|If a cubic salt crystal is broken, the pieces still have flat faces
that intersect at 90o angles. On the other hand, if you shatter
a piece of glass, the pieces often have surfaces that are not flat. Instead,
they tend to be smooth and curved. This behaviour illustrates a major difference
between crystalline solids, like NaCl, and noncrystalline solids, or amorphous
solids such as glass.
|The word amorphous is derived from the Greek word amorphos,
which means "without form". Amorphous solids do not have long-range repetitive
internal structures such as those found in crystals. In some ways their structures,
being jumbled, are more like liquids than solids. Examples of amorphous solids
are ordinary glass and many plastics. In fact, glass is sometimes
used as a general term to refer to any amorphous solid.
|Substances that form amorphous solids usually consist of long, chainlike
molecules that are intertwined in the liquid state somewhat like long strands
of cooked spaghetti. To form a crystal from the melted material, these long
molecules would have to become untangled and line up in specific patterns.
But as the liquid is cooled, the molecules move more slowly. Unless the liquid
is cooled extremely slowly, the molecular motion decreases too rapidly for
the untangling to take place, and the substance solidifies with the molecules
|Compared with substances that produce crystalline solids, those that
form amorphous solids behave quite oddly when they are cooled. Those that
form crystals solidify at a constant temperature. As the liquid is cooled,
it eventually reaches the substances's freezing point, and crystals begin
to form. Even though more heat continues to be removed, the temperature remains
constant until all of the liquid has frozen. Only then does the temperature
of the solid begin to drop.
|Sometimes a liquid can be cooled below its freezing point. As the
temperature approaches the freezing point, particles of the liquid may not
be aligned in just the proper way for them to form a crystal. Thus, the temperature
may continue to fall below the freezing point until, by chance, the particles
in some small portion of the liquid suddenly find themselves properly arranged
for a small crystal to form. This crystal grows rapidly, and the temperature
rises again to the freezing point. The temperature then stays constant until
all the liquid has frozen. While the liquid has a temperature below its normal
freezing point, it is said to be supercooled.
|With substances that form amorphous solids, the solidification of
the melted material into highly ordered crystals never occurs because the
molecules can't become untangled before they are frozen in place at a low
temperature. Sometimes amorphous solids are therefore described as supercooled
liquids. This term puts an emphasis on the kind of structural disorder found
in liquids. It also suggests that the material's constituent molecules retain
some residual ability at least to flex their chains into somewhat less random
patterns and over a long period of time, achieve an improved degree of crystalline
orderliness. But the rate at which such change occurs is typically so small
that under ordinary conditions it cannot be observed and the material is
fully rigid. Glass, for instance, which is a typical amorphous solid, over
a very long time will develop regions of higher and higher order.
|Amorphous solids also soften gradually when they are heated. This
is the reason you can heat glass tubing in a flame to soften it so that it
can be bent. By contrast, if you warm an ice cube (crystalline water), it
won't become soft gradually - at 0OC it will suddenly melt and
drip all over you!