15. Air Around Us
Introduction: Air is present everywhere around us. Even in an "empty" bottle, there is air. We cannot see air, but we can feel its presence when leaves rustle or clothes sway.
Properties of Air
- Air occupies space: Push an empty bottle upside down into water. Water doesn't enter. Tilt it, bubbles come out (air escaping) and water enters.
- Air is transparent: We can see through it.
- Atmosphere: The layer of air surrounding the earth. It extends many kilometres above the surface.
Example: Mountaineers
As we move higher, air gets thinner. That is why mountaineers carry oxygen cylinders with them for climbing high mountains.
Composition of Air
Air is a mixture of many gases:
- Nitrogen (78%): The major part of air. Does not support burning.
- Oxygen (21%): Essential for respiration and burning.
- Carbon Dioxide (Small amount): Produced during burning and respiration. Used by plants for photosynthesis.
- Water Vapour: Formed by evaporation. Important for water cycle.
- Dust and Smoke: Fine particles present in air.
Air in Water and Soil
Air in Water: Heat some water in a pan. Before it boils, small bubbles appear. This is air dissolved in water escaping. This dissolved air allows fish to breathe.
Air in Soil: Pour water on dry soil. Bubbles come out. This shows soil has air trapped between particles. Earthworms and roots use this air.
Oxygen Replacement in Atmosphere
Balance of Oxygen:
- Animals and humans inhale Oxygen and exhale Carbon Dioxide (Respiration).
- Plants take in Carbon Dioxide and release Oxygen (Photosynthesis).
This shows the interdependence of plants and animals.
Windmill
Wind (moving air) rotates the windmill. It is used to draw water from tube wells, run flour mills, and generate electricity.
Chapter Summary
- Air is found everywhere. We cannot see air, but we can feel it.
- Air in motion is called wind.
- Air occupies space.
- Air is present in water and soil.
- Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and a few other gases. Some dust particles may also be present in it.
- Oxygen supports burning and is necessary for living organisms.
- The envelope of air that surrounds the earth is known as atmosphere.
- Atmosphere is essential for life on earth.
- Aquatic animals use dissolved air in water for respiration.
- Plants and animals depend on each other for exchange of oxygen and carbon dioxide from air.
Exercise Q1
Q: What is the composition of air?
- Nitrogen (78%)
- Oxygen (21%)
- Carbon dioxide (less than 1%)
- Water vapour
- Dust and smoke particles
Exercise Q2
Q: Which gas in the atmosphere is essential for respiration?
Exercise Q3
Q: How will you prove that air supports burning?
Exercise Q4
Q: How will you show that air is dissolved in water?
Exercise Q5
Q: Why does a lump of cotton wool shrink in water?
Exercise Q6 & Q7: Fill in the blanks
- The layer of air around the earth is known as Atmosphere.
- The component of air used by green plants to make their food is Carbon Dioxide.
Exercise Q8
Q: List five activities that are possible due to the presence of air.
- Respiration (Breathing).
- Burning of fuels.
- Flying of kites and birds.
- Winnowing (Separating husk from grain).
- Movement of sailing yachts and windmills.
Exercise Q9
Q: How do plants and animals help each other in the exchange of gases in the atmosphere?
- Plants: Take in Carbon Dioxide from the air to make food (Photosynthesis) and release Oxygen.
- Animals/Humans: Take in Oxygen for breathing (Respiration) and release Carbon Dioxide.
Key Facts and Definitions
- Atmosphere: Envelope of air surrounding the earth.
- Composition of Air: Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and few other gases.
- Wind: The air in motion.
| Gas | Main Function |
|---|---|
| Oxygen | Breathing, Burning |
| Nitrogen | Growth of plants, Controls burning |
| Carbon Dioxide | Photosynthesis by plants |