---
title: "Short note: Acid rain"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:acid-rain"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acid-rain
site: BCT Engineering
---

# Short note: Acid rain

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2073 Bhadra, 2.5 marks; 2067 Mangsir, 2 marks (asked 2 times)
- Video: [Acid Rain | Environmental Chemistry | Chemistry | FuseSchool](https://www.youtube.com/watch?v=Nf8cuvl62Vc) — FuseSchool - Global Education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acid-rain
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

Write a short note on acid rain.

## Short answer

Acid rain is precipitation with pH below 5.6, mainly from SO2 and NOx forming H2SO4 and HNO3. It acidifies lakes and soils, damages forests, corrodes marble buildings (e.g., Taj Mahal), and causes respiratory issues. Control: flue gas desulphurisation, catalytic converters, use of low-sulfur fuels, and renewable energy.

## Answer

#### Acid rain
**Acid rain** is rain (or snow, fog or dust) with a pH below 5.6. Normal rain water has a pH of about 5.6; it is slightly acidic because of dissolved CO₂ (CO₂ + H₂O ⇌ H₂CO₃). Acid rain is caused by the oxides of sulphur and nitrogen released into the air.

#### Causes
SO₂ comes from burning coal and oil and from smelters; NO and NO₂ come from vehicle engines and power plants (N₂ + O₂ → 2NO at high temperature). In the air they are oxidised and dissolve in cloud water:

```text
2SO₂ + O₂ → 2SO₃          SO₃ + H₂O → H₂SO₄
2NO + O₂ → 2NO₂           4NO₂ + O₂ + 2H₂O → 4HNO₃
```
These strong acids fall as acid rain.

#### Effects
- Acidifies lakes and rivers, killing fish and other aquatic life.
- Damages forests and leaches nutrients (Ca²⁺, Mg²⁺) and releases toxic Al³⁺ from the soil.
- Corrodes metals and attacks marble and limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), for example the Taj Mahal.
- Causes respiratory problems in humans.

#### Control
- Use low-sulphur coal and oil; remove sulphur before burning (desulphurisation).
- Flue gas desulphurisation: scrubbing with lime, CaCO₃ + SO₂ + ½O₂ → CaSO₄ + CO₂.
- Catalytic converters in vehicles; control of NOx by modern combustion.
- Use renewable energy (hydropower, solar), and treat acidified lakes with lime.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is sulphuric acid formed in the air from SO2?
   A. SO2 + CO2 -> H2SO4
   B. 2SO2 + O2 -> 2SO3, then SO3 + H2O -> H2SO4
   C. SO3 + O2 -> H2SO4
   D. SO2 reacts with N2 to give H2SO4
   Answer: B. SO2 is oxidised to SO3, which dissolves in water to give H2SO4. (Key point: Acid formation)

2. How does NO2 give nitric acid?
   A. NO2 + CO2 -> HNO3
   B. NO2 + N2 -> HNO3
   C. 4NO2 + O2 + 2H2O -> 4HNO3
   D. NO2 loses oxygen to form NH3
   Answer: C. NO2 with oxygen and water gives HNO3. (Key point: Nitric acid)

3. Acid rain is rain with pH below
   A. 7.0, neutral water
   B. 8.4, like sea water
   C. 2.0 always
   D. 5.6, the pH of normal rain (slightly acidic due to dissolved CO2)
   Answer: D. Normal rain is about pH 5.6 because of carbonic acid, so acid rain is below this. (Key point: pH of rain)

4. What does acid rain do to soil and lakes?
   A. Leaches Ca2+ and Mg2+, releases toxic Al3+ and kills fish
   B. Adds nutrients and helps fish
   C. Makes lakes alkaline
   D. Has no effect on soil
   Answer: A. Acid rain removes nutrients and releases toxic aluminium. (Key point: Effects)

## Related in Engineering Chemistry

- [NO₂ and SO₂ sources; role in acid rain](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ano2-so2-acid-rain) — Engineering Chemistry, chapter 3
- [Acid rain; sources and control of CO and SO₂](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aacid-rain-co-so2) — Engineering Chemistry, chapter 3

## Related in other subjects

- [Short note: Acid rain causes and impacts](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Aacid-rain) — Technology, Environment and Society, chapter 5, asked 2×