Air and Atmosphere
1. Composition of Air
Air is a MIXTURE of gases (not a compound).
Percentage Composition by Volume
| Gas | Percentage (%) | Properties |
|---|---|---|
| Nitrogen (N₂) | 78.08% (~78%) | Relatively inert, does not support combustion |
| Oxygen (O₂) | 20.95% (~21%) | Supports combustion and respiration |
| Argon (Ar) | 0.93% | Noble gas, inert |
| Carbon dioxide (CO₂) | 0.03-0.04% | Greenhouses gas, essential for photosynthesis |
| Neon (Ne) | 0.0018% | Used in neon signs |
| Helium (He) | 0.0005% | Light gas, used in balloons |
| Other gases | Trace | Hydrogen, ozone, krypton, xenon |
| Water vapour | Variable (0-4%) | Humidity |
| Dust particles | Variable | Condensation nuclei |
Experiments to Show Air Composition
Presence of Oxygen: A burning candle placed in a closed jar goes out after some time (oxygen is used up). Water rises to ~1/5 of the jar volume — proving oxygen is ~21% of air.
Presence of Water Vapour: Ice cold water in a glass causes condensation on the outside — shows water vapour in air.
Presence of Carbon Dioxide: Limewater turns MILKY when exposed to air showing CO₂.
2. Layers of the Atmosphere
The ATMOSPHERE is the GASEOUS ENVELOPE surrounding the Earth.
Layers (Bottom to Top)
| Layer | Altitude Range | Key Features |
|---|---|---|
| Troposphere | 0-12 km | WEATHER occurs here. Temperature DECREASES with height. Contains ~80% of air. |
| Stratosphere | 12-50 km | OZONE LAYER present. Temperature INCREASES with height. Jet aircraft fly here. |
| Mesosphere | 50-80 km | Temperature DECREASES with height. Meteors BURN UP here. |
| Thermosphere | 80-700 km | Temperature INCREASES sharply. Auroras occur. Radio waves reflect. |
| Exosphere | 700+ km | Outermost layer. Air VERY thin. Satellites orbit here. |
The Ozone Layer
- Located in the STRATOSPHERE (20-30 km altitude).
- Ozone (O₃) ABSORBS harmful ULTRAVIOLET (UV) radiation from the Sun.
- Protects living organisms from skin cancer, cataracts, and genetic damage.
- Ozone depletion: Caused by CFCs (chlorofluorocarbons) from refrigerators, air conditioners, and aerosols.
- Montreal Protocol (1987): International agreement to phase out CFCs.
3. Air Pollution
Major Air Pollutants
| Pollutant | Source | Harmful Effects |
|---|---|---|
| Carbon monoxide (CO) | Incomplete burning of fuels | Reduces oxygen-carrying capacity of blood |
| Sulphur dioxide (SO₂) | Burning of coal, oil | Acid rain, respiratory problems |
| Nitrogen oxides (NOₓ) | Vehicle emissions | Acid rain, smog, breathing issues |
| Particulate matter | Dust, smoke, construction | Lung diseases, asthma |
| Lead compounds | Old petrol engines | Brain damage (especially in children) |
| Chlorofluorocarbons (CFCs) | Refrigerators, ACs | Ozone layer depletion |
Effects of Air Pollution
- Acid rain: SO₂ and NOₓ dissolve in rainwater to form acids. Damages buildings, soil, water bodies.
- Smog: Mixture of smoke and fog. Reduces visibility. Causes respiratory problems.
- Global warming: Rise in Earth's average temperature due to increased greenhouse gases.
- Health issues: Asthma, bronchitis, lung cancer, heart disease.
4. The Greenhouse Effect
The GREENHOUSE EFFECT is the NATURAL process by which certain gases trap heat in the Earth's atmosphere.
How It Works
- Sunlight passes through the atmosphere and warms the Earth's surface.
- The Earth radiates heat (infrared radiation) back towards space.
- GREENHOUSE GASES trap some of this heat, keeping the Earth WARM.
- Without this effect, Earth would be too cold for life (~ -18°C).
Greenhouse Gases
| Gas | Source | Contribution |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels, deforestation | ~60% of enhanced effect |
| Methane (CH₄) | Agriculture, livestock, landfills | ~20% |
| Nitrous oxide (N₂O) | Fertilizers, industrial processes | ~6% |
| Water vapour (H₂O) | Natural | Most abundant but natural |
| CFCs | Refrigerants | Small but powerful |
Enhanced Greenhouse Effect (Global Warming)
Human activities are INCREASING greenhouse gas levels, trapping MORE heat → rising global temperatures.
Effects:
- Melting of polar ice caps → rising sea levels.
- Extreme weather events (floods, droughts, storms).
- Changes in agricultural patterns.
- Loss of biodiversity.
5. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Composition of air (percentages) | 2-3 marks | Very High |
| Layers of atmosphere | 2-3 marks | High |
| Air pollution (pollutants and effects) | 3 marks | Very High |
| Greenhouse effect and global warming | 3 marks | Very High |
| Ozone layer and depletion | 2-3 marks | High |
Common Mistakes
- Thinking air is a COMPOUND — it is a MIXTURE of gases.
- Confusing the layers of atmosphere (troposphere vs stratosphere).
- Writing CO₂ percentage as 21% (CO₂ is just 0.03-0.04%, oxygen is 21%).
- Saying greenhouse effect is always bad — it is NATURAL and necessary. The ENHANCED greenhouse effect is the problem.
Self-Test (5 Questions)
Q1. Which gas makes up the LARGEST percentage of air? (1 mark)
- A) Oxygen
- B) Nitrogen
- C) Carbon dioxide
- D) Argon
Q2. 'Name the layer of atmosphere that contains the ozone layer.' (1 mark)
Q3. List TWO air pollutants and their sources. (2 marks)
Q4. 'What is the greenhouse effect?' (2 marks)
Q5. 'Why is the ozone layer important for life on Earth?' (2 marks)
Answers
A1. B) Nitrogen (~78%). A2. Stratosphere. A3. SO₂ (burning coal), CO (incomplete burning of fuels), NOₓ (vehicle emissions), CFCs (refrigerators). (Any two valid.) A4. The greenhouse effect is the natural trapping of heat by greenhouse gases (CO₂, CH₄, etc.) in the Earth's atmosphere, keeping the planet warm enough for life. A5. The ozone layer ABSORBS harmful ULTRAVIOLET (UV) radiation from the Sun, protecting living organisms from skin cancer, eye damage, and genetic mutations.
