Chemistry Reference
In-Depth Information
Solids will undergo phase changes when they encounter energy changes.
Fusion, commonly called melting, is the process by which a solid becomes a
liquid. Solidification, or freezing, is the process by which a liquid becomes a
solid. Sublimation is an unusual process by which a solid goes directly to the
gas phase, without turning to a liquid first. Sublimation is seen in substances
such as carbon dioxide, which have relatively high vapor pressure and rela-
tively low intermolecular forces. Deposition, the opposite of sublimation, is
when a gas vapor goes directly into the solid phase without becoming a
liquid first.
Very few of the elements listed on the Periodic Table of Elements exist
as liquids at standard temperature and pressure. On the other hand, ap-
proximately three-fourths of our planet is covered with the liquid known as
water, so you should be very familiar with the properties of liquids. Unlike
solids, liquids do not have definite shape. If you pour a liquid from a cylin-
drical bottle into a square container, it changes shape to match the con-
tainer. This is possible because the motion of the individual particles within
the liquid is much less restricted than in a solid. The particles are not locked
into fixed positions, and they push past each other, allowing the liquid sample
to flow. Some liquids, such as water, flow readily, whereas other liquids,
such as molasses, are said to be viscous and flow slowly. The viscosity of a
liquid is its relative resistance to flow. Regardless of how fluid a liquid is,
the space that a liquid occupies is more fixed, and it will not expand to
occupy an entire vessel the way a gas will.
When you change the temperature and/or the pressure of a liquid, you
get phase changes to occur. Vaporization, or boiling, is the process by which
a liquid becomes a gas. The temperature and pressure at which a substance
undergoes vaporization depend upon the intermolecular forces between
its particles. When a substance such as gasoline evaporates at a relatively
low temperature, it is an indication that the forces between its molecules
are not as strong as those between water molecules. Vaporization takes
place when the vapor pressure of the liquid is equal to the pressure of the
atmosphere on its surface. Condensation is the process by which gas be-
comes a liquid. We see condensation form on the outside of a cold glass, as
water vapor in the air turns into a liquid.
An interesting question to explore at this point involves another pro-
cess that you are familiar with, called evaporation. Evaporation is when the
liquid on the surface of a sample changes to the gas phase. If the normal
boiling point of water is 100
o
C or 373 K, how is it possible for water to
evaporate at room temperature? The answer to this question can be found
