Unit Twenty-Three
　　Passage 2
　　The difference between a liquid and a gas is obvious under the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fill it to the level of a free surface. A gas forms no free surface but tends to diffuse throughout the 11 available; it must therefore be kept in a closed container or held by a gravitational field, as in the 12 of a planet’s atmosphere. The distinction was a 13 feature of early theories describing the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be "dissolved" in a vapor without losing its identity, and another theory 14 that the two phases are made up of different kinds of molecules. The theories now prevailing take a quite different approach by emphasizing what liquids and gases have in 15 They are both forms of matter that have 6 structure, and they both flow readily.
　　The fundamental similarity of liquids and gases becomes clearly apparent when the temperature and pressure are 17 somewhat. Suppose a closed container 18 filled with a liquid is heated. The liquid expands, or in other words becomes less dense; some of it evaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are 19 to it. The combination of temperature and pressure at which the densities become 20
　　is called the critical point.

A. added	B. case	C.	prominent	D.	held
E. equal	F. partially	G.	example	H.	previous
' I. space	J, lifted	K.	permanent	L.	particularly
M. extended	N. raised	0.	common

买知识，卖知识，就上233考试大！还可以免费赚取现金哦！
·您手头有好东西吗？上传上来和大家一起分享吧！你也可以求购你自己想要的资料。 　　·上传您的精品资料，可获得收入，既可在网站上进行消费，也可提取现金。 　　·若被评为资料，还可以增加任务分值。了解更多>>

　　编辑推荐：
　　2012年12月大学英语四级报名时间
　　2012年12月英语四级考试时间及流程