View Full Version : Glass not a liquid?
08-16-1999, 10:56 PM
(Referring to article www.straightdope.com/mailbag/mglass.html (http://www.straightdope.com/mailbag/mglass.html) )
David claims that glass isn't a liquid, but an amorphous solid, and that we should deal with it. I'm perfectly willing to so deal, but you are at odds with a chemistry teacher, professor, and textbook that I had previously found reliable. They all claim that glass is a supercooled liquid. That is, it is cold enough for the SiO2 to solidify, but hasn't crystallized into a solid. Are we just arguing semantics, or is there a real difference between a supercooled liquid and an amorphous solid?
08-17-1999, 12:25 AM
Depends on where you draw the line. Or whether there is a line (there isn't).
If there are three distinct states of matter at STP, then solids are materials that have a crystalline structure, where atoms are bonded to one another in a regular, repeating pattern. Liquids are materials in which the atoms (or individual molecules) are not bonded to one another, and slide past one another when subjected to stress.
The truth however, is that there is a broad DMZ at the boundary between solids and liquids, and glass lives here. Each silicon atom is bonded to several oxygen atoms; there is no regular, repeating pattern to the structure (it is not crystalline), but the individual atoms are not free to slide about when the material is subjected to moderate stresses.
In this irregular structure, the bonds are weaker than those in crystalline SiO2 (quartz). This means, for instance, that glass is not as hard as quartz. It also means that the bonds can slip past one another under stress, but very, very slowly. Left on its own, after millions of years, glass will flow a bit under stress; but eventually the system of atoms will fall to their lowest energy state, and the system will begin to crystallize.
Sorry to trash your science education Greg, but it's happened to us all. If you'd like the rest of the structure swept from beneath you, go to: http://www.ems.psu.edu/~fraser/BadScience.html
08-17-1999, 07:10 AM
Greg said:I'm perfectly willing to so deal, but you are at odds with a chemistry teacher, professor, and textbook that I had previously found reliable. They all claim that glass is a supercooled liquid. That is, it is cold enough for the SiO2 to solidify, but hasn't crystallized into a solid. Are we just arguing semantics, or is there a real difference between a supercooled liquid and an amorphous solid? Yes, there is a difference. Supercooled liquids will, for one thing, act as liquids do. For another, they will tend to crystallize quickly if a seed crystal is dropped into them (if memory serves -- I may be thinking of a supersaturated solution on this part).
Please allow me to quote from a message I posted on alt.folklore.urban several years ago, and which I reposted to the "Melding Glass" thread on General Questions a short while ago:
For what it's worth, glass did used to be commonly referred to as a super-cooled liquid. However, as Doremus points out in Glass Science (incidentally a book Copyrighted in 1973), "the difficulty with this view is that glasses can be prepared without cooling from the liquid state. Glass coatings are deposited from the vapor or liquid solution, sometimes with chemical reactions. Thus sodium silicate glass can be made by evaporating an aqueous solution of sodium silicate (water glass) and baking the deposit to remove water. The product of this process is indistinguishable from sodium silicate glass of the same composition made by cooling from the liquid." (p. 1)
In other words, you can make glass via methods other than cooling a liquid -- so it would be wrong to call it a supercooled liquid from that standpoint alone.
08-17-1999, 07:11 AM
Incidentally, the General Questions thread is at http://www.straightdope.com/ubb/Forum3/HTML/001917.html if you're interested.
C K Dexter Haven
08-17-1999, 07:33 AM
This is one of those topics that independently found its way to two different forums, which is fine, we're just being sure to reference the other forum... where, BTW, Cecil himself commented on the supercooled liquid vs amorphous solid.
08-17-1999, 08:34 AM
David B writes:For another, they will tend to crystallize quickly if a seed crystal is dropped into them (if memory serves -- I may be thinking of a supersaturated solution on this part).
No, both supercooled liquids and supersaturated solutions act this way.
It is possible to supercool water to, I believe, about -6°C -- if the water is very pure, the container is very smooth, etc. One particle (of ice or otherwise), and, bingo! it's all over.
In essence, the water is not looking for a particle of ice [i]per se (otherwise, there could be no freezing of water that was not in contact with already-solid water). It's looking for something with something that has a similar crystalline structure as ice. Adsorbed water molecules on the substrate form an ice crystal, other water molecules align themselves in that structure, and so forth.
Water molecule will not spontaneously align themselves in a structure large enough and longevous enough to permit crystallization for some degrees below the nominal freezing point.
See http://www.newswise.com/articles/1998/7/TIP72898.NSC.html for references to supercooling (not necessarily to freezing of supercooled water).
08-22-1999, 04:53 AM
"Despite its appearance, glass is really a highly viscous liquid rather than a solid, and you can see through it for the same reasons that you can see through water."
That is from Straight Dope Classic, "How come you can see through glass?"
Cecil seems to think glass is a liquid and I'm with him.
C K Dexter Haven
08-22-1999, 06:28 AM
Lance, if you would please check out the thread in GENERAL QUESTIONS forum:
you will find that Cecil has retracted his earlier statement, supporting DavidB's comments that glass is not liquid.
Cecil doesn't post often here, I doubt we could get him to repeat himself in this thread.
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