Read the following passage and fill in each blank with one
word. Choose the correct word in one of the following three ways : according to
the context, by using the correct form of the given word, or by using the given
letters of the word. Remember to write the answers on the answer sheet.
The difference between a liquid and a gas is obvious (66)
the conditions of temperature and pressure commonly found at the
surface of the Earth. A liquid can be kept in an open container and f
(67) it to the level of a free surface. A gas forms no free
surface but tends to diffuse throughout the space available ; it must
(68) be kept in a closed container, as in the (69)
of a planet’s atmosphere. The distinction was a prominent feature of
early theories (70) (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 t (71) a quite different approach by
emphasizing (72) liquids and gases have in common. They are
both forms of matter (73) have no permanent structure, and
they both flow easily. They are fluids. The f (74)
similarity of liquids and gases becomes clearly apparent when the
temperature and pressure are raised somewhat. S (75) a
closed container partially filled with a liquid is heated. The liquid expands,
or in other (76) , becomes less dense; some of it
evaporates. In contrast, the vapor above the liquid surface becomes dense as the
evaporated molecules are added (77) it. The combination of
temperature and pressure (78) which the densities become
equal is c (79) the critical point. Above the critical
point the liquid and the gas can no longer be dis (80) ;
there is a single, undifferentiated fluid phase of uniform density.