Chemistry Reference
In-Depth Information
You are probably familiar with most change-of-phase operations. In
Lesson 1-2 we discussed the process of sublimation, which you may not
have been familiar with. Recall that sublimation is the process of a sub-
stance changing from a solid phase directly into a vapor phase. Even if you
don't know the term, you have probably seen sublimation taking place.
When “dry ice,” which is frozen carbon dioxide, appears to give off a white
“smoke,” you see evidence of the solid carbon dioxide turning directly into
vapor. Carbon dioxide is actually an invisible gas. The white “smoke” is
actually water vapor condensing because of the cold gas coming off the
solid block of dry ice.
Sublimation happens more readily with “dry ice” than frozen water
because the forces between carbon dioxide molecules are not as strong as
the forces between water molecules. An example of sublimation can be
represented by the following equation:
CO
2(s)
+ energy
CO
2(g)
Experimentally, we can determine the freezing point or boiling point
of a pure substance by determining at what temperature the substance stops
getting hotter or colder for a period of time. If we graph our data for heating
or cooling, the “plateaus” or flat areas represent where a change-of-phase
operation is occurring. It is also important to note that, for a pure substance,
the freezing point and melting point are exactly the same. For example, the
normal freezing point of pure water is 0
o
C, and the normal melting point
for pure ice (frozen water) is also 0
o
C. The same relationship holds true
for the boiling point and condensation point of a pure substance.
Examples of Physical Changes
Melting wax, cutting wood, breaking glass, boiling water, bending
metal, freezing carbon dioxide, dissolving salt, cutting hair.
Chemical changes, also called chemical reactions, are more complex,
and they tend to be harder to reverse than physical changes. There are
many different types of chemical changes. Some happen very quickly, such
as an explosion, and some happen slowly, such as rusting, but they all result
in the production of one or more new substances. Look at the equation for
the decomposition of water, shown on page 28, and compare it to the equa-
tion for melting water, shown previously.
This reaction, called the electrolysis of water, produces two new sub-
stances. Hydrogen and oxygen are both colorless gases, and they both have
properties that are different from the properties of water. When liquid
