DAV Board Class 7 Social Science (We and Our World) | 100% Authentic Textbook Solutions

DAV Class 7 SST Chapter 4 Air Around Us Solutions

Get complete, 100% authentic textbook solutions for DAV Class 7 Social Science (We and Our World – Geography) Chapter 4: Air Around Us. This comprehensive study guide features detailed chapter summaries and core concepts (Atmospheric composition, Nitrogen, Oxygen, Carbon Dioxide, Ozone shield, Atmospheric layers: Troposphere, Stratosphere, Mesosphere, Thermosphere/Ionosphere, Normal Lapse Rate, Atmospheric Pressure & Barometer), key scientific definitions, complete textbook exercise solutions (Section A: MCQs, Section B: Fill in the Blanks, Section C: Column Matching, Section D: Short Answer Questions, Section E: Detailed Explanations with Diagram), Value Based Questions (VBQ), Higher Order Thinking Skills (HOTS), and project activities strictly adhering to the DAV Education Board curriculum.

1. Detailed Chapter Summary and Core Concepts

A. The Atmosphere and Its Importance

• Enveloping Blanket: The Earth is enveloped by a thick layer of air composed of colorless, odorless, and transparent gases, designated as the atmosphere.
• Vertical Extent & Mass Distribution: The atmosphere extends up to approximately 1,600 kilometers above the Earth’s surface. Gravitational pull compresses 99 percent of its total atmospheric mass within the lowest 32 kilometers.
• Origin: The atmosphere originated approximately 4,600 million years ago through gradual volcanic degassing and chemical modifications energised by solar radiation.
• Life Support System: Without this gaseous blanket, the Earth would experience extreme sub-zero night freezes and scorching daytime radiation, rendering organic life impossible.

B. Composition of Air

Air is a mechanical mixture of uniform proportions of gases up to an altitude of 90 km, above which lighter gases (helium, hydrogen) become predominant:

C. Concentric Layers of the Atmosphere

• 1. Troposphere (0 to 8-18 km):
  • The lowest, densest layer where living beings dwell.
  • Contains almost all water vapour and dust particles; consequently, all meteorological and weather phenomena (clouds, storms, fog, rain) occur exclusively here.
  • Normal Lapse Rate: Temperature drops steadily with elevation at an average rate of 1 degree Celsius for every 165 meters of ascent.
  • Separated from the next layer by a transitional boundary termed the tropopause.
• 2. Stratosphere (up to 50 km):
  • Characterized by calm, dry, cloudless conditions free from violent convective air currents, providing ideal and safe visibility for flying jet aircrafts.
  • Houses the ozone layer, which absorbs destructive ultraviolet radiation from the sun.
• 3. Mesosphere (50 to 80 km):
  • The coldest atmospheric zone where temperatures plummet as low as -100 degrees Celsius.
  • Protects Earth by burning up incoming space debris and meteoroids through frictional resistance.
• 4. Thermosphere / Ionosphere (80 to 1,600 km):
  • Air density is exceedingly low; temperature increases rapidly with altitude due to absorption of intense solar X-rays.
  • Its lower portion, the ionosphere, contains electrically charged ion particles that reflect high-frequency radio waves back to Earth, enabling global wireless telecommunication.
  • Beyond 1,600 km, the atmosphere merges imperceptibly into interplanetary outer space (exosphere).

D. Atmospheric Pressure, Weather and Climate

• Atmospheric Pressure: The weight exerted by the column of air upon a unit area of Earth’s surface. Measured using a scientific instrument called a barometer.
• Standard Pressure: The normal mean atmospheric pressure at sea level is 1,013 millibars (mb). Atmospheric pressure decreases progressively with rising altitude.
• Weather vs. Climate:
  • Weather: Day-to-day or hour-to-hour state of the atmosphere at a specific local station regarding temperature, pressure, wind, and rain.
  • Climate: The composite statistical average of weather conditions observed over an extensive geographical territory over decades (30-35 years). India experiences a tropical monsoonal climate.

2. Key Terms and Definitions

3. Complete Textbook Exercise Solutions

Section A: Tick (✓) the Correct Option

1. In normal lapse rate, temperature decreases at the rate of 1°C with every:
(a) 165 m ascend [✓]
(b) 175 m ascend
(c) 185 m ascend
(d) 195 m ascend
Answer: (a) 165 m ascend

2. Which layer is safe for flying aircrafts?
(a) troposphere
(b) stratosphere [✓]
(c) mesosphere
(d) thermosphere
Answer: (b) stratosphere

3. The role of ion-particles in the ionosphere is to:
(a) absorb ultra violet rays
(b) enable wireless communication [✓]
(c) burn meteors
(d) enable rainfall
Answer: (b) enable wireless communication

4. Which one of the following is the main cause of global warming?
(a) air pollution [✓]
(b) water pollution
(c) soil pollution
(d) land degradation
Answer: (a) air pollution

5. Smog causes:
(a) breathing problems [✓]
(b) purification of air
(c) heavy rainfall
(d) reduction in temperature
Answer: (a) breathing problems

Section B: Fill in the Blanks

• 1. Atmosphere is mainly composed of three major gases: nitrogen, oxygen and carbon dioxide.
• 2. Gases like argon, helium, methane, etc., are present in small amounts in the air.
• 3. All weather phenomena occur in the troposphere.
• 4. Atmospheric conditions over a long period of time and on a larger area is called climate.
• 5. The normal air pressure at sea level is 1,013 millibar.

Section C: Match the Following

Section D: Answer the Following Questions in Brief

Question 1. Name the layers of the atmosphere.
Answer: The four primary concentric layers of the atmosphere from Earth’s surface upwards are:
1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere (which encompasses the Ionosphere and merges into the Exosphere).

Question 2. How do plants absorb nitrogen?
Answer: Plants cannot ingest gaseous nitrogen directly from ambient air. Instead, nitrogen is converted into soluble nitrates in the soil by nitrogen-fixing soil bacteria and lightning discharges, which plant roots absorb dissolved in soil moisture to manufacture essential plant proteins.

Question 3. What is the importance of water vapours in the atmosphere?
Answer: Water vapour is critically important because it is the primary medium for the hydrological cycle. Upon rising and cooling, it condenses around dust particles to form clouds and triggers all forms of precipitation (rain, snow, hail), thereby supplying fresh water to continents and regulating Earth’s thermal balance.

Question 4. Name the different variables of atmosphere.
Answer: The chief variables that define changing atmospheric conditions are: temperature, atmospheric pressure, wind direction and velocity, humidity, and precipitation (rainfall).

Question 5. What type of climate does India have?
Answer: India has a tropical monsoonal climate, characterized by high temperatures and rhythmic seasonal reversals of wind patterns that govern wet monsoon summers and dry winter seasons.

Section E: Answer the Following Questions in Detail

Question 1. What is the importance of carbon dioxide and oxygen for living beings?
Answer:

• Importance of Oxygen (21%):
  1. Oxygen is the indispensable breath of life for humans and animals, essential for cellular respiration to burn food and release bodily energy.
  2. It is vital for natural combustion (burning of fuels) and is constantly replenished by autotrophic plants via photosynthesis.
• Importance of Carbon Dioxide (0.03%):
  1. Green plants absorb carbon dioxide from the atmosphere during daytime photosynthesis to synthesize glucose and sustenance, which feeds the entire biosphere.
  2. It acts as a natural terrestrial greenhouse insulator by absorbing longwave infrared heat radiated from Earth, preventing the planet from freezing into a solid ice desert at night.

Question 2. Name three major sources of pollutants in the air. Also state the ill-effects of these air pollutants.
Answer:

• Three Major Sources of Pollutants:
  1. Vehicular Emissions: High-density automobiles releasing toxic carbon monoxide, nitrogen oxides, unburnt hydrocarbons, and lead particles.
  2. Agricultural & Forest Burning: Open burning of paddy crop residue (parali/husk) by farmers and uncontrolled wildfires discharging immense ash and particulate matter.
  3. Industrial & Thermal Smoke: Coal-based thermal plants and factories discharging sulphur dioxide, fly ash, and carbon soot.
• Ill-Effects of Air Pollutants:
  1. Severe Human Health Hazards: Inhalation triggers chronic asthma, bronchitis, reduced lung capacity, eye burns, and cardiovascular disorders.
  2. Formation of Toxic Smog: Pollutants fuse with winter fog to create dense smog, causing widespread road and air traffic accidents and choking urban populations.
  3. Global Climate Warming & Acid Rain: Sulphur and nitrogen oxides dissolve into rainwater forming acid rain, destroying historic monuments and aquatic life, while surplus greenhouse gases escalate global warming.

Question 3. What is the importance of ozone gas? Why is global warming a threat to life on the earth?
Answer:

• Importance of Ozone Gas: Ozone (O3) concentrated in the stratosphere forms an invaluable planetary shield that intercepts and filters out harmful ultraviolet (UV) solar radiation. Without this shield, high-energy UV rays would penetrate to the surface, triggering rampant skin carcinomas, cataract-induced blindness, and fatal damage to foundational marine food chains (plankton).
• Global Warming as an Existential Threat:
  1. Excessive emissions of greenhouse gases (carbon dioxide, methane, nitrous oxide, and CFCs) trap surplus outgoing heat, artificially elevating Earth’s mean temperature.
  2. This thermal rise causes rapid melting of polar ice caps and Himalayan glaciers, driving catastrophic sea-level rise that inundates low-lying coastal cities and island nations.
  3. It destabilizes planetary weather patterns, causing frequent unseasonal droughts, catastrophic flash floods, heatwaves, and mass extinction of vulnerable wildlife species.

Question 4. How is rainfall caused? Explain it with the help of a diagram.
Answer:

• Mechanism of Rainfall Formation:
  1. Evaporation: Solar thermal energy warms oceans, rivers, and lakes, transforming liquid surface water into invisible water vapour.
  2. Ascent & Cooling: Being warm and buoyant, the moist air ascends into higher altitudes of the troposphere, expanding and cooling under the Normal Lapse Rate.
  3. Condensation: Upon reaching its dew point, water vapour condenses around suspended microscopic dust and salt particles (hygroscopic nuclei), aggregating into visible cloud masses.
  4. Precipitation: When millions of tiny cloud droplets coalesce into heavier water droplets that can no longer remain buoyed by rising air currents, they fall to the surface under gravity as rainfall.

Question 5. Write the difference between weather and climate.
Answer:

4. Value Based Questions (VBQ) and HOTS

Value Based Question (VBQ)

Context: ‘Global warming is threatening the life support system of the earth.’

1. How can we relate our life with it? Give examples in support of your answer.
Answer: Global warming is not a remote theory but directly threatens our daily survival and quality of life. For instance, scorching summer heatwaves cause severe dehydration, heatstrokes, and astronomical electricity bills due to constant air conditioning. Furthermore, erratic rainfall patterns damage agriculture, causing steep food price inflation and water scarcity in urban areas.

2. Explain any two measures to combat this problem.
Answer:

• 1. Massive Afforestation: Undertaking extensive community tree-planting campaigns, since mature forests act as natural carbon sinks that absorb enormous quantities of excess atmospheric carbon dioxide during photosynthesis.
• 2. Energy Transition and Emissions Reduction: Replacing fossil fuel combustion with clean renewable energy sources (solar, wind, hydroelectric), adopting electric public transit, strictly curbing stubble and waste burning, and eliminating ozone-depleting chlorofluorocarbons (CFCs).

HOTS (Higher Order Thinking Skills)

Q1. Why does temperature decrease as we ascend in the troposphere, but increase in the thermosphere?
Answer: The troposphere is heated primarily from the ground upwards by longwave terrestrial radiation re-emitted by Earth’s surface; thus, as altitude increases, distance from this primary heat source widens and thinner air holds less heat, resulting in a temperature drop (Normal Lapse Rate). Conversely, in the thermosphere, the extremely rarefied air molecules and ions absorb intense, unfiltered high-energy shortwave solar radiation (ultraviolet and X-rays) directly from the sun, causing temperatures to surge rapidly with altitude.

Q2. If the ozone layer were completely depleted from the stratosphere, how would it affect life on Earth?
Answer: If the stratospheric ozone layer were eliminated, raw and lethal ultraviolet solar radiation would bombard Earth’s biosphere unabated. This would trigger unprecedented epidemics of malignant skin melanoma, immune deficiencies, and eye cataracts across human populations. In nature, it would destroy phytoplankton at the base of marine aquatic food webs, stunt agricultural crops, and precipitate catastrophic ecological collapse.

5. Something To Do / Project Activities

A. Weekly Weather Log Record

B. Presentation Slide Blueprints

• Topic 1: Global Warming
  • Slide 1: Conceptual Definition of the Greenhouse Effect and Global Warming.
  • Slide 2: Primary Drivers (Deforestation, fossil fuel burning, vehicular smoke, and industrial gases).
  • Slide 3: Severe Consequences (Polar glacier retreat, rising oceans, ecological loss, and heatwaves).
  • Slide 4: Sustainable Solutions (Reforestation, solar/wind adoption, and green urban planning).
• Topic 2: Air Pollution
  • Slide 1: Definition and Criteria of Toxic Atmospheric Pollutants.
  • Slide 2: Key Emission Sources (Automotive exhausts, industrial chimneys, crop residue burning).
  • Slide 3: Health Hazards (Respiratory diseases, winter smog, and acid rain).
  • Slide 4: Practical Countermeasures (Public transit, EV transition, strict industrial filtration).

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