A. solid B. made up of C. equal D. fills E. describing F. vapor G. therefore H. fundamental I. elementary J. distinguished K. flow L. refuting M. consisted of N. however O. dense 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 1 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 case of a planet's atmosphere. The distinction was a prominent feature of early theories 3 the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be "dissolved" in a 4 without losing its identity, and another theory held that the two phases are 5 different kinds of molecules. The theories now prevailing take a quite 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 6 easily. They are fluids. The 7similarity 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 words, becomes less 8; some of it evaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and pressure at which the densities become 9 is called the critical point. Above the critical point the liquid and the gas can no longer be 10 ; there is a single, undifferentiated fluid phase of uniform density.