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Water pollutants, sources; oxides making 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.

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

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:

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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