acidic due to dissolved CO₂.
• 3. Thinking acid rain only affects the area where pollutants are released. Acid rain can fall hundreds of kilometres from its source.
• 4. Forgetting the role of NO₂. NO₂ plays a significant role in the catalytic oxidation of SO₂ to SO₃.
• 5. Thinking acid rain only affects buildings. It also kills fish, destroys trees, and acidifies lakes and rivers.
• 6. Confusing the pH values. Normal rain = 5.6; acid rain < 5.6; extreme acid rain can be as low as 2.1.

Quick Revision

• Acid rain: rain having pH less than 5.6.
• Normal rainwater is saturated with carbon dioxide and has a pH of 5.6.
• Acidity of rain increases during thunderstorms in polluted areas.
• Fossil fuels contain compounds of sulphur and nitrogen.
• Sulphur dioxide from fossil fuels and nitrogen oxides from car exhausts dissolve in rainwater, creating acids such as sulphuric acid, nitrous acid, and nitric acid.
• pH of acid rain: can be as low as 2.1 — lower than the pH of vinegar or lemon juice.
• Effects of acid rain:
• Corrodes metals, stone structures, and statues.
• Disintegrates marble buildings and statues.
• Kills fish and destroys trees.
• Lakes and rivers may become too acidic for living things to survive.
• Equations:
• Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)
• CaCO₃(s) + H₂SO₄(aq) → CaSO₄(aq) + H₂O(g) + CO₂(g)
• CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(g) + CO₂(g)
• Transboundary: acid rain can fall hundreds of kilometres from its source.

60-Second Revision

• Acid rain = rain with pH < 5.6.
• **Normal rain** = pH 5.6.
• Causes: SO₂ and NOₓ.
• Acids formed: H₂SO₄ and HNO₃.
• Effects: corrodes metals, damages marble, kills fish, destroys trees.
• Warning: acid rain can travel hundreds of kilometres from its source.