Noncrystalline Solids
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 still intertwined.
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!
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