|I did a science fair project using glass squares
cut the same size and thickness. I fractured the
glass by dropping the same weight at the same
spot. Why didn't it break the same way?
|Question Date: 2004-01-28|
First of all, glass
does not have crystal planes, so any time you hit
it, it will break in an irregular pattern that
sort of looks like circular fractures. We call
this "conchoidal fracture".
crystalline solid, like calcite, will always break
along its crystal planes, no matter where or how,
or how hard you hit it.
You might say that
glass breaks "chaotically". That means that a very
minute change in the way you hit the glass will
result in an unpredictable way in which it breaks.
You can try to set up the breaking conditions
very carefully: you can take a polished pointed
weight, drop it from a precisely measured height
in precisely the same way every time, but there
are tiny variations in exactly the angle at which
it hits the glass that you cannot control. Because
glass is not a crystalline solid, the way it
breaks will be sensitive to the very minute
variations in the breaking conditions. And so it
will never break the same way twice.
you have coined a new saying: "Glass never breaks
the same way twice."
There are two reasons I can see that your glass
did not break the same way. One is that even
though you tried your best to cut the squares
exactly and drop the weight exactly the same they
are probably still slightly different.
other reason is in the actual formation of the
glass. When glass is made the particles will
arrange in a slightly different manner making each
piece stronger or weaker in different spots. So,
the weaker places are more likely to fracture,
each time creating a different pattern
This is a very interesting experiment.
breaks (fractures) in what is called a conchoidal
manner and the reason for this is related to the
fact that glass has a very special kind of
disorder in the structure.
One of the
characteristics of conchoidal fracture is that it
is difficult to predict how exactly it will take
place. Researchers are still working on precisely
the same experiment you have described so many
congratulations for having picked a fascinating
problem to work on.
Your question is a good geology question.
Glass is amorphous, meaning that it's atoms are
not arranged in an orderly manner. Minerals, on
the other hand, form in an organized way and their
atoms are lined up in flat sheets or in cubes, for
example. This pattern repeats itself over and over
as the crystal grows.
Glass is sort of like a
mineral in that it is hard and can occur naturally
(like volcanic glass) but glass is not crystalline
(doesn't grow in crystals made up of atoms with a
In a mineral crystal,
some atoms are bonded more strongly than others.
Where the bonds are weak, the crystal is most
likely to fracture if you smash it.
halite (salt), for example. It's weakest planes
are all 90 degrees from each other so that the
mineral breaks into perfect cubes. The cubes may
be different sizes but they always have the same
You can test me. Use a magnifying
glass and take a look at some salt from your salt
shaker at home. You'll see that all or most of the
salt pieces are little, tiny cubes. (They may not
be perfect cubes anymore if they rubbed on each
other too much, but they'll be close.) Because
glass is not crystalline, it breaks into many
different shaped pieces.
I hope this helps!
This is because glass has a property called
concoidal fracturing. That means that it doesn't
break in a straight line, but in almost a spiral
pattern. If you break a square of say halite (salt
crystals) they always break into smaller squares.
This is due the arrangement of atoms within. Glass
has an arrangement of atoms that is equally strong
in all directions so it won't break on the same
line each time.
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