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| 作者：不明 来源：www.pccode.net 整理 发布时间：2007-1-25 16:40:20 发布人：wongrs | |
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a gas is obvious ( 26 ) the conditions of temperature and pressure commonly found at the sur face of the Earth. A liquid can be kept in an open container and ( 27 ) it to the level of a free surface. A gas forms no free surface but ( 28 ) to diffuse throughout the space available; it must ( 29 ) be kept in a closed container, as ( 30 )a planet’s atmosphere. The distinction was a prominent feature of early theories ( 31 )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 ( 32 ) different kinds o f molecules(分子). The theories now prevailing ( 33 ) a quit e different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both flow ea sily. They are fluids. The ( 34 ) similarly of liquids and gases becomes clear ly apparent when the temperature and pressure are raised somewhat.( 35 ) a closed container partially filled with a liquid is heated. The li quid expands or ( 36 ), becomes less dense; some of it evapor ates.( 37 ), the&
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