Figure 24. Vapor Pressure-Temperature Relationship for Carbon Tetrachloride
and WaterCRITICAL TEMPERATURE AND PRESSURE
There are conditions for particular substances when it is impossible for the liquid
or gaseous phase to exist. Since the kinetic energy of a molecular system is
directly proportional to the Kelvin temperature, it is logical to assume that there
is a temperature at which the kinetic energy of the molecules is so great that the
attractive forces between molecules are insufficient for the liquid phase to
remain. The temperature above which the liquid phase of a substance cannot exist
is called its critical temperature. Above its critical temperature, no gas can be
liquefied regardless of the pressure applied. The minimum pressure required to
liquefy a gas at its critical temperature is called its critical pressure.
SOLIDS
Whereas particles in gases have the highest degree of disorder, the solid state has
the most ordered system. Particles are fixed in rather definite positions and
maintain definite shapes. Because of their variation in packing, solids can be
divided into three categories: Crystalline solids have a three-dimensional
representation much like a brick wall. They have a regular structure, in which the
particles pack in a repeating pattern from one edge of the solid to the other.
Amorphous solids (literally, “solids without form”) have a random structure, with
little if any long-range order. Polycrystalline solids are an aggregate of a large
number of small crystals or grains in which the structure is regular, but the
crystals or grains are arranged in a random fashion.
Particles in solids do vibrate in position, however, and may even diffuse
through the solid. (Example: Gold clamped to lead shows diffusion of some gold
atoms into the lead over long periods of time.) Other solids do not show diffusion