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The Ocean of Air |
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As we know that we cannot live without air even for a single minute. We live in a vast ocean of air, which extends upwards for over 1000 km. Apart from providing us air to breathe, this blanket of air traps the sun's heat that is essential for life on the earth, and also acts as a barrier against harmful and excessive radiation. The lowest part of this ocean of air is known as the troposphere. It contains about 75% of the total mass of the atmosphere and 90% of water vapour. It is in this troposphere, where all the weather is formed. |
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Composition of Air |
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Air is a mixture of various gases. The main gases present in air are nitrogen (78.1% by volume) and oxygen (20.9% by volume). These two gases together form 99% of the air. Besides these two, air contains small amounts of carbon dioxide (0.03 to 0.04%), water vapour (variable), noble gases (0.94%), and dust particles. The noble gases present in the air are helium, neon, argon, krypton and xenon. Since the percentage of these gases is very small so, they are also called "Rare gases". |
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Air is a Mixture of Different Gases |
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The percentage composition of air changes from place to place and time to time. The percentage of oxygen becomes lesser and lesser as you go higher and higher; hence mountaineers carry oxygen cylinders while climbing. The vapour density of oxygen is 16, while that of air is 14.4. So, the slightly heavier oxygen is found more in the plains than at higher altitudes. Also the air in a green belt or a seaside contains more oxygen and less carbon dioxide than in the air elsewhere. In highly populated and industrial areas, the percentage of oxygen is lesser when compared to the rural areas and countryside. |
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Air Pollution |
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We know that air is one of the most essential requirements for all the living organisms. It is therefore imperative that we try and keep the air pure. Technological and industrial progress has vastly affected the quality of air that we breathe in. Because of the dense population and the presence of industries in large cities, air gets polluted in urban areas. This is becoming a major environmental problem and health hazard. |
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Air - Essential for Burning |
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We know that without air, we cannot burn anything. In the laboratory, if the flame of a spirit lamp is to be put out, we just place the lid of the spirit lamp. |
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Oxygen is One-fifth by Volume in Air |
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The part of air that helps in combustion or burning is oxygen gas.
You can perform the following experiment to determine the percentage of oxygen in air. You will need a trough, a bell jar, an evaporation dish and a small piece of yellow phosphorus. |
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Presence of Carbon Dioxide in Air |
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Another important constituent of air is carbon dioxide. As stated before, plants use it to synthesize carbohydrates, a basic food for all of us. It is also one of the green house gases that help to retain the warmth of the suns rays on earth, making weather conducive for life. You can perform the following simple experiment to show the presence of carbon dioxide in air. |
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Presence of Water vapor in Air |
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Ordinary air seems to be dry. It does not cause wetness on our body or clothes. However, you may have some idea about the huge quantity of rain, snow, dew, etc. that gets formed in a year throughout the world. All of it comes only from the air. Thus, the apparent dryness of air is very deceptive. Actually, air has a tremendous capacity for holding water vapour. The extent of this capacity can be gauged by the fact that all the rain that falls on the earth any time is less than three percent of the water present in the atmosphere. |
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Noble Gases |
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The amount of all the noble gases put together (Helium, Neon, Argon, Krypton, Xenon and radon) is extremely low in air. These form only 0.94% of the volume of air and as such can be treated as negligible. These gases were found chemically inert (i.e. did not mix with other elements nor were effected by oxidizing or reducing agents) and therefore their detection in air was very difficult. They were only discovered at the turn of the 20th century and due to their chemical inertness, were named as inert gases. |
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Nitrogen |
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Nitrogen forms nearly 78% of the volume of air. Earlier, the useful significance of nitrogen was not known and it was considered to be inactive. At that time, nitrogen was considered important only because it was known to moderate the processes of respiration and combustion, by diluting the effect of oxygen. Its real importance has now been realized. |
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Summary of the Constituents of Air and their Significance |
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Nitrogen : 78.1% Dilutes the air, moderates respiration and combustion; essential for plants to produce proteins. |
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Combustion Process |
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Combustion is a chemical reaction in which burning of substances takes place. In this process, a substance combines with oxygen to burn (decompose) itself liberating heat and sometimes light and flame. Thus, combustion reactions are exothermic or heat dissipating reactions. Burning or combustion is one of the most important reactions, which supply most of the energy required by human consumption. For example, the burning of firewood, coal or L.P.G. gas used in modern houses of today all employ the combustion reaction to generate heat energy. |
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Change in Mass on Combustion |
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When a substance burns in air, it undergoes a change in mass.
For instance, if 3 g of magnesium is completely burnt, the magnesium oxide will have a mass of 5 g. Magnesium oxide, shows an increase of 2 g, which is the mass of the oxygen that gets combined with magnesium.
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Apparent Loss of Mass |
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You must have seen that when substances like wood, wax, coal, oil, etc. are burnt, they seem to disappear, or leave a very small quantity of residue. It looks as if the mass is much lesser. Well, the loss of some mass of these substances is only apparent. As per the Law of conservation of masses, matter is neither created nor destroyed but can be transformed from one form to other. The products formed after burning are mostly gaseous e.g., carbon dioxide and water vapour, which escape into the air. As we do not measure the amount of the gases liberated, an apparent loss of mass is noticed. |
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Respiration |
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Respiration is also a type of combustion. The red blood cells in blood pick up oxygen from the lungs and send it to other cells in different tissues. There, oxygen combines with glucose to release energy. Carbon dioxide and water are formed as a result of this reaction. The resulting products, water and carbon dioxide, are expelled when we exhale.
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Comparison of Burning of Hydrocarbons and Respiration |
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Burning in Absence of Air |
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Usually air is associated with burning, and without air or oxygen, substances do not burn. But air not essentially used for all types of burning. For example, when charcoal powder is mixed with any of the substances like chlorates or nitrates of potassium or potassium permanganate (KMnO4), and heated, it will catch fire and burn even in the absence of air. When heated, these compounds decompose and liberate oxygen, which helps the charcoal powder to burn. We have seen earlier that only one-fifth of the total air i.e. that part which is oxygen, is useful for burning. |
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Dry Air |
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We know that there is water vapour in air. In other words, air contains moisture and so it is wet. This quality of wetness is referred to as humidity. For our physical comfort, humidity should not be too high or too low. Usually when the atmospheric temperature is high, more evaporation of water takes place and hence humidity is high. During such conditions, when you do any physical activity, you perspire profusely. It is because of the high humidity your perspiration does not get evaporated as the air cannot absorb more moisture. You feel sticky, hot and uncomfortable. Besides, due to lack of evaporation, your body does not get a chance to cool down. During winter season, due to low temperature, the rate of evaporation is lower. So the air is comparatively less humid. Hence your perspiration gets evaporated, your body gets cooled, and you feel more comfortable. |
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Summary |
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Troposphere is the blanket of air in which we live where all the weather is formed.
Air is a mixture of 78.1% of nitrogen, 20.9% oxygen, 0.03 - 0.04% carbon dioxide, 0.94% noble gases and water vapour. |
