Is glass a liquid or a solid

I was having lunch today with my girlfriend and my daughter, and the subject of glass came up.

My GF stated that her science teacher told her that glass is a liquid in a chemical way. She felt this made sense since older glass has that droopy look as it ages it is basically sliding down.

Is this true? It didn’t sound right–so I told her I would ask on the Dope. Seems we always find our answers here!

so what say the doper masses?

The Master Speaks.

have to wait for a materials scientist to come by, but from what I have heard glass is considered a solid now. I grew up with the story that it was a liquid because it doesn’t form cyrstals. Turns out that the characteristic isn’t enough to make it lose place in the world of solids. And the stories of glass panes made in the middle ages being thicker on the bottom-it is true! But has nothing to do with glass flowing. That is the way panes of glass were made back then (they didn’t float them like is done now), and logically enough when they were installed they were put in thick end down.

You might also want to do a search on the boards, as this question has already been discussed.

Regarding the “glass flows as it ages” meme (typically speaking about old windows), it’s pretty much bunk. For example, you might want to take a look at much older glass (e.g., this Roman cage cup) and note that even though its glass is considerably older than windows, it shows no sign of drooping.

Ahem. A test in the not-so-scientific method.

Hypothesis: Glass is a solid.

Equipment required: cookie sheet(solid), tumbler of water(liquid), window pane(???).

cookie sheet- Tap, tap, tap

                 tumbler of water-Sploooooosh!(shouldn't have tossed that there rock in it.)
                 window pane-Tap, tap, tap 

seems pretty cut and dried to me. :smiley:

True solids have a melting point: a narrow temperature range where it goes from solid to liquid. Glass doesn’t. Heat it, and it starts to soften, getting less and less viscous as you increase its temperature. Ergo, glass is not a true solid.

I have to agree with the master on this one: there is no cut-and-dry description of glass as solid or liquid. And it does spring from the fact that there is 1.) no crystalline arrangement of molecules, which leads to 2.) no definative melting point. Although glass might have the brittle-ness and rigidity we would say defines a solid, when heated, it slowly loses viscosity until it flows like how we describe a liquid to. There is not “transition temperature” at which it becomes liquid.

Compare with ice. We know that it has a very “sharp” melting point. Raise its temp from -0.1 to +0.1 (Degrees C) and bingo: fluid. The other part is that it will flow. So do most rocks, btw. It just happens really slowly. IANA materials scientist, but I do have a BA in chemistry.

Near as I can tell, its very similar to making candy with sugar. Pure crystalline sugar would be rock candy. Lollipops are amorphous. What’s the difference? Sugar molecules in the rock candy are held in place by intermolecular bonds. Those in lollipops aren’t. (Or at least there aren’t as many.) I don’t know about you, but I always though rock candy was harder. (No scientific test there, just the mouthfeel.)

Well I am a Materials Scientist, and with the full weight of my Master’s Degree, PhD and years of industrial experience I can confidently declare that you won’t find a better argument than Inner Stickler’s.

It depends how you want to define liquid and solid. If you want to define them in terms of flow properties, then water is liquid and glass is solid. Personally I like the things I call “liquids” to flow and drip and splash, so I regard glass as solid.

You can argue glass is liquid based on phase changes and microstructure, and if you want to call it a liquid, fair enough. It’s just a liquid that doesn’t flow, fractures in a brittle manner and can be used to make blades and springs.

Equally non-committal answer here:

Why should that matter? You can come up with a definition like that, but I don’t see the value in it. That sort of a definition is arbitrary and not useful in describing things.

Technically, any real solid above 0 K also flows really slowly. In a crystal lattice, you get flaws - vacancies, dislocations and grain boundaries. At the right-hand tail of the energy distribution of the atoms in the solid, some will occasionally have enough energy to jump from a lattice site to a vacancy. This allows the solid to gradually change shape.

For high temperature applications, the slow flow of solids over time can become a problem, and is called creep. Gas turbine blades can elongate over their life by creep. Pure tungsten lightbulb filaments can creep under their own weight, which is one reason why the filaments also contain thorium oxide dispersions. Lead, which is a crystalline solid, creeps measureably at room temperature.

Now, as you lower the temperature, the creep rate becomes so small it’s immeasurable. But theoretically it’s still happening, even if it’s only a few dozen atoms per year. Same with the “flow” of glass. Extrapolating viscosity vs. temperature, you can claim that glass is flowing at room temperature if you want. But you’re not exactly going to be plugging that viscosity into a fluid dynamics equation and pumping room-temperature glass down a pipe!

I’ve only encountered the flow of solid rock in a geological sense, where it happens under very high hydrostatic pressure. In terms of flow, a liquid is a substance of fixed volume with zero shear strength, and a solid is a substance of fixed volume with non-zero shear strength. If you apply a pressure to a solid hundreds of times greater than its shear strength, the difference between zero shear strength and comparatively tiny shear strength becomes neglible and the solid will obey the hydrodynamic equations for liquids quite happily.

Bit of a sidetrack, but you may be family with the HEAT-type anti-tank round, or “shaped charge”. This uses an explosive block with a conical hollow cavity, lined with steel or copper. The explosion squeezes the liner like a lemon pip between the fingers, firing a “jet” that punches through the tank armour like a screwdriver through styrofoam.

What is interesting about the HEAT round is that the jet isn’t liquid metal - flash X-ray crystallography shows that it’s solid crystalline copper or steel, although under such huge pressure that it flows exactly as if it were liquid. Also interesting is that glass can function nicely as a cavity liner, a hollow glass cone happily squishing into a glass “carrot”.

Substances like glass are called amorphous solids, meaning solids without crystalline form.

“Liquid” and “solid” are not fundamental phenomena - they’re the human brain’s abstractions of a large range of behavior. So there’s no point in looking for the “true” definitions - you go with what works. The microstructural definitions of liquid given by physicists and materials scientists do not have any definite connection to most people’s intuitive idea of what a liquid is. Is toothpaste a liquid or a solid? It’s a complex mixture of many things, some of which are clearly solid, like abrasives, and some of which are clearly liquid, like water. Looking at the microstructure of toothpaste tells you nothing about whether the mixture as a whole is liquid or solid.

A better way to look at it is from a mechanical point of view. The most common definition of a fluid (which generally comprises what physicists refer to as liquids and gases) is a material whose shape can be changed without affecting the way it responds to subsequent shape changes, i.e., it has no “mechanical memory”. A solid is defined as a material that responds differently depending on how much you stretch it. (The real definitions are given in terms of group theory. I translated them into plain English with some loss of clarity.)

You then try to fit real materials into these idealized definitions, but of course, the fit is never exact. Also, the same material can be fluid under one set of conditions but solid under another. In the case of glass, it seems that you would have to consider very, very long time scales before the fluid definition would be at all useful. Under ordinary conditions, glass is definitely a solid.

A snailboy said, glass is an example of an “amorphous solid,” as opposed to “crystalline solid,” which is what I think of when someone says “solid,” since they exhibit the classic proprties of a solid: incompressibility, rigidity, and characteristic geometry. This “arbitrary” definition is taken from University Chemistry by Bruce Mahan, 3rd edition, which goes on to say:

Couldn’t have said it better myself.

To (maybe) clarify the “mechanical memory” argument above – Take a tub if water. Apply hammer. Wait a few seconds. Other than maybe some spillage, you can’t tell a hammer has been applied – no “memory”.

Now put a few squares of glass inside a tub. Apply hammer… not quite the same behavior, eh?

Gotta disagree. Science only works by way of very specific definitions that everyone agrees to. Included in this are words and concepts such as “liquid” and “solid” which have been given very concrete definitions which encompass both microscopic and macroscopic properties and behaviors.

If you want to debate whether glass is a solid or a liquid, you have to comply with the nomenclature. If you’re gonna propose that a solid is whatever you feel it should be, if you want to to go with what * works*, then all bets are off. This is chemistry, not psychology. You don’t change definitions to better fit people’s ignorance.

By the way, is it me, or do we go through this exercise in hair-splitting roughly every 6 months?

So tell me, why are they called amorphous solids, if they’re liquid?

To distinguish them from “true” solids?

But the alternative form is crystalline solid.

OK…so, what’s you’re take, then?