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Introduction to Environment
Water is a necessity for life. Living organisms comprise at least 70 percent of water. It is the only substance present on Earth and in the atmosphere in its three phases – solid, liquid and gaseous – at the same time. Water covers about 71% of the earth's surface. 97% of the earth's water is found in the oceans and it is too salty for drinking, growing crops, and most industrial uses except cooling. Only 3% of the earth's water is fresh in which 2.5% of the earth's freshwater is unavailable: i.e. locked up in glaciers, polar ice caps, atmosphere, and soil or highly polluted or lies too far under the earth's surface to be extracted at an affordable cost. Thus only 0.5% of the earth's water is available as fresh water. 

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