高中二年级英语(人教版)下册 Unit 13 Reading

　　THE PROPERTIES OF WATER

　　Earth is an ocean planet. About three billion years ago, life on our planet began in the deep blue seas. Ninety-nine percent of the living space on earth is in the oceans. Marine life is incredibly rich and varied. There are about five million species in the oceans and we have yet tolearn much about them. Life in the oceans ranges from the tiniest plankton all the way up to giants like sharks and whales. What makesthe ocean such a great place to live? The answer is water.

　　CHEMICAL STRUCTURE

　　The chemical structure of water makes it an excellent medium for life. Water is actually quite simple, but the way the water molecule is formed gives water its unique properties. The water molecule is made up of two hydrogen atoms and one oxygen atom. They form a polarmolecule, that is, one with a slightly positive end and a slightly negative end. Because water is polar, it can break down both solids and gases.The nutrients in whatever falls into the ocean quickly become available to other living creatures. The chemical structure of water also makes itdifferent from almost everything else on earth. Water is a liquid at room temperature and has a relatively high freezing point.

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