Ask A Scientist Chemistry Archive

Question:  We all know that glass at STP is a very very slow moving LIQUID.
When I recently asked a teacher of mine whet the melting point was he gave 
me an answer that was well above room temperature. When heard this I was 
shocked and I was told to find out what the melting point was since "I 
knew everything." I think the melting point is somewhere around 100 Kelvin. 
If some one would be kind enough to shed some light on the actual temperature 
I would be very grateful.
yours truly,
"lost"
Richard P. Bak

Answer 1:
"Glasses" are amorphous solids, that is, they don't have a regular shape.
Essentially the opposite of crystalline solids which have a defined shape.
When you break a crystalline solid, it will give flat, well defined faces.
Crystals melt or decompose at definite temps and the change from s to l
is usually abrupt.  Glass (say window glass) will shatter into irregular
faces and curves.  There are many different kinds of glass and none really
have a melting point.  All pretty much have silicon dioxide as a principle
ingredient then something else added:  Here are some numbers...keep in mind
these are all SOFTENING  points and NOT melting points:
silica glass: 1580 Celsius   soda-lime glass: 695 C
borosilicate glass: 820 C    aluminosilicate glass: 915 C
leaded glass: 630 C
What I'm trying to say is:  glass isn't a liquid.  It's a solid with so
many 'defects' that the order inherent in a crystal is absent.  When I
say defects, I'm talking at the molecular level, not physical scratches,
and so on. You may want to recheck that 100 K number you presented.
That's 173 degrees Celsius below zero (-280 F)!
Am I going to get some arguments about my stance?  I'll find out more
from our resident glassblower here at the university.
-Joe Schultz

Answer 2:
I very much agree with J. Schultz's response...  Here are a couple
of other observations:
Glass is formed by melting silicon dioxide (SiO2) which has a
melting point of 1710 deg. C..  In its crystalline form, SiO2
has a very ordered structure with each Si atom bonded to 4 O atoms
and each O atom attached to two Si atoms.  If the SiO2 is quickly
cooled after being melted, the SiO2 "molecules" do not have a chance
to get completely lined up in their solid crystalline form and thus
an amorphous solid (glass) is formed.
Often we like to categorize substances as gas, solid, or liquid..
obviously, glass is somewhere between a solid and a liquid --
some scientists refer to it as a pseudo-solid
-Dr. Brown