The difference between a liquid and a gas is obvious【46】______the conditions of temperatur

e and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fills it to the level of a free surface. A gas forms no free surface but tends to diffuse throughout the space available; it must【47】______be kept in a closed container, as in the【48】______of a planet' s atmosphere. The distinction was a prominent feature of early theories【49】______(describe) 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 held that the two phases are made up of different kinds of molecules (分子). The theories now prevailing【50】 t______a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter【51】______have no permanent structure , and they both flow easily. They are fluids.

The【52】______(fundament) similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. Suppose a closed container partially filled with a liquid is heated. The liquid expands, or in other【53】______, becomes less dense; some of it evaporates. In contrast, the vapor above the liquid surface becomes dense as the evaporated molecules are added to it. The combination of temperature and pressure【54】______ which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be【55】______(distinguish) ; there is a single, undifferentiated fluid phase of uniform. density.

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