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ICSE Class 9 Chemistry Question 17 of 31

Atmospheric Pollution — Question 29

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Question 29

Name the chemicals responsible for destruction of the ozone layer. State the main chemical from these chemicals, which is responsible for more than 80% ozone depletion. State the man-made applications which make use of that chemical.

Answer

Chemicals responsible for ozone depletion are :

  1. Chlorofluorocarbons [CFC i.e., CFCl3]
  2. Methyl chloride [CH3Cl]
  3. Methyl bromide [CH3Br]
  4. Carbontetrachloride [CCl4]
  5. Bromofluorocarbons
  6. Methane [CH4]
  7. Nitrous oxide [N2O]

Out of the above chemicals, Chlorofluorocarbons (CFCs) is the main chemical responsible for more than 80% ozone depletion.

Man-made applications making use of Chlorofluorocarbons (CFCs) are:

  1. Aerosol sprays
  2. Coolants in refrigeration and A.C.'s
  3. Drycleaning fluids
  4. Cleansing agents for cleaning electronic components

Chapter Overview: Atmospheric Pollution

This chapter examines the causes, effects, and prevention of atmospheric pollution. The atmosphere is composed of nitrogen (78%), oxygen (21%), argon (0.93%), carbon dioxide (0.04%), and trace amounts of other gases and water vapour. Air pollution occurs when harmful substances (pollutants) contaminate the atmosphere. Major air pollutants include carbon monoxide (CO), sulphur dioxide (SO2), nitrogen oxides (NOx), particulate matter, and chlorofluorocarbons (CFCs). Acid rain forms when SO2 and NOx dissolve in rainwater to form sulphuric acid and nitric acid. The greenhouse effect is the trapping of heat by gases like CO2, CH4, and N2O, leading to global warming. Ozone depletion is caused by CFCs breaking down the ozone (O3) layer in the stratosphere. Students must understand the sources of each pollutant, its harmful effects on health and environment, and measures to control pollution. This chapter connects chemistry concepts to real-world environmental issues.

Key Concepts

Pollutant / Issue Details
CO (Carbon Monoxide)From incomplete combustion; binds to haemoglobin, reducing O2 transport
SO2From burning fossil fuels with sulphur; causes acid rain and respiratory problems
Acid RainSO2 + H2O → H2SO3; 2NO2 + H2O → HNO3 + HNO2; pH < 5.6
Greenhouse EffectCO2, CH4, N2O trap infrared radiation, warming Earth's surface
Ozone DepletionCFCs release Cl atoms that destroy O3: Cl + O3 → ClO + O2
Global WarmingRise in Earth's average temperature due to enhanced greenhouse effect
SmogSmoke + fog; photochemical smog contains O3, NO2, and organic compounds

Must-Know Concepts

  • Composition of clean air: N2 (78%), O2 (21%), Ar (0.93%), CO2 (0.04%)
  • CO is a silent killer - colourless, odourless, and highly toxic
  • Acid rain (pH < 5.6) damages buildings (especially marble: CaCO3 + H2SO4 → CaSO4 + H2O + CO2), kills aquatic life, and harms vegetation
  • Greenhouse gases in order of impact: CO2 > CH4 > N2O > CFCs
  • Montreal Protocol (1987) aims to phase out CFCs to protect the ozone layer
  • Control measures: catalytic converters in vehicles, using clean fuels, afforestation, emission regulations

Greenhouse Effect vs Ozone Depletion

Feature Greenhouse Effect Ozone Depletion
CauseExcess CO2, CH4, N2OCFCs and halons
LocationLower atmosphere (troposphere)Upper atmosphere (stratosphere)
EffectGlobal warming, sea level riseMore UV radiation, skin cancer
PreventionReduce fossil fuel use, afforestationBan CFCs (Montreal Protocol)

Important Diagrams to Practice

  • Greenhouse effect diagram showing incoming solar radiation and trapped infrared radiation
  • Acid rain formation cycle (from factory emissions to acid precipitation)
  • Ozone layer depletion mechanism (CFC breakdown and ozone destruction cycle)

Common Mistakes

  • Confusing greenhouse effect (lower atmosphere, warming) with ozone depletion (upper atmosphere, UV radiation)
  • Saying the greenhouse effect is entirely bad (it is natural and essential; the problem is the enhanced effect)
  • Calling CO2 a pollutant in all contexts (it is a natural atmospheric component; excess is the problem)
  • Confusing ozone at ground level (harmful pollutant in smog) with stratospheric ozone (protective)

Scoring Tips

  • For essay-type questions, organise into: causes → effects → prevention
  • Include chemical equations wherever possible (acid rain formation, ozone destruction)
  • Mention specific international agreements: Montreal Protocol, Kyoto Protocol, Paris Agreement
  • Use specific data: normal rain pH = 5.6, acid rain pH < 5.6; CO2 level now ~420 ppm

Frequently Asked Questions

Is the greenhouse effect good or bad?

The natural greenhouse effect is essential - without it, Earth's average temperature would be about −18°C instead of the current +15°C. The problem is the enhanced greenhouse effect caused by human activities increasing CO2 and other greenhouse gases, leading to global warming.

Why is the ozone hole mainly over Antarctica?

The extreme cold of the Antarctic winter creates polar stratospheric clouds where CFCs accumulate. When spring sunlight returns, UV light breaks CFC molecules, releasing chlorine atoms that rapidly destroy ozone. One chlorine atom can destroy up to 100,000 ozone molecules.

How does acid rain damage buildings?

Acid rain reacts with marble and limestone (CaCO3) in buildings: CaCO3 + H2SO4 → CaSO4 + H2O + CO2. This dissolves the stone surface, causing the "stone cancer" effect. The Taj Mahal is a famous example of acid rain damage.