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• Air is everywhere around us. It cannot be seen but its presence can be felt
• Properties of air
  • Air is transparent
  • Air is compressible
  • Air expand on heating and contracts on cooling
  • Air has mass
  • Atmospheric pressure decreases with the increase in height
  • Air scatters light
• Composition of air
  • Air around the earth is known as earth’s atmosphere
  • Amount of air present in different layer. E.g. Troposphere consists of 80% of the total air, stratosphere consists of 19% of the total air. Further, the remaining 1% of air is present in mesosphere and ionosphere.
  • Air is mixture of gases containing Nitrogen, Oxygen, and Argon, Water Vapour, Carbon dioxide, Smoke and Dust,

  Component	Percent
  Nitrogen	78%
  Oxygen	21 %
  Argon	0.9 %
  Carbon dioxide	0:03 %

  • Oxygen – Oxygen needed by organisms for respiration. Oxygen is responsible for burning of fuel. Plants take up carbon dioxide and release oxygen during photosynthesis. This is how oxygen in maintained in atmosphere.
  • Nitrogen – It is major component of atmosphere. It is essential for living organisms to produce proteins and food. It is useful for the production of ammonia and in airtight packaging of foodstuffs.
  • Carbon dioxide – Plants need carbon dioxide for producing food. It used as fire extinguisher. Increase in the quantity of carbon dioxide in air causes global warming
  • Water vapour – Water is present in air in the form of water vapour.Actually the water vapours get cooled to form liquid water. Evaporation of water from water bodies forms water vapours. Up in the sky the water vapours form cloud. From clouds water comes on earth in the form of rain. So, water is continually cycled through earth’s atmosphere. In any region amount of water vapours present in air can be found by measuring humidity level. Near sea the humidity is higher than the humidity in the desert.
  • Dust and Smoke: Air contains dust and smoke particles
• Importance of air
  • Air is required for Respiration
  • Air help the plants to prepare food for everyone
  • Air protects life from harmful UV Rays
  • Air Maintain temperature on Earth
  • Air brings water back on Earth
  • Hearing is possible due to air
  • Air is energy source for windmill
  • Birds can fly due to presence of air