Class 11 Chemistry Environmental Chemistry — Samacheer Kalvi Notes

Quick Summary

Environmental chemistry examines how human activities and natural processes affect air, water, and soil. It covers pollutants, their sources, effects, and control. This chapter is important for board exams and for environmental awareness. NEET tests definitions and specific facts from this chapter.

Atmospheric Layers

Air Pollution

Primary pollutants: Directly emitted (SO₂, NOₓ, CO, particulates, hydrocarbons). Secondary pollutants: Formed by reactions in atmosphere (O₃, PAN, H₂SO₄).

Key Air Pollutants

Acid Rain

• Cause: SO₂ + H₂O → H₂SO₃; SO₃ + H₂O → H₂SO₄; NO₂ + H₂O → HNO₃
• Effects: Corrodes buildings (marble, limestone), damages crops, acidifies water bodies, kills fish
• Normal rain pH: ~5.6 (CO₂ + H₂O → H₂CO₃); Acid rain pH < 5.6

Photochemical Smog

• Forms in sunny weather with NOₓ and unburnt hydrocarbons.
• Contains PAN (peroxyacetyl nitrate), O₃, NO₂ — causes eye irritation, respiratory issues.
• Reducing smog (London type): SO₂ + particulates in cold, damp weather.

Ozone Depletion

• Ozone layer absorbs harmful UV-B and UV-C radiation.
• CFCs (refrigerants, aerosols) release Cl• radicals in stratosphere.
• Mechanism: Cl• + O₃ → ClO• + O₂; ClO• + O• → Cl• + O₂ (Cl• regenerated — catalytic)
• Effects of UV: skin cancer, cataracts, reduced crop yield, damage to marine phytoplankton.
• Control: Montreal Protocol (1987) — global ban on CFCs.

Greenhouse Effect and Global Warming

• Greenhouse gases: CO₂ (most important), CH₄, N₂O, CFCs, H₂O vapour
• Effects: Rising sea levels, melting glaciers, extreme weather events, habitat loss
• Control: Reduce fossil fuel use, increase forest cover, renewable energy

Water Pollution

Pollutants and sources:

BOD: Higher BOD = more polluted water. Clean water BOD < 5 mg/L.

Eutrophication: Excess nutrients (N, P) → algal bloom → O₂ depletion → death of aquatic life.

Minamata disease: Mercury poisoning (organic mercury from industrial waste) — Japan, 1950s.

Soil Pollution

Important Questions

One Mark

• Define BOD.
• Name the protocol related to ozone layer protection.
• What is eutrophication?
• Write the chemical formula of PAN.
• Define an air pollutant.

Short Answer (3 marks)

• Explain how acid rain is formed. What is its effect on the Taj Mahal?
• Explain the mechanism of ozone depletion by CFCs.
• What is photochemical smog? How is it different from industrial smog?

Long Answer (5 marks)

• Describe the greenhouse effect. Name the greenhouse gases, explain enhanced greenhouse effect, and suggest measures to reduce global warming.
• Explain water pollution with examples of three major types of water pollutants, their sources, and harmful effects.

Common Mistakes

• Acid rain pH: students write pH = 7 for normal rain — it is ~5.6 (acidic due to dissolved CO₂).
• Greenhouse effect: confusing ozone depletion (UV problem) with greenhouse effect (IR/warming problem) — they are different phenomena.
• BOD: writing that high BOD = clean water — it is the opposite; high BOD means high pollution.
• Ozone mechanism: forgetting that Cl• is regenerated — it acts as a catalyst, not a consumed reactant.

Board Exam Tips

• Acid rain and ozone depletion are almost guaranteed 3-mark questions — know the chemistry (equations) not just the effects.
• Greenhouse gases and global warming: list all gases and specific human activities causing them.
• BOD and eutrophication are frequently tested definitions in 1-mark and 2-mark questions.
• NEET: Environmental chemistry contributes 2–3 direct factual questions — every mark counts.