---
title: "Water pollutants, sources; oxides making water acidic"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:water-pollutants-acidic"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/water-pollutants-acidic
site: BCT Engineering
---

# Water pollutants, sources; oxides making water acidic

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2073 Bhadra, 1+2+2 marks
- Video: [Sulfur, sulfur oxides and sulfuric acid (acid rain)](https://www.youtube.com/watch?v=iL95R6TBIP8) — Explained Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/water-pollutants-acidic
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

What are water pollutants? Write the major sources of water pollution. How do the oxides of sulphur and nitrogen make water acidic?

## Short answer

Water pollutants are substances that make water unfit for use. Major sources: domestic sewage, industrial effluents, agricultural runoff. SO₂ and NOx dissolve in rainwater to form H₂SO₄ and HNO₃, producing acid rain (pH < 5.6) that acidifies water bodies.

## Answer

#### Water pollutants
**Water pollutants** are substances (physical, chemical or biological) that, when present in water above a permissible level, make it unfit for use. Examples: pathogens, sewage, detergents, pesticides, heavy metals, nitrates and phosphates, oil, heat and radioactive substances.

#### Major sources of water pollution
- **Domestic sewage:** human waste, kitchen waste, detergents.
- **Industrial effluents:** acids, alkalis, dyes, heavy metals (Pb, Hg, Cd, As, Cr), oils and phenols, from tanneries and from paper, textile, sugar and chemical industries.
- **Agricultural runoff:** fertilisers (nitrates, phosphates), pesticides (DDT) and animal wastes.
- **Mining and oil:** acid mine drainage, oil spills.
- **Thermal pollution:** hot water from power plants and industries.
- **Others:** radioactive wastes, landfill leachate, siltation from soil erosion, religious and cultural offerings, municipal solid waste dumping into rivers.

#### How oxides of sulphur and nitrogen make water acidic
SO₂ and NOx released by burning fuels and vehicles dissolve in atmospheric water and rain, forming strong acids:

```text
SO₂ + H₂O → H₂SO₃ ;   2SO₂ + O₂ → 2SO₃ ;   SO₃ + H₂O → H₂SO₄
2NO + O₂ → 2NO₂ ;   3NO₂ + H₂O → 2HNO₃ + NO
```
These acids fall as acid rain (pH < 5.6) into lakes and rivers, increasing [H⁺] and lowering the pH of the water, which kills fish and aquatic plants.

## Check yourself

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

1. What are water pollutants?
   A. Only toxic metals
   B. Substances that above a permissible level make water unfit for use
   C. Only substances that are coloured
   D. Only sewage
   Answer: B. Physical, chemical or biological substances above permissible level. (Key point: Definition)

2. Which is an agricultural source of water pollution?
   A. Hot cooling water from power plants
   B. Acid mine drainage
   C. Runoff of fertilisers (nitrates, phosphates), pesticides and animal wastes
   D. Tannery effluents with phenols
   Answer: C. Agricultural runoff carries fertilisers, pesticides and animal wastes; the others are thermal, mining and industrial sources. (Key point: Sources)

3. How do oxides of sulphur make water acidic?
   A. They react to produce NaOH
   B. They form CO2 that makes water neutral
   C. They lower [H+]
   D. SO3 dissolves to give H2SO4 (and SO2 gives H2SO3), raising [H+] in rain that falls into water
   Answer: D. The acids increase H+ and lower the pH. (Key point: Acid formation)

4. How does NO2 form nitric acid?
   A. 3NO2 + H2O -> 2HNO3 + NO
   B. NO2 + H2 -> NH3
   C. 2NO2 -> N2O4 only
   D. NO2 + CO2 -> HNO3
   Answer: A. NO2 disproportionates in water to HNO3 and NO. (Key point: NO2)

## Related in Engineering Chemistry

- [Oxides of sulphur and nitrogen as pollutants](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Asox-nox-pollutants) — Engineering Chemistry, chapter 3, asked 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
- [Water pollution, pollutants and adverse effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects) — Engineering Chemistry, chapter 3
- [Major pollutants of water](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-list) — Engineering Chemistry, chapter 3
- [Sources, effects and remedies of water pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollution-sources) — Engineering Chemistry, chapter 3, asked 2×
- [Sources and impacts of water pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-sources-impacts) — Engineering Chemistry, chapter 3

## Related in other subjects

- [Water pollution: sources and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Awater-pollution-sources-effects) — Environment and Pollution Control, chapter 4
- [Water pollution: sources, causes and impacts](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Awater-pollution-sources-impacts) — Technology, Environment and Society, chapter 5
- [Sources and types of water pollution](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Awater-pollution-sources-types) — Technology, Environment and Society, chapter 5
- [Sources and effects of SO2, NOx and CO](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Agaseous-pollutants-so2-nox-co) — Environment and Pollution Control, chapter 1