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7. The air around the glass with ice contains water vapour. The cold surface, of the glass containing ice cools the air around it and when the water, vapour of the air comes in contact with a cool surface it condenses on, the outer cool surface of the glass. This occurs due to condensation., , D. Long Answer Questions., , 1. Animals inhale oxygen from air and exhale carbon dioxide which goes into, the air. Burning of fuels uses oxygen from the air and gives out carbon, dioxide. Green plants use carbon dioxide from the air for photosynthesis, during the daytime and give out oxygen. The process of respiration in, plants takes place during day as well as at night. The oxygen released, during photosynthesis is more than the oxygen used in respiration., Therefore, there is a net release of oxygen in the atmosphere. In the, above mentioned ways, plants and animals maintain the balance of, oxygen and carbon dioxide in the air and remains unchanged., , 2. To show that air is dissolved in water, Things needed: A beaker, tripod stand, a burner, some water, Method: Take some water in a beaker and keep it on a tripod stand., Heat the water slowly by using a burner., , Observation: You observe that, much before the water begins to boil,, , tiny bubbles appear on the inner side of the beaker. These bubbles, , come from the air dissolved in water. Actually, when water is heated,, , the solubility of air in water decreases. As a result, the air dissolved in, , water comes out in the form of tiny bubbles., , When the dissolved gas is removed through heating and the same water, , is heated again after a short interval of time, we do not observe bubbles., , Conclusion: Air is dissolved in water., , 3. (a) No, all organisms do not breathe through lungs. Most of the, , terrestrial organisms breathe through lungs., , (b) The aquatic organisms take the oxygen dissolved in water for, breathing., , 4. The air is useful to us in many ways. The major uses are as follows—, , (i) Oxygen is used by all living organisms for respiration., , (ii) Oxygen is necessary for burning of fuels., , (iii) Compressed air is used for inflating tyres of bicycles, scooters, cars, and buses. It is also used in balloons and footballs., , (iv) Moving air has force. This force helps yachts (sailing boats),, parachutes and gliders to move., , (v) Moving air (wind) is used to turn the blades of windmills., , (vi) Wind helps in the dispersal of seeds of plants. It also helps in, pollination.