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__1__available; it must therefore be kept in a closed container or held by a gravitational field, as in the__2__of a planet's atmosphere. The distinction was a __3__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__4 __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 __5 __They are both forms of matter that have no __6 __structure, and they both flow readily.
The fundamental similarity of liquids and gases becomes clearly apparent when the temperature and pressure are __7__somewhat. Suppose a closed container __8__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 __9__to it. The combination of temperature and pressure at which the densities become __10__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 O. common
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