The Atmosphere chapter examines the composition, structure, and importance of Earth’s gaseous envelope. Students learn about atmospheric layers (troposphere, stratosphere, mesosphere, thermosphere), the roles of major gases (N2, O2, CO2, water vapour, ozone), and essential environmental issues such as air pollution, the greenhouse effect, ozone depletion, and acid rain. This chapter also covers major air pollutants, their sources and effects, and basic measures for pollution control and protection of the atmosphere. MCQs below test conceptual understanding and environmental application aligned to Class 10 syllabus.
1. Which gas makes up the largest proportion of dry air?
- A. Oxygen
- B. Nitrogen (N2) ✅
- C. Carbon dioxide
- D. Argon
Explanation: Approximately 78% of Earth’s dry atmosphere is nitrogen by volume.
2. Which atmospheric layer contains the ozone layer?
- A. Troposphere
- B. Stratosphere ✅
- C. Mesosphere
- D. Thermosphere
Explanation: Ozone (O3) is concentrated mainly in the stratosphere, providing protection from harmful UV radiation.
3. Ozone in the stratosphere is formed by which simplified reaction (photochemical)?
- A. O2 + O → O3
- B. O2 + hv (UV) → 2O; O + O2 → O3 ✅
- C. 2O3 → 3O2
- D. O + O → O2
Explanation: UV light splits O2 into O atoms; these combine with O2 to make O3.
4. Which pollutant is primarily responsible for respiratory problems and is a major product of incomplete combustion?
- A. Sulfur dioxide (SO2)
- B. Carbon monoxide (CO) ✅
- C. Ozone (O3)
- D. Nitrogen gas (N2)
Explanation: CO is produced by incomplete combustion and binds to hemoglobin, reducing oxygen transport.
5. Acid rain is mainly caused by the atmospheric conversion of which gases?
- A. CO2 and N2
- B. SO2 and NOx (NO, NO2) ✅
- C. O2 and H2
- D. NH3 and CH4
Explanation: SO2 and nitrogen oxides react with water to form sulfuric and nitric acids, causing acid rain.
6. Which of the following reactions shows formation of sulfuric acid from sulfur dioxide?
- A. SO2 + H2O → H2SO4
- B. 2SO2 + O2 → 2SO3; SO3 + H2O → H2SO4 ✅
- C. SO2 + O → SO3
- D. SO2 → S + O2
Explanation: SO2 oxidizes to SO3, which forms H2SO4 with water.
7. What is ‘smog’ commonly associated with?
- A. High ozone concentration in stratosphere
- B. Photochemical pollution in urban areas (NOx + VOCs + sunlight) ✅
- C. Only volcanic ash
- D. Dissolved gases in rainwater
Explanation: Photochemical smog forms from NOx and volatile organic compounds under strong sunlight, producing ground-level ozone and particulates.
8. Ground-level ozone (O3) is harmful because:
- A. It protects against UV
- B. It irritates lungs and damages vegetation ✅
- C. It is inert and harmless
- D. It removes CO from air
Explanation: Unlike stratospheric ozone, ground-level ozone is a pollutant that harms respiratory health and plants.
9. The greenhouse effect is primarily caused by which gases?
- A. Oxygen and nitrogen
- B. Carbon dioxide (CO2), methane (CH4), water vapour, and others ✅
- C. Argon and neon
- D. Helium and hydrogen
Explanation: Greenhouse gases trap infrared radiation and warm Earth’s surface; CO2 and CH4 are major contributors.
10. Which human activity is the largest source of increased atmospheric CO2?
- A. Natural respiration of plants
- B. Burning of fossil fuels (coal, oil, gas) ✅
- C. Photosynthesis
- D. Ocean evaporation
Explanation: Fossil fuel combustion is the major anthropogenic source of CO2 emissions.
11. Which pollutant is a major product of the combustion of sulfur-containing fuels?
- A. NO
- B. SO2 ✅
- C. CO2
- D. O3
Explanation: Burning sulfur-containing coal or oil produces sulfur dioxide (SO2), a gas that contributes to acid rain.
12. Which simple practice can help reduce atmospheric pollution locally?
- A. Increase burning of biomass
- B. Use more two-stroke engines
- C. Promote public transport and reduce vehicle use ✅
- D. Remove all trees in cities
Explanation: Reducing vehicle use lowers emissions of NOx, CO, VOCs and particulates.
13. Which of the following is NOT a primary air pollutant?
- A. Particulate matter (PM)
- B. Carbon monoxide (CO)
- C. Ozone (O3) ✅
- D. Sulfur dioxide (SO2)
Explanation: Primary pollutants are emitted directly; ozone at ground level is typically a secondary pollutant formed from reactions in air.
14. What is the major environmental effect of acid rain on freshwater ecosystems?
- A. Increase in pH making water alkaline
- B. Decrease in pH harming aquatic life (fish kills) ✅
- C. Increase in dissolved oxygen
- D. Immediate desalination
Explanation: Acid rain lowers the pH of lakes and rivers, which can be lethal to many aquatic organisms.
15. The depletion of stratospheric ozone is mainly linked to which group of chemicals?
- A. CO2
- B. NOx
- C. Chlorofluorocarbons (CFCs) ✅
- D. Sulfur compounds
Explanation: CFCs release Cl atoms in the stratosphere that catalytically destroy ozone molecules.
16. Which international agreement aims to reduce substances that deplete the ozone layer?
- A. Kyoto Protocol
- B. Paris Agreement
- C. Montreal Protocol ✅
- D. Basel Convention
Explanation: The Montreal Protocol (1987) phased out many ozone-depleting substances like CFCs.
17. Photochemical smog formation is favoured by which weather conditions?
- A. Cool, rainy and windy
- B. Sunny, hot, and stagnant air (little wind) ✅
- C. Cold and snowy
- D. High humidity only
Explanation: Strong sunlight drives photochemical reactions; stagnant conditions allow pollutants to accumulate.
18. Which measurement is commonly used to indicate small-particle air pollution harmful to health?
- A. pH of rain
- B. CO2 concentration
- C. PM2.5 or PM10 concentration ✅
- D. O3 in stratosphere
Explanation: PM2.5/PM10 are particulate matter size categories commonly used to assess air quality and health risks.