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．Marine¹ life is incredibly rich and varied²．
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 plankton³ 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 molecule⁴ 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 nutrients⁵ 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 relatively⁶ high freezing point．
Salinity⁷
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．
Density⁸
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．Dense⁹ water sinks and less dense water is pushed to the surface．This circulation adds energy to the marine ecosystems¹⁰ 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．

1 marine   

adj.海的；海生的；航海的；海事的；n.水兵

参考例句：

• Marine creatures are those which live in the sea. 海洋生物是生存在海里的生物。
• When the war broke out,he volunteered for the Marine Corps.战争爆发时，他自愿参加了海军陆战队。

2 varied   

adj.多样的，多变化的

参考例句：

• The forms of art are many and varied.艺术的形式是多种多样的。
• The hotel has a varied programme of nightly entertainment.宾馆有各种晚间娱乐活动。

3 plankton   

n.浮游生物

参考例句：

• Plankton is at the bottom of the marine food chain.浮游生物处于海洋食物链的最底层。
• The plankton in the sea feeds many kinds of animals. 海的浮游生物成为很多种动物的食物。

4 molecule   

n.分子，克分子

参考例句：

• A molecule of water is made up of two atoms of hygrogen and one atom of oxygen.一个水分子是由P妈̬f婘̬ 妈̬成的。
• This gives us the structural formula of the molecule.这种方式给出了分子的结构式。

5 nutrients   

n.（食品或化学品）营养物，营养品( nutrient的名词复数 )

参考例句：

• a lack of essential nutrients 基本营养的缺乏
• Nutrients are absorbed into the bloodstream. 营养素被吸收进血液。 来自《简明英汉词典》

6 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

7 salinity   

n.盐分；咸度；盐浓度；咸性

参考例句：

• In the sea water sampled the salinity is two parts per thousand.在取样的海水中，盐度为千分之二。
• In many sedimentary basins the salinity of the formation water increases with depth or compaction.在许多沉积盆地中，地层水的含盐量随深度或压实作用而增高。

8 density   

n.密集，密度，浓度

参考例句：

• The population density of that country is 685 per square mile.那个国家的人口密度为每平方英里685人。
• The region has a very high population density.该地区的人口密度很高。

9 dense   

a.密集的，稠密的，浓密的；密度大的

参考例句：

• The general ambushed his troops in the dense woods. 将军把部队埋伏在浓密的树林里。
• The path was completely covered by the dense foliage. 小路被树叶厚厚地盖了一层。

10 ecosystems   

n.生态系统( ecosystem的名词复数 )

参考例句：

• There are highly sensitive and delicately balanced ecosystems in the forest. 森林里有高度敏感、灵敏平衡的各种生态系统。 来自《简明英汉词典》
• Madagascar's ecosystems range from rainforest to semi-desert. 马达加斯加生态系统类型多样，从雨林到半荒漠等不一而足。 来自辞典例句