Chapter 18: Air Around Us 🌬️
A Comprehensive Guide for PSTET Paper-2 (Science)
Chapter Overview
| Section | Topic | PSTET Weightage | Page No. |
|:---:|:---|::---:|:---:|
| 18.1 | Composition of Air | High | 2 |
| 18.2 | Role of Atmosphere (Air for Respiration, for Burning, by Plants) | High | 8 |
| 18.3 | Air Pollution: Causes and Effects | High | 14 |
| Practice Zone | MCQs & Pedagogical Questions | - | 22 |
Learning Objectives 🎯
After studying this chapter, you will be able to:
✅ Describe the composition of air with accurate percentages of major and minor gases
✅ Explain the essential roles of the atmosphere in supporting life—respiration, combustion, and photosynthesis
✅ Identify major causes of air pollution from natural and anthropogenic sources
✅ Analyze the effects of air pollution on human health, plants, animals, and the environment
✅ Understand the balance between oxygen and carbon dioxide maintained by natural processes
✅ Apply pedagogical strategies to teach air-related concepts effectively to upper primary students
Pedagogical Link 🔗
For Teachers: This chapter directly aligns with:
Class 6 Science NCERT Chapter 15: "Air Around Us"
Class 7 Science NCERT Chapter 18: "Wastewater Story" (related to environmental pollution)
Class 8 Science NCERT Chapter 18: "Pollution of Air and Water"
Teaching Tips:
Begin with simple experiments like the candle burning under a glass to show oxygen consumption
Use demonstrations with lime water to test for carbon dioxide
Create a "pollution journal" where students observe and record local air quality indicators
Use videos and animations to show the greenhouse effect and ozone layer depletion
Connect to current events—discuss air quality index (AQI) in your city
Section 18.1: Composition of Air 🔬
Introduction
Air is all around us—invisible, tasteless, and odorless. But what exactly is air? It is not a single substance but a mixture of gases that forms the Earth's atmosphere . The composition of air is remarkably uniform near the Earth's surface due to constant mixing by winds and atmospheric motions .
18.1.1 Air is a Mixture, Not a Compound
Key Concept: Air is a mixture of gases, not a chemical compound . This means:
The components retain their individual properties
They can be separated by physical methods
The composition can vary (especially variable components)
18.1.2 Major Components of Air
The atmosphere consists of gases with nearly constant concentrations (the major components) and gases with variable concentrations (trace gases) .
Table 18.1: Composition of Dry Air (Constant Components)
| Gas | Chemical Symbol | Percentage by Volume | Role |
|---|---|---|---|
| Nitrogen | N₂ | 78.084% | Essential for plant nutrition; dilutes oxygen |
| Oxygen | O₂ | 20.946% | Supports respiration and combustion |
| Argon | Ar | 0.934% | Inert gas; no significant biological role |
| Carbon Dioxide | CO₂ | 0.033% (average 0.04%) | Essential for photosynthesis; greenhouse gas |
| Neon | Ne | 0.0018% | Trace inert gas |
| Helium | He | 0.000524% | Trace inert gas |
| Methane | CH₄ | 0.0002% | Greenhouse gas |
| Krypton | Kr | 0.000114% | Trace inert gas |
| Hydrogen | H₂ | 0.00005% | Trace gas |
| Nitrous Oxide | N₂O | 0.00005% | Greenhouse gas |
| Xenon | Xe | 0.0000087% | Trace inert gas |
📝 PSTET Note: Nitrogen and oxygen together make up approximately 99% of dry air . Only 3 out of every 10,000 particles are carbon dioxide, yet it plays a crucial role in Earth's heat balance and photosynthesis .
18.1.3 Variable Components of Air
Some atmospheric gases have concentrations that vary significantly with location, time, and conditions .
Table 18.2: Variable Components of Air
| Gas | Chemical Symbol | Typical Range (%) | Importance |
|---|---|---|---|
| Water Vapor | H₂O | 0 to 7% | Source of precipitation; absorbs infrared radiation |
| Carbon Dioxide | CO₂ | 0.01 to 0.1% | Photosynthesis; greenhouse gas; increasing due to human activities |
| Ozone | O₃ | 0 to 0.01% | Absorbs harmful UV radiation (in stratosphere) |
| Sulfur Dioxide | SO₂ | 0 to 0.0001% | Pollutant from industrial processes; causes acid rain |
| Nitrogen Dioxide | NO₂ | 0 to 0.000002% | Pollutant from vehicles; causes respiratory problems |
18.1.4 Understanding the Numbers: Particles Representation
Table 18.3: Air Composition in Particles per 10,000
| Substance | Number of Particles in Every 10,000 Particles of Air |
|---|---|
| Nitrogen | 7,800 |
| Oxygen | 2,100 |
| Argon | 93 |
| Carbon Dioxide | 3 |
| Other Substances | 4 |
This representation helps students visualize that for every 10,000 air particles, only about 3 are carbon dioxide—yet those 3 are critically important for life on Earth.
18.1.5 Why Is the Composition Uniform?
The uniformity of air composition near Earth's surface is maintained by constant mixing associated with atmospheric motions—winds, convection currents, and turbulence . However, above about 90 km (55 miles), diffusional processes become more important than mixing, and lighter gases like hydrogen and helium become more abundant .
18.1.6 "Pure Air" vs. "Clean Air"
Important Distinction: The term "pure air" has no precise scientific meaning because air composition is naturally variable . In atmospheric chemistry, "clean air" is commonly considered to be air that is:
Zero Air: Laboratories use specially prepared "zero air" as a reference sample, which is free of most reactive anthropogenic pollutants (NO, NO₂, SO₂, non-methane hydrocarbons, etc.) .
18.1.7 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Pie Chart Activity | Students draw pie charts showing air composition | Visual learning |
| Particle Model | Use 10,000 counters to represent particles and sort by gas type | Concrete representation |
| Candle Experiment | Burn candle under glass—water rises showing oxygen consumed | Hands-on demonstration |
| Lime Water Test | Test exhaled air for CO₂ (lime water turns milky) | Scientific investigation |
Section 18.2: Role of Atmosphere (Air for Respiration, for Burning, by Plants) 🌍
Introduction
The atmosphere is not just a passive blanket of gases—it is essential for life on Earth . Every breath we take, every fire we light, and every plant that grows depends on the air around us. The atmosphere maintains a delicate balance that makes our planet habitable .
18.2.1 The Atmosphere's Essential Functions
Table 18.4: Essential Roles of the Atmosphere
| Function | Description | Why It Matters |
|---|---|---|
| Provides Oxygen for Respiration | Animals and humans inhale oxygen to release energy from food | Without oxygen, aerobic life cannot exist |
| Provides Carbon Dioxide for Photosynthesis | Plants use CO₂ and sunlight to produce food and oxygen | Maintains oxygen-CO₂ balance |
| Supports Combustion (Burning) | Fire requires oxygen to burn | Essential for cooking, heating, industry |
| Nitrogen for Plants | Nitrogen is fixed by bacteria and used by plants | Essential for protein synthesis |
| Protects from UV Radiation | Ozone layer absorbs harmful ultraviolet rays | Prevents skin cancer, protects ecosystems |
| Regulates Temperature | Greenhouse gases trap heat, maintaining habitable temperatures | Prevents extreme temperature swings |
| Enables Water Cycle | Water vapor condenses to form clouds and precipitation | Provides fresh water |
18.2.2 Air for Respiration
Respiration is the process by which living organisms release energy from food .
The Respiration Equation:
Glucose (C₆H₁₂O₆) + Oxygen (O₂) → Carbon Dioxide (CO₂) + Water (H₂O) + Energy
Key Points:
All aerobic organisms—animals, humans, and even plants—respire
During respiration, oxygen is taken in and carbon dioxide is released
This is the opposite of photosynthesis
Interesting Fact: Plants respire too! They take in oxygen and release carbon dioxide continuously—both day and night. However, during daylight, photosynthesis dominates, so overall plants produce more oxygen than they consume .
18.2.3 Air for Burning (Combustion)
Definition: Combustion is a chemical reaction in which a substance reacts rapidly with oxygen, producing heat and light .
Requirements for Burning:
Fuel (wood, coal, gas, candle wax)
Oxygen (from air)
Ignition temperature (heat to start the reaction)
Simple Demonstration: The Candle Experiment
| Step | Procedure | Observation | Conclusion |
|---|---|---|---|
| 1 | Light a candle and let it burn | Candle burns steadily | Air supports burning |
| 2 | Cover the candle with a glass jar | Flame flickers and goes out | Burning stops when oxygen is depleted |
| 3 | Observe water rise (if set up) | Water rises inside jar | Oxygen was consumed, creating partial vacuum |
This classic experiment demonstrates that:
Air (oxygen) is essential for burning
Only about 1/5 of air (oxygen) supports combustion—the remaining 4/5 (mostly nitrogen) does not
18.2.4 Air for Plants: The Oxygen-CO₂ Balance
The atmosphere and the biosphere are locked in a dynamic equilibrium maintained by life itself .
A. Photosynthesis: Plants Using Carbon Dioxide
The Photosynthesis Equation:
Carbon Dioxide (CO₂) + Water (H₂O) ───sunlight───→ Glucose (C₆H₁₂O₆) + Oxygen (O₂)
(chlorophyll)Key Points:
Plants take in CO₂ from the atmosphere
Using sunlight energy and chlorophyll, they convert CO₂ and water into glucose (food)
This process is the foundation of almost all food chains
B. The Delicate Balance
Respiration (animals and plants) and photosynthesis (plants) are opposite processes that together maintain atmospheric balance .
Table 18.5: Respiration vs. Photosynthesis
| Process | Takes In | Releases | Occurs In | Energy |
|---|---|---|---|---|
| Respiration | Oxygen | Carbon Dioxide | All living organisms | Releases energy |
| Photosynthesis | Carbon Dioxide | Oxygen | Green plants (only in light) | Stores energy |
C. The Biosphere 2 Lesson
The Biosphere 2 experiment (a sealed ecological facility in Arizona) dramatically demonstrated the importance of this balance. When oxygen levels began dropping unexpectedly, scientists discovered that microbes in the soil were respiring more than expected, consuming oxygen faster than plants could produce it . This showed how delicate the balance can be when the system is small and enclosed.
Key Insight: Earth's atmosphere has such a vast volume that all respiration on the planet would only reduce oxygen by 1% over a century if it weren't continually replenished by photosynthesis .
18.2.5 Nitrogen's Role
Although nitrogen (78%) is the most abundant gas, it is not directly usable by most organisms. However, it is essential because :
| Process | Description |
|---|---|
| Nitrogen Fixation | Certain bacteria (in soil and root nodules of legumes) convert atmospheric nitrogen into usable compounds (ammonia, nitrates) |
| Plant Nutrition | Plants absorb these nitrogen compounds to make proteins |
| Animal Nutrition | Animals get nitrogen by eating plants or other animals |
📝 PSTET Note: Without nitrogen-fixing bacteria, plants could not access the vast reservoir of nitrogen in the atmosphere.
18.2.6 The Ozone Layer: Atmospheric Protection
Ozone (O₃) is present mainly in the atmospheric region 10 to 50 km above Earth's surface (the stratosphere) .
18.2.7 Temperature Regulation
The atmosphere acts as a thermal blanket :
| Greenhouse Effect | Importance |
|---|---|
| Greenhouse gases (CO₂, water vapor, methane) trap heat | Maintains Earth's average temperature at about 15°C |
| Without greenhouse effect | Earth would be about -18°C—too cold for most life |
18.2.8 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| Candle Experiment | Demonstrate that burning requires oxygen | Hands-on learning |
| Lime Water Test | Show exhaled air contains CO₂ | Scientific investigation |
| Plant Respiration Demo | Cover leaves with polythene bag—observe water droplets | Observation skills |
| Balance Diagram | Draw the oxygen-CO₂ cycle between plants and animals | Visual learning |
Section 18.3: Air Pollution: Causes and Effects 🏭
Introduction
Air pollution is the presence of harmful substances in the air that we breathe . It is a global concern driven by growing population, rising energy consumption, transportation, and industrial activities . In 2021 alone, air pollution caused 8.1 million premature deaths worldwide .
18.3.1 What is Air Pollution?
Definition: Air pollution refers to the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere .
Two Main Types of Air Pollution :
| Type | Sources | Affected Areas |
|---|---|---|
| Indoor Air Pollution | Burning of wood, charcoal, animal dung ("solid fuels") for cooking | Homes in developing countries (billions of people affected) |
| Outdoor (Ambient) Air Pollution | Vehicles, factories, power plants, construction, forest fires, dust storms | Urban and industrial areas worldwide |
📝 PSTET Note: 99% of people on Earth breathe air that the World Health Organization considers polluted .
18.3.2 Major Air Pollutants
Table 18.6: Primary Air Pollutants and Their Sources
| Pollutant | Symbol | Major Sources | Characteristics |
|---|---|---|---|
| Particulate Matter (PM10) | PM₁₀ | Vehicles, factories, construction, dust storms, wildfires | Particles <10 microns; can penetrate deep into lungs |
| Fine Particulate Matter | PM₂.₅ | Burning of fossil fuels, industrial processes | Particles <2.5 microns; can enter bloodstream |
| Carbon Monoxide | CO | Incomplete combustion of wood, kerosene, charcoal, petrol | Odorless, colorless gas; reduces oxygen delivery in body |
| Ground-level Ozone | O₃ | Formed when sunlight reacts with pollutants (NOx, VOCs) | Major component of smog; harmful to breathe |
| Nitrogen Dioxide | NO₂ | Fossil-fuel-burning factories and vehicles | Respiratory irritant; contributes to acid rain |
| Sulfur Dioxide | SO₂ | Burning of coal and oil in power plants, industrial processes | Causes acid rain; respiratory problems |
| Volatile Organic Compounds | VOCs | Vehicle emissions, solvents, industrial processes | Contribute to ozone formation |
| Methane | CH₄ | Agriculture (livestock), landfills, fossil fuel extraction | Greenhouse gas; affects climate |
| Black Carbon | - | Incomplete combustion of biomass and fossil fuels | Contributes to global warming; health hazard |
18.3.3 Sources of Air Pollution
A. Anthropogenic (Human-made) Sources
| Source Category | Specific Sources | Major Pollutants |
|---|---|---|
| Transportation | Cars, buses, trucks, motorcycles, ships, airplanes | CO, NOx, VOCs, PM, SO₂ (from diesel) |
| Industrial Activities | Factories, power plants, refineries, manufacturing | SO₂, NOx, PM, VOCs, heavy metals |
| Residential | Cooking with solid fuels (wood, dung, coal), heating | PM, CO, black carbon |
| Agriculture | Livestock, fertilizer application, crop residue burning | Methane, ammonia, PM |
| Waste Management | Landfills, waste incineration | Methane, toxic gases |
B. Natural Sources
| Source | Pollutants |
|---|---|
| Wildfires | PM, CO, VOCs |
| Dust storms | PM (mineral dust) |
| Volcanic eruptions | SO₂, ash, PM |
| Pollen and spores | Biological particulates |
18.3.4 Effects of Air Pollution on Human Health
Air pollution poses significant threats to human health, with vulnerable groups including children, the elderly, and people with pre-existing conditions being most at risk .
How Pollutants Enter the Body:
Table 18.7: Health Effects of Air Pollution
📝 PSTET Note: Children, the elderly, and people with pre-existing health conditions are particularly vulnerable to air pollution effects .
18.3.5 Effects of Air Pollution on the Environment
Air pollution has devastating impacts on ecosystems, plants, and animals .
A. Acid Rain
| Formation | Effects |
|---|---|
| SO₂ and NOx react with water, oxygen in atmosphere | • Harms aquatic ecosystems (fish die) |
| Forms sulfuric and nitric acids | • Damages soil quality (leaches nutrients) |
| Falls as rain, snow, fog ("acid rain") | • Destroys forests |
| • Corrodes buildings and monuments (especially marble/limestone) |
B. Climate Change
| Mechanism | Impact |
|---|---|
| Greenhouse gases (CO₂, CH₄, black carbon) trap heat | Global warming |
| Black carbon absorbs sunlight, heats atmosphere | Accelerates ice melt |
| Methane is a potent greenhouse gas (~25× CO₂ over 100 years) | Temperature rise, extreme weather |
C. Ecosystem Damage
| Effect | Description |
|---|---|
| Eutrophication | Nitrogen deposition from air pollution over-fertilizes water bodies, causing algal blooms and dead zones |
| Ozone Damage to Plants | Ground-level ozone damages leaf tissue, reduces photosynthesis, stunts growth, reduces crop yields |
| Biodiversity Loss | Sensitive species die; habitats degrade |
D. Agricultural Impacts
| Impact | Cause |
|---|---|
| Reduced crop yields | Ground-level ozone inhibits plant growth |
| Forest dieback | Acid rain and nutrient depletion |
| Soil degradation | Acidification, nutrient loss |
18.3.6 The Air Pollution-Climate Change Connection
Many of the same substances that make people sick also trap heat near Earth's surface, stoking climate change .
| Pollutant | Health Impact | Climate Impact |
|---|---|---|
| Black carbon | Respiratory disease | Absorbs sunlight, warms atmosphere |
| Methane | (indirect, ozone formation) | Potent greenhouse gas |
| Ground-level ozone | Lung damage | Greenhouse gas |
| CO₂ | (indirect) | Primary long-lived greenhouse gas |
📝 PSTET Note: These "super pollutants" are responsible for a large percentage of global warming to date .
18.3.7 Solutions and Control Measures
A. Policy and Regulatory Approaches
| Measure | Examples |
|---|---|
| Emission Standards | Clean Air Act (USA), Bharat Stage VI (India) |
| Air Quality Monitoring | Real-time monitoring networks, public AQI reporting |
| Vehicle Regulations | Phasing out old vehicles, promoting electric vehicles |
| Industrial Controls | Scrubbers, filters, cleaner production technologies |
| Urban Planning | Green spaces, emission-free transport zones |
B. Individual Actions
| Action | Impact |
|---|---|
| Use public transport, carpool, bike | Reduces vehicle emissions |
| Reduce energy consumption | Less fossil fuel burning |
| Avoid burning waste | Prevents toxic emissions |
| Plant trees | Absorb CO₂, filter particulates |
| Choose clean cooking fuels | Reduces indoor air pollution |
C. International Cooperation
| Initiative | Purpose |
|---|---|
| Climate and Clean Air Coalition | Reduce short-lived climate pollutants |
| BreatheLife | Public health campaign for clean air |
| Global Partnership on Air Quality | International collaboration |
| UN Common Approach to a Pollution-Free Planet | UN-wide framework |
📝 PSTET Note: Air pollutants can travel across borders, with ozone and particulate matter capable of radiating out hundreds or thousands of kilometers from their source, making international cooperation essential .
18.3.8 India-Specific Context
| Fact | Details |
|---|---|
| Major Sources in India | Vehicles, industrial emissions, biomass burning, construction dust, crop residue burning |
| National Air Quality Index (AQI) | Launched in 2014; monitors 8 pollutants across 240+ cities |
| Bharat Stage VI (BS-VI) | Equivalent to Euro 6; implemented nationwide since 2020 |
| National Clean Air Programme (NCAP) | Aims to reduce particulate pollution by 20-30% by 2024 |
18.3.9 Pedagogical Implications
| Teaching Strategy | Description | PSTET Focus |
|---|---|---|
| AQI Monitoring | Check daily AQI for your city; discuss with students | Real-world connection |
| "Pollution Detectives" | Identify local pollution sources (idling cars, construction, burning) | Observation skills |
| Health Connection | Discuss respiratory illnesses, especially during smog episodes | Health awareness |
| Poster Campaign | Students create awareness posters about pollution prevention | Creative learning |
Chapter Summary: Key Points for Revision 📝
Quick Revision Table
Practice Zone: PSTET-Style Questions 🎯
Content-Based MCQs
Q1. The most abundant gas in Earth's atmosphere is:
a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Argon
Q2. The approximate percentage of oxygen in dry air is:
a) 78%
b) 21%
c) 0.9%
d) 0.04%
Q3. Air is best described as:
a) A compound
b) An element
c) A mixture of gases
d) A single substance
Q4. The process by which plants release oxygen into the atmosphere is called:
a) Respiration
b) Transpiration
c) Photosynthesis
d) Decomposition
Q5. Which gas is essential for burning (combustion)?
a) Nitrogen
b) Oxygen
c) Carbon dioxide
d) Argon
Q6. A candle burning under a glass jar eventually goes out because:
a) All oxygen is consumed
b) Carbon dioxide is used up
c) Nitrogen is used up
d) The glass absorbs light
Q7. Which pollutant is small enough to enter the bloodstream and affect every organ?
a) PM10
b) PM2.5
c) Carbon monoxide
d) Sulfur dioxide
Q8. The layer of the atmosphere that protects us from harmful UV radiation is:
a) Troposphere
b) Stratosphere (ozone layer)
c) Mesosphere
d) Thermosphere
Q9. Which of the following is NOT a greenhouse gas?
a) Carbon dioxide
b) Methane
c) Nitrogen
d) Water vapor
Q10. How many premature deaths were attributed to air pollution in 2021?
a) 1 million
b) 3.5 million
c) 8.1 million
d) 15 million
Pedagogical MCQs
Q11. A teacher wants to demonstrate that air contains oxygen. The best activity would be:
a) Show a diagram of air composition
b) Light a candle and cover it with a glass jar—observe flame goes out
c) Lecture about oxygen
d) Show a video of a rocket launch
Q12. To teach students about air pollution sources in their community, the most effective approach is:
a) Give a list of sources to memorize
b) Take students on a neighborhood walk to observe and record potential sources
c) Show pictures of polluted cities
d) Read from textbook
Q13. A student asks, "Why do plants need carbon dioxide?" The best explanation is:
a) "They breathe it like we breathe oxygen"
b) "Plants use CO₂ and sunlight to make their food through photosynthesis, and they release oxygen"
c) "CO₂ helps plants grow bigger"
d) "I don't know"
Q14. While teaching the oxygen-CO₂ balance, a teacher should emphasize that:
a) Only animals need oxygen
b) Plants and animals depend on each other to maintain atmospheric balance
c) Plants produce oxygen only at night
d) Respiration and photosynthesis are unrelated
Q15. The most effective way to help students understand the scale of 78% nitrogen vs. 21% oxygen is:
a) Write percentages on board
b) Use a visual representation like a pie chart or 100 counters
c) Give a formula to memorize
d) Show a textbook diagram
Answer Key with Explanations
Pedagogical Reflection for Teachers 🤔
Think-Pair-Share Activity:
Think: How would you explain to students that air is not "nothing" but a mixture of gases with mass and weight?
Pair: Discuss with a colleague how you would design a "Clean Air Week" for your school with activities for different grade levels.
Share: Design a 15-minute activity to demonstrate that exhaled air contains more carbon dioxide than inhaled air (using lime water).
NCERT Textbook Linkages 📚
| Class | Chapter | Topic |
|---|---|---|
| Class 6 | Chapter 15 | Air Around Us |
| Class 7 | Chapter 18 | Wastewater Story |
| Class 8 | Chapter 18 | Pollution of Air and Water |
| Class 9 | Chapter 14 | Natural Resources |
Chapter End Notes
Key Terminology Glossary
| Term | Definition |
|---|---|
| Atmosphere | Layer of gases surrounding Earth |
| Mixture | Combination of substances that retain individual properties |
| Nitrogen fixation | Process of converting atmospheric nitrogen into usable compounds |
| Respiration | Process of releasing energy from food using oxygen |
| Photosynthesis | Process by which plants make food using CO₂, water, and sunlight |
| Combustion | Burning; rapid reaction with oxygen producing heat and light |
| Pollutant | Harmful substance in the environment |
| Particulate Matter (PM) | Tiny particles suspended in air |
| PM10 | Particles ≤10 microns; can penetrate deep into lungs |
| PM2.5 | Fine particles ≤2.5 microns; can enter bloodstream |
| Acid rain | Rain containing acids formed from SO₂ and NOx |
| Greenhouse effect | Trapping of heat by certain gases in atmosphere |
| Ozone layer | Stratospheric region with high ozone concentration; absorbs UV |
Quick Tips for PSTET Aspirants ⚡
✅ Memorize with Mnemonics:
Air Composition: "Nice Oxford Argon College" = Nitrogen (78%), Oxygen (21%), Argon (0.9%), Carbon dioxide (0.04%)
Major Pollutants: "PM, CO, NO₂, SO₂, O₃, VOC" = Particulate Matter, Carbon monoxide, Nitrogen dioxide, Sulfur dioxide, Ozone, Volatile Organic Compounds
Health Effects: "Respiratory, Cardiovascular, Cancer, Brain" = Respiratory diseases, Cardiovascular diseases, Cancer, Brain effects
PM Sizes: "10 = Lungs; 2.5 = Blood" = PM10 reaches Lungs; PM2.5 reaches Bloodstream
✅ Common Exam Traps:
Air is a MIXTURE, not a compound—don't call it a substance
Nitrogen is MOST abundant, but oxygen is most critical for respiration
Plants respire too—they take in oxygen and release CO₂ (but photosynthesis dominates during day)
PM2.5 is more dangerous than PM10 because it enters bloodstream
Ground-level ozone is a pollutant; stratospheric ozone protects us—same gas, different location!
8.1 million deaths in 2021—this is a current statistic for PSTET 2026
✅ Important Facts:
78% Nitrogen, 21% Oxygen, 0.9% Argon, 0.04% CO₂
Oxygen-CO₂ balance maintained by plants and animals over millions of years
Biosphere 2 showed how delicate this balance can be in a closed system
Ozone layer at 10-50 km altitude
PM2.5 can cross from lungs into bloodstream
Answers to "Check Your Understanding"
[To be filled by student]
📝 Note for Self-Study: After completing this chapter, ensure you can:
List the major components of air with percentages
Explain why air is a mixture, not a compound
Describe the role of oxygen in respiration and combustion
Describe the role of carbon dioxide in photosynthesis
Explain how plants and animals maintain the oxygen-CO₂ balance
Define air pollution and list 5 major pollutants
Describe the health effects of air pollution (respiratory, cardiovascular, cancer)
Describe the environmental effects of air pollution (acid rain, climate change)
Identify major sources of air pollution in India
Suggest individual and policy-level solutions to reduce air pollution
