The difference between a liquid and a gas is obvious (1)____

The difference between a liquid and a gas is obvious (1)____ the conditions of temperature 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 (2)____ be kept in a closed container, as in the (3)____ of a planet’s atmosphere. The distinction was a prominent feature of early theories (4)____(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 (5) t____ a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter (6)____ have no permanent structure, and they both flow easily. They are fluids.The (7)____(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 (8)____, 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 (9)____ which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be (10)____(distinguish); there is a single, undifferentiated fluid phase of uniform density.