Class 6 Science Notes Chapter 15 (Air Around Us) – Science Book

Detailed Notes with MCQs of Chapter 15, 'Air Around Us'. This is a fundamental chapter, and understanding the properties and composition of air is crucial, not just for your exams but also for understanding many natural phenomena. Pay close attention as we break down the key concepts for your government exam preparation.

Chapter 15: Air Around Us - Detailed Notes

1. Introduction: What is Air?

• Air is a mixture of gases that surrounds the Earth, forming the atmosphere.
• It is invisible, tasteless, and odourless (in its pure form).
• Air is essential for the survival of most living organisms on Earth.

2. Presence of Air:

• Air is Everywhere: Although invisible, air occupies space and is present all around us. It fills seemingly empty containers, spaces in soil, and is dissolved in water.
• Evidence:
  • An empty bottle inverted into water shows bubbles escaping, indicating air was present inside.
  • Moving air (wind) makes leaves rustle, flags flutter, and windmills turn.

3. Composition of Air:
Air is primarily a mixture of several gases. The main components are:

• Nitrogen (N₂): Approximately 78%.
  • Does not support burning (combustion).
  • Essential nutrient for plant growth (plants cannot take it directly from the air; it needs to be 'fixed' in the soil by bacteria or other processes).
• Oxygen (O₂): Approximately 21%.
  • Essential for respiration (breathing) in almost all living organisms (plants and animals).
  • Necessary for combustion (burning).
• Carbon Dioxide (CO₂): Approximately 0.03 - 0.04%. (Small but vital amount).
  • Used by plants during photosynthesis to make food.
  • Released by animals and plants during respiration.
  • Produced during burning (combustion) of fuels.
  • Contributes to the greenhouse effect, helping regulate Earth's temperature.
• Water Vapour: The amount varies depending on the weather and location.
  • It's the gaseous form of water.
  • Its presence leads to humidity.
  • Plays a crucial role in the water cycle (formation of clouds, rain).
• Other Gases: Argon, Neon, Helium, etc., make up a very small percentage (less than 1% combined).
• Dust Particles and Smoke:
  • Air also contains fine dust particles and smoke (from burning).
  • The amount varies greatly depending on location, weather, and human activities (factories, vehicles).
  • Smoke contains fine solid particles and some gases.
  • These are considered impurities in the air. Breathing air with excessive dust/smoke can cause respiratory problems.

4. Oxygen Availability in Water and Soil:

• In Water: Air (including oxygen) is dissolved in water. Aquatic animals (like fish) use this dissolved oxygen for respiration through specialized organs (like gills).
  • Evidence: Heating water slowly shows tiny bubbles appearing on the inner surface of the container before boiling – this is dissolved air escaping.
• In Soil: Soil particles have spaces between them which contain air. Organisms living in the soil (earthworms, insects, bacteria) and plant roots use this trapped air for respiration.
  • Evidence: Pouring water onto a lump of dry soil causes bubbles to emerge as water displaces the air trapped within.

5. Balancing Oxygen in the Atmosphere:

• Oxygen is constantly being used up by living organisms for respiration and during combustion.
• Photosynthesis: Green plants replenish the oxygen in the atmosphere. During photosynthesis, plants use carbon dioxide, water, and sunlight to produce food (glucose) and release oxygen as a byproduct.
• This creates a natural balance between oxygen consumption (respiration, combustion) and oxygen production (photosynthesis), highlighting the interdependence of plants and animals.

6. Uses of Air/Wind:

• Respiration: Essential for life.
• Combustion: Needed for burning.
• Windmills: Moving air (wind) is used to:
  • Generate electricity.
  • Draw water from tubewells.
  • Operate flour mills.
• Movement/Transportation: Helps in the movement of sailing yachts, gliders, parachutes, and aeroplanes.
• Dispersal: Wind aids in the dispersal of seeds and pollen of many plants.
• Flight: Enables birds, bats, and insects to fly.
• Water Cycle: Air movement plays a vital role in carrying water vapour and forming clouds.

7. The Atmosphere:

• The thick layer or envelope of air surrounding the Earth is called the atmosphere.
• It extends several hundred kilometres above the Earth's surface.
• It is held in place by Earth's gravity.
• Importance: Protects life from harmful solar radiation (like UV rays), helps regulate temperature, and contains the air we breathe.

Key Terms Recap: Atmosphere, Components of Air, Nitrogen, Oxygen, Carbon Dioxide, Water Vapour, Dust Particles, Respiration, Combustion, Photosynthesis, Windmill, Dissolved Air.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the chapter for your practice:

1. Which gas makes up the largest percentage of the air we breathe?
   a) Oxygen
   b) Carbon Dioxide
   c) Nitrogen
   d) Argon

2. The process by which plants release oxygen into the atmosphere is called:
   a) Respiration
   b) Combustion
   c) Photosynthesis
   d) Transpiration

3. Which component of air is essential for burning (combustion)?
   a) Nitrogen
   b) Oxygen
   c) Carbon Dioxide
   d) Water Vapour

4. Aquatic animals like fish breathe using:
   a) Lungs that filter air from water
   b) Oxygen dissolved in the water
   c) Air bubbles trapped under rocks
   d) Coming to the surface to breathe air directly

5. What happens when water is poured onto a lump of dry soil?
   a) The soil dissolves completely.
   b) Bubbles emerge from the soil.
   c) The soil turns into rock.
   d) The water immediately evaporates.

6. Windmills utilize the energy of:
   a) Sunlight
   b) Moving water
   c) Moving air
   d) Geothermal heat

7. Which of the following is NOT a primary use of windmills mentioned in the context of air?
   a) Generating electricity
   b) Operating flour mills
   c) Purifying air
   d) Drawing water from tubewells

8. The blanket of air surrounding the Earth is known as the:
   a) Hydrosphere
   b) Lithosphere
   c) Atmosphere
   d) Biosphere

9. Which gas is used by plants during photosynthesis?
   a) Oxygen
   b) Nitrogen
   c) Carbon Dioxide
   d) Hydrogen

10. Which component of air varies significantly depending on weather conditions?
    a) Nitrogen
    b) Oxygen
    c) Argon
    d) Water Vapour

Answer Key for MCQs:

1. c) Nitrogen
2. c) Photosynthesis
3. b) Oxygen
4. b) Oxygen dissolved in the water
5. b) Bubbles emerge from the soil.
6. c) Moving air
7. c) Purifying air
8. c) Atmosphere
9. c) Carbon Dioxide
10. d) Water Vapour

Make sure you revise these notes thoroughly. Understanding the composition and importance of air is key. Good luck with your preparation!