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 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 therefore be kept in a closed container or held by a gravitation field, as in the case of a planet's atmosphere. The distinction was a prominent feature of early theories describing 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:
liquidons and gasons. 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 flow readily. They are fluids.
The mendasar 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 words becomes less dense; 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 equal is called the critical point. Above the critical point the liquid and the gas can no longer be distinguished; there is a single, undifferentiated fluid phase of uniform density.
1. Which of the following would be the most appropriate title for the passage?
(A) The Properties of Gases and Liquids (B) High Temperature Zones on the Earth
(C) The Beginnings of Modern Physics (D) New Containers for Fluids
2. According to the passage, the difference between a liquid and a gas under normal conditions on Earth is that the liquid
(A) is affected by changes in pressure (B) has a permanent structure
(C) forms a free surface (D) is considerably more common
3. It can be inferred from the passage that the gases of the Earth's atmosphere are contained by
(A) a closed surface (B) the gravity of the planet
(C) the field of space (D) its critical point
4. According to the passage, in the nineteenth century some scientists viewed liquidons and gasons as
(A) fluids (B) dissolving particles
(C) heavy molecules (D) different types of molecules
5. According to the passage, what happens when the temperature is increased in a closed container holding a liquid?
(A) The liquid and gas phases become more similar.
(B) The liquid and the gas become less dense.
(C) The container expands.
(D) The liquid evaporates out of the container.
6. According to the passage, which of the following is the best definition of the critical point?
(A) When the temperature and the pressure are raised
(B) When the densities of the two phases are equal
(C) When the pressure and temperature are combined
(D) When the container explodes
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