Class 6 Science Notes Chapter 15 (Chapter 15) – Examplar Problem Book

Detailed Notes with MCQs of Chapter 15, 'Air Around Us'. This is a fundamental chapter, and understanding the concepts well is important not just for your regular studies but also for various competitive government exams where basic science is often tested.

Chapter 15: Air Around Us - Detailed Notes for Competitive Exams

1. What is Air?

• Air is present everywhere around us. It fills all apparently empty spaces.
• It is invisible and transparent. We can feel its presence when it moves (wind) or when we fill objects like balloons.
• Air is a mixture of several gases, not a single compound.

2. Composition of Air:
This is a very important section for exams. Pay close attention to the components and their approximate percentages:

• Nitrogen (N₂): Makes up about 78% of the air.
  • It is the most abundant gas.
  • It does not support burning (combustion).
  • Crucial for plant growth, but plants cannot use it directly from the air (requires nitrogen fixation by bacteria).
• Oxygen (O₂): Makes up about 21% of the air.
  • Essential for respiration in almost all living organisms (plants and animals).
  • Necessary for combustion (burning).
• Carbon Dioxide (CO₂): Makes up a small percentage, about 0.03%.
  • Used by green plants during photosynthesis to produce food.
  • Released by organisms during respiration and during the burning of fuels.
• Water Vapour: The amount varies depending on the weather and location. It's the gaseous form of water and is responsible for humidity, clouds, fog, and rain.
• Dust Particles: Tiny solid particles suspended in the air. The amount varies greatly. They can act as nuclei for water vapour to condense upon, forming clouds/fog. Smoke from factories, vehicles, and fires also adds particles to the air (pollutants).
• Other Gases: Trace amounts (making up less than 1% combined) include Argon, Neon, Helium, Krypton, etc. These are often referred to as inert or noble gases.

3. Properties of Air:

• Air Occupies Space: Demonstrated by experiments like pushing an empty inverted glass into a bucket of water; water doesn't enter fully because air is trapped inside.
• Air Has Mass/Weight: An inflated balloon or football weighs slightly more than a deflated one, showing air has mass.
• Air Exerts Pressure: Air pushes on all surfaces from all directions. This pressure is called atmospheric pressure. We don't usually feel it because the pressure inside our bodies balances the outside pressure.
• Air is Transparent: We can see through it clearly.
• Air Supports Combustion: Specifically, the oxygen component is required for burning. If oxygen supply is cut off (e.g., covering a burning candle with a glass), the flame extinguishes.
• Air is Essential for Respiration: Living beings need oxygen from the air to break down food and release energy.
• Moving Air is Wind: Air movement creates wind, which has energy and can be harnessed (e.g., by windmills).

4. Importance and Availability of Components:

• Oxygen Cycle: Oxygen is constantly consumed during respiration and combustion. It is replenished in the atmosphere primarily through photosynthesis carried out by green plants. This maintains a balance.
• Oxygen for Aquatic Life: Animals and plants living in water breathe oxygen dissolved in the water. The amount of dissolved oxygen is crucial for their survival.
• Oxygen for Soil Organisms: Organisms like earthworms, insects, and microbes living in the soil get oxygen from the air trapped in the spaces (pores) between soil particles. This is why waterlogged soil (where air spaces are filled with water) is harmful to many soil organisms, forcing them to the surface.
• Role of Nitrogen: Although abundant, atmospheric nitrogen isn't directly usable by most organisms. It needs to be converted into compounds (like nitrates) through processes like nitrogen fixation (often involving bacteria in soil or root nodules of leguminous plants) before plants can absorb it.
• Role of Carbon Dioxide: Essential raw material for photosynthesis. Its concentration in the atmosphere influences global temperatures (greenhouse effect, though this concept is more detailed in higher classes).

5. The Atmosphere:

• The thick layer or envelope of air surrounding the Earth is called the atmosphere.
• It extends hundreds of kilometres above the Earth's surface, becoming thinner with increasing altitude.
• It protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun and regulating temperature.

6. Windmills:

• Devices designed to harness the kinetic energy of wind (moving air).
• Traditional uses include drawing water from wells/tubewells and grinding grain ('flour mills').
• Modern windmills (wind turbines) are widely used to generate electricity.

7. Air Pollution:

• The presence of harmful substances (pollutants) in the air, such as excessive smoke, dust, harmful gases (like sulphur dioxide, nitrogen oxides) from industries, vehicles, burning of garbage, etc.
• Air pollution adversely affects the health of humans, animals, plants, and the environment.

Key Points to Remember for Exams:

• Composition percentages (N₂ ~78%, O₂ ~21%, CO₂ ~0.03%).
• Gases essential for Respiration (O₂) and Photosynthesis (CO₂).
• Gas essential for Combustion (O₂).
• Air occupies space, has mass, exerts pressure.
• How aquatic and soil organisms obtain oxygen.
• The role of Photosynthesis in maintaining oxygen levels.
• Definition of Atmosphere.
• Function of Windmills.

Multiple Choice Questions (MCQs):

1. What is the approximate percentage of oxygen in the air?
   (a) 78%
   (b) 21%
   (c) 1%
   (d) 0.03%

2. Which gas is used by green plants to make their food during photosynthesis?
   (a) Oxygen
   (b) Nitrogen
   (c) Carbon Dioxide
   (d) Water Vapour

3. The presence of air can be felt when it moves. Moving air is known as:
   (a) Atmosphere
   (b) Pressure
   (c) Wind
   (d) Vapour

4. Which component of air is necessary for the process of burning (combustion)?
   (a) Nitrogen
   (b) Carbon Dioxide
   (c) Oxygen
   (d) Argon

5. How do organisms living deep inside the soil primarily obtain oxygen for respiration?
   (a) From water trapped in the soil
   (b) From air present in the spaces between soil particles
   (c) By coming to the surface frequently
   (d) They do not need oxygen

6. The thick blanket of air surrounding the Earth is called the:
   (a) Biosphere
   (b) Hydrosphere
   (c) Lithosphere
   (d) Atmosphere

7. Which property of air is primarily used by windmills to generate electricity or draw water?
   (a) Air has mass
   (b) Air exerts pressure
   (c) Air occupies space
   (d) Air moves (wind)

8. Fish absorb oxygen dissolved in water through their:
   (a) Lungs
   (b) Skin
   (c) Gills
   (d) Fins

9. What happens to a burning candle if it is covered completely with an inverted glass jar?
   (a) It burns brighter
   (b) It continues to burn indefinitely
   (c) It extinguishes after some time
   (d) It produces more smoke

10. Which of the following is generally considered an air pollutant when present in excess?
    (a) Oxygen
    (b) Nitrogen
    (c) Water Vapour
    (d) Smoke particles

Answer Key for MCQs:

1. (b)
2. (c)
3. (c)
4. (c)
5. (b)
6. (d)
7. (d)
8. (c)
9. (c)
10. (d)