Water is one of the most important and abundant compounds of the ecosystem. All living organisms on the earth need water for their survival and growth. It rarely occurs in its pure form in nature (Ababio, 2005). It is the only substance that exists naturally on Earth in all three physical states of matter; gas, liquid, and solid. The Earth has oceans of liquid water and polar regions covered by solid water and the gaseous water vapour, a greenhouse gas which traps energy radiated from the surface of the planet and provides the planet with warmth. Energy from the sun is absorbed by liquid water in oceans, lakes, and rivers and gains enough energy for some of it to evaporate and enter the atmosphere as an invisible gas, water vapour. As the water vapor rises in the atmosphere it cools and condenses into tiny liquid droplets that scatter light and become visible as clouds. Under the proper conditions, these droplets further combine and become heavy enough to precipitate (fall out) as drops of liquid or, or if the air is cold enough, flakes of solid, thus returning to the surface of the Earth to continue this cycle of water between its condensed and vapor phases. This cycle is known as the Hydrologic cycle.
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