Environmental Chemistry — Complete Mnemonic
Target: mnemonic for environmental chemistry pollution ozone depletion
Why is this hard to memorize?
Environmental chemistry covers air pollution (SOx, NOx, CO, particulates, smog), water pollution (BOD, COD, eutrophication), soil pollution (pesticides, industrial waste), and stratospheric ozone depletion (CFCs). NEET tests this chapter as direct factual recall — knowing the pollutant, its source, effects, and solutions is enough for full marks. Key facts: BOD (Biological Oxygen Demand) measures organic pollution in water, higher BOD = more polluted. Ozone depletion is caused by CFCs (Freon) releasing Cl atoms that catalytically destroy O₃. The greenhouse effect is caused by CO₂, CH₄, N₂O, and CFCs.
Classic mnemonics you should know
"Primary: directly emitted (CO, SO₂, NO, particulates). Secondary: formed by reaction (O₃, PAN, smog)"
Primary pollutants are emitted directly from sources (vehicles, factories). Secondary pollutants form in the atmosphere by reaction of primary pollutants with sunlight or other chemicals. Photochemical smog = O₃ + PAN + aldehydes (from NOx + hydrocarbons + sunlight).
"CFCs → Cl atoms in stratosphere. Cl + O₃ → ClO + O₂. ClO + O → Cl + O₂. One Cl destroys ~100,000 O₃ molecules (catalytic cycle)"
Chlorofluorocarbons (CFCs like Freon) are stable in the troposphere but break down in the stratosphere under UV radiation, releasing Cl atoms. Each Cl atom catalytically destroys tens of thousands of ozone molecules. Montreal Protocol (1987) banned CFC production.
"BOD = mg of O₂ consumed per litre in 5 days at 20°C. Clean water: BOD < 5 mg/L. Polluted: BOD > 17 mg/L"
BOD measures how much dissolved oxygen microorganisms need to decompose organic matter. High organic pollution → microbes consume more O₂ → higher BOD → fish die (oxygen depletion). COD (Chemical Oxygen Demand) is always higher than BOD because it includes non-biodegradable organics too.
The complete list
- Primary pollutants: CO, SO₂, NO, particulates
- Secondary pollutants: O₃, PAN, smog
- Greenhouse gases: CO₂, CH₄, N₂O, CFCs
- Ozone depletion: CFCs release Cl (catalytic)
- BOD: O₂ needed by microbes (organic pollution)
- Eutrophication: excess nutrients → algal bloom
- Acid rain: SO₂ + NOx → H₂SO₄ + HNO₃
- Montreal Protocol (1987): banned CFCs
Frequently asked questions
What causes photochemical smog and how is it different from classical smog?
Photochemical smog (Los Angeles type): NOx + hydrocarbons + sunlight → O₃ + PAN + aldehydes. Occurs in warm, dry, sunny climate. Oxidizing in nature. Classical smog (London type): SO₂ + particulates in cool, humid conditions → smoke + fog. Reducing in nature. Photochemical smog is more common today due to vehicle emissions.
What is eutrophication?
Excess nutrients (phosphates from detergents, nitrates from fertilizers) enter water bodies → excessive algal growth (algal bloom) → algae block sunlight → aquatic plants die → decomposition consumes dissolved O₂ → fish and aquatic life die → water body becomes "dead." This is why phosphate-free detergents and controlled fertilizer use are important.
What is the greenhouse effect?
Greenhouse gases (CO₂, CH₄, N₂O, CFCs, O₃) allow visible light from the sun to pass through but trap infrared (heat) radiation re-emitted by Earth's surface. This natural effect keeps Earth warm enough for life (~15°C average). But human activities have increased CO₂ (from 280 to 420 ppm) causing enhanced greenhouse effect → global warming → climate change. CO₂ is the biggest contributor by volume, but CH₄ and CFCs are more potent per molecule.
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