---
title: "Oxides of sulphur and nitrogen as pollutants"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:sox-nox-pollutants"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sox-nox-pollutants
site: BCT Engineering
---

# Oxides of sulphur and nitrogen as pollutants

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2075 Baisakh, 2 marks; 2070 Magh, 2 marks; 2069 Poush, 2 marks (asked 3 times)
- Video: [4. Sulfur dioxide and nitrogen oxides (HSC chemistry)](https://www.youtube.com/watch?v=eUiyn3yf3do) — Letslearnscience
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/sox-nox-pollutants
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

Why are the oxides of sulphur and nitrogen assumed as air pollutants?

## Short answer

SO2 and NOx are air pollutants because they form acid rain (H2SO4, HNO3), cause respiratory diseases (asthma, bronchitis), contribute to photochemical smog, damage plants and materials, and N2O is a greenhouse gas that also depletes ozone.

## Answer

Oxides of sulphur (SO₂, SO₃) and nitrogen (NO, NO₂, N₂O) are major air pollutants because:

1. **Sources are large:** SO₂ is produced by burning coal and oil, smelting of sulphide ores and brick kilns; NOx by vehicle engines and power plants (N₂ + O₂ → 2NO at high temperature).
2. **Acid rain:** they form H₂SO₄ and HNO₃ in the air (2SO₂ + O₂ + 2H₂O → 2H₂SO₄; 4NO₂ + O₂ + 2H₂O → 4HNO₃), which acidify lakes and soils and damage forests and buildings.
3. **Health effects:** they irritate the eyes, throat and lungs, causing asthma, bronchitis and reduced lung function; NO₂ is toxic.
4. **Smog:** NO₂ with hydrocarbons in sunlight forms photochemical smog and ground-level ozone; SO₂ with smoke forms classical smog.
5. **Damage to plants and materials:** they bleach leaves and corrode metals, stone, paper and textiles.
6. N₂O is also a greenhouse gas and depletes ozone in the stratosphere.

## Check yourself

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

1. Why are SOx and NOx pollutants in terms of acid rain?
   A. They form carbonic acid that is harmless
   B. They form bases that make soil alkaline
   C. They form H2SO4 and HNO3, acidifying lakes and soils and damaging buildings
   D. They form ozone which cools the earth
   Answer: C. SO2 and NO2 oxidise and dissolve to strong acids. (Key point: Acid rain)

2. How is NO produced in engines?
   A. N2 + H2 -> 2NH3
   B. 2NO2 -> N2 + 2O2 at low temperature
   C. NO is released by plants
   D. N2 + O2 -> 2NO at high temperature
   Answer: D. High temperature combustion makes nitrogen and oxygen combine. (Key point: NOx source)

3. Which smog is formed from NO2 with hydrocarbons in sunlight?
   A. Photochemical smog with ground-level ozone
   B. Classical smog from SO2 and smoke
   C. Volcanic smog
   D. Acid fog containing only CO
   Answer: A. NO2 with hydrocarbons gives photochemical smog; SO2 with smoke gives classical smog. (Key point: Smog)

4. What is a special point about N2O?
   A. It is only an acid rain gas
   B. It is a greenhouse gas and depletes stratospheric ozone
   C. It is harmless
   D. It forms CO
   Answer: B. N2O causes warming and ozone loss. (Key point: N2O)

## Related in Engineering Chemistry

- [Water pollutants, sources; oxides making water acidic](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-acidic) — Engineering Chemistry, chapter 3
- [Air pollution, pollutants, sources and remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollution-sources-remedies) — Engineering Chemistry, chapter 3
- [Air pollutants: effects and remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollutants-effects) — Engineering Chemistry, chapter 3
- [Classification of air pollutants; sources, effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollutants-classified) — Engineering Chemistry, chapter 3

## Related in other subjects

- [Health impacts of indoor air pollutants](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aindoor-air-pollutants-health) — Energy Environment and Society, chapter 4
- [Reducing indoor air pollutants](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aindoor-air-pollutant-reduction) — Energy Environment and Society, chapter 4
- [Classification and sources of air pollutants](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aair-pollutants-classification-sources) — Environment and Pollution Control, chapter 1
- [Indoor air pollutants and their sources](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aindoor-air-pollutants-sources) — Environment and Pollution Control, chapter 3