Air and Water Quality

Air and Water Quality

Overview

Air and Water Quality is a core environmental science topic for WB TET Paper II, bridging chemistry, biology and environmental awareness. Questions typically test your knowledge of atmospheric composition, types and sources of pollution, water treatment basics and conservation strategies. This topic connects directly to the broader Environmental Studies and Science pedagogy themes in the syllabus.

For the exam, you must know the exact percentage composition of atmospheric gases, distinguish between primary and secondary pollutants, understand the water cycle and its disruption by human activity, and recall specific conservation measures.

Mastering this topic also prepares you for pedagogy questions on teaching environmental concepts through experiments, field visits and real-life examples—a key NCF emphasis for upper-primary science.

Key Concepts

• Composition of air: Air is a mixture of gases—nitrogen (78%), oxygen (21%), argon (0.93%), carbon dioxide (0.04%) and traces of water vapour, neon, helium and ozone.
• Air pollution defined: Introduction of harmful substances (particulates, gases, biological molecules) into the atmosphere that cause disease, damage to living organisms or climate change.
• Primary vs secondary pollutants: Primary pollutants are emitted directly (CO, SO₂, NOₓ, particulate matter). Secondary pollutants form by reactions in air (ozone at ground level, PAN—peroxyacetyl nitrate).
• Water distribution on Earth: About 97% is saline (oceans); only 3% is freshwater, of which nearly 69% is locked in glaciers and ice caps, 30% is groundwater and less than 1% is surface freshwater.
• Water pollution defined: Contamination of water bodies (rivers, lakes, groundwater, oceans) by substances that make water harmful for humans, animals, plants or ecosystems.
• Potable water criteria: Safe drinking water must be colourless, odourless, free from pathogens and harmful chemicals, with acceptable pH (6.5–8.5) and dissolved solids (< 500 mg/L TDS recommended).
• Water cycle disruption: Deforestation, urbanisation and pollution alter evaporation, infiltration and runoff patterns, reducing groundwater recharge and freshwater availability.
• Conservation principle: Reduce consumption, reuse where possible, recycle wastewater and protect natural water bodies and air sheds from contamination.

Formulas / Key Facts

Worked Examples

Example 1 – Identifying pollutant type

Question: Classify the following as primary or secondary pollutant: (a) Carbon monoxide from vehicles (b) Ground-level ozone.

Solution:

1. Carbon monoxide (CO) is released directly from incomplete combustion in vehicle engines → Primary pollutant.
2. Ground-level ozone (O₃) is not emitted directly; it forms when NOₓ and volatile organic compounds react in sunlight → Secondary pollutant.

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Example 2 – Water quality indicator

Question: A river sample shows BOD of 8 mg/L. Another sample shows 2 mg/L. Which is more polluted and why?

Solution:

1. BOD measures the oxygen microorganisms need to decompose organic matter.
2. Higher BOD means more organic waste, hence more pollution.
3. Sample with 8 mg/L BOD is more polluted than the one with 2 mg/L.

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Example 3 – Conservation measure

Question: Suggest two methods to conserve groundwater in a village.

Solution:

1. Rainwater harvesting: Construct percolation pits or rooftop collection systems to recharge aquifers.
2. Afforestation: Plant trees to increase infiltration, reduce runoff and maintain the water table.

Common Mistakes

• Confusing nitrogen and oxygen percentages → Remember "78-21" (Nitrogen first, then Oxygen). Nitrogen is roughly four times oxygen.
• Mixing up stratospheric ozone (protective) with ground-level ozone (pollutant) → Ozone in the stratosphere blocks UV; ozone near the ground causes smog and respiratory harm.
• Thinking all CO₂ increase is "pollution" → CO₂ is a natural atmospheric component; it becomes problematic at elevated concentrations causing the greenhouse effect. Precision matters in exam answers.
• Assuming freshwater is abundant → Students often forget that usable freshwater (rivers, lakes, accessible groundwater) is less than 1% of total water. This statistic is frequently tested.
• Ignoring units for BOD and TDS → Always mention mg/L (milligrams per litre) when stating permissible limits or measured values.

Quick Reference

1. Air = 78% N₂ + 21% O₂ + 0.93% Ar + 0.04% CO₂ + traces.
2. Primary pollutants emitted directly; secondary form by atmospheric reactions.
3. PM₂.₅ and PM₁₀ are key air-quality indicators; smaller particles are more harmful.
4. Only ~1% of Earth's water is accessible freshwater.
5. Higher BOD = more organic pollution in water.
6. Safe drinking water: pH 6.5–8.5, TDS < 500 mg/L, free of pathogens.
7. Conservation: Reduce, Reuse, Recycle + Rainwater harvesting + Afforestation.