2011年考研英语作文之美文范例：油和水

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 Oil and Water

　　To understand the emulsifying process, we must first accept the scientific principle that oil and water do not naturally mix. Quite literally, they find each other's presence repulsive. A good illustration of  this aversion  is homemade  oil and vinegar salad dressing.When you shake or beat your salad dressing, you do more than disperse the oil throughout the vinegar: you also break down the oil into droplets minute enough to remain temporarily suspended in the vinegar (which from now on we will call water, because that tart condiment is in effect mainly water). The second you  stop  agitating  the  dressing,  the  oil  droplets  start  to  combine  into  units  too  large  to  be suspended in the water, and thus slither their way upward, separating from the water in the process. The oil rises to the top and the water sinks because oil has a lower specific density than water. If you want a stable emulsion, you need an emulsifying agent which prevents the oil droplets from combining  into  larger  units.  Emulsifying  agents  occur  naturally  in  many  animal  substances including egg yolks and milk. An emulsifying agent helps to keep the oil particles from combining in three basic ways. First, the agent coats the oil, serving as a physical barrier between the droplets. Second, it reduces the water's surface tension, which, in turn, reduces the water's ability to repulse oil. Third, the agent gives the surfaces of the oil droplets identical electrical charges; since like charges repel each other the
droplets repel each other.
　　全文翻译：油和水为理解乳化过程的发生，我们必须首先认识到这样一条科学原理：水和油不能自 然混合，它们实实在在互相排斥。 家制的油醋沙拉调料就是一个好例子。 当你搅拌沙拉调 料时，你不仅使油扩散到醋里，而且使油滴被搅得很小可以悬浮在醋中(自此我们改称它为 水，因为事实上这种酸性调味品的主要成份就是水)。 一旦你停止了搅动，油滴就会凝聚， 大到再也不能悬浮在水中时，它们就向上滑动出来而与水分离了。  因为油的密度小，所以 油会浮在水上。 如果想得到稳定的乳化，就需要一种乳化剂。 它可以不让油形成大的油滴。 自然界中，很多动物体中含有乳化剂，如蛋黄和牛奶。乳化剂可以从以下三方面阻止油滴 凝聚。一、  乳化剂包裹住油滴，在油滴间形成一道物质屏障。二、乳化剂降低水的表面 张力，从而降低了水排斥油的能力。  三、  乳化剂使油滴表面带上了同种电荷，因为同种电 荷互相排斥，油滴之间也相互排斥。