高中课本听力人教版 第二册课本13

　　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 1 life is incredibly rich and varied 2．

There are about five million species in the oceans and we have yet to learn much about them．Life in the oceans ranges from the tiniest plankton 3 all the way up to giants like sharks and whales．What makes the 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 4 is formed gives water its unique properties．The water molecule is made up of two hydrogen atoms and one oxygen atom．They form a polar molecule，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 5 in whatever falls into the ocean quickly become available to other living creatures．The chemical structure of water also makes it different from almost everything else on earth．Water is a liquid at room temperature and has a relatively 6 high freezing point．

Salinity 7

When dissolved gases and solids mix with pure water, the result is sea water, or salt water．The salinity of the earth’s oceans is about thirty-five parts per thousand，meaning that there are about thirty-five grammes of dissolved solids and gases in one kilogramme of water．Even though the pure water is what gives sea water its main properties，the salinity of sea water affects both its weight and freezing point．Salt water has a lower freezing point and is heavier than pure water．

Density 8

Density is the relationship between mass and volume and is measured in kilogrammes per cubic metre ( kg/m3 )．The density of pure water is 1,000 kg/m3，meaning that one cubic metre of water weighs one thousand kilogrammes. If a substance has a higher density, say 5,000 kg/m3，it will not float on water．Marine animals and plants take advantage of the density of water．With the right kind of body, it is possible to float around in the ocean and let the water do the work．When water freezes，its density decreases．If it did not．the oceans would be frozen solid．

Heat Capacity

Heat capacity is the amount of energy it takes to raise the temperature of a substance by one degree centigrade．The heat capacity of water is relatively high．Water can absorb and give off a lot of heat without big changes in temperature，thus creating a stable environment．Most animals and plants are sensitive to large or sudden changes in temperature．so the ocean is a safe and comfortable habitat．The water in the oceans also keeps the temperature of the earth steady by absorbing and giving off heat．

Ocean Motion

Since changes in salinity and temperature affect water’s density, the water in the ocean is always moving．Dense 9 water sinks and less dense water is pushed to the surface．This circulation adds energy to the marine ecosystems 10 and moves nutrients around．

Isn’t it amazing that a single substance can be so important to our planet and even the whole universe? Water，which seems so simple and common, is what makes life possible．Of all the resources on earth — oil，gas，gold and so on — nothing is as precious as a drop of rain．Learning about the properties of water helps us

understand life on our planet．The most important thing we can learn about water, however, is that we must protect it and use it wisely．Our future depends on it．