Liquid or Solid?

How were you taught to distinguish between a liquid and a solid? Sounds obvious, but is glass a liquid or solid? I was taught a solid has a definite crystalline structure. My daughter’s elementary science book teaches of amorphous solids including glass and plactics. While I am not sure how atoms are arranged in glass, plastics (even clear plastics) have a definite structure to their monomer (or polymer) strands.

This led to questions like what is a stress ball? Liquid or solid? Shaving cream - liquid or amorphous solid? Is there any true way to distinguish?

Last, if you were taught that glass is a highly viscous liquid, how long would it take for a window to show signs of deformation (like a liquid) showing signs of flowing out of its windowframe…or, even signs of thinning at the top and thickening at the base? My chemistry teacher claimed glass will do this over time, but she did not go into further detail.

What’s the SD say about this deceptively simple question?

Glass is a solid, practically speaking. It would probably take millions of years for a glass pane to flow enough to be noticable.

Some say, “Oh yea? Well people have taken measurements on old panes of glass! And they found they’re thicker on the bottom! So there!!”

Perhaps. But that doesn’t prove the glass flowed.

Pitch is a liquid. It’s been proven by experiment.

My Ceramic Engineering professor taught us that glass is a solid, and raved up and down when anyone claimed it was a liquid. That is because he was a practical, hard-nosed realist who used materials to solve problems and achieve results. Thinking of glass as a liquid does not contribute to those goals.

He never mentioned shaving cream.

According to the Master:

As you indicate, this is now widely regarded as dubious. From this link:

Glass shows every mechanical property of a solid; rigidity (it has a definite unconfined shape), measurable elasticity and definable hysteresis (it flexes and returns to the original shape), toughness (resistance to intrusion or local deformation), and fracture (when you break it, it cleves into separate, solid pieces). Not all solids are crystalline; most people, including chemists, would agree that many polymers are “solid” for all intents and purposes even though they exist as long chain molecules with little or no definable macrostructure.


It depends on how you’re defining your “liquid”: in terms of mechanical properties, or microstructure.

A “newtonian” liquid, that is, one whose behaviour is predicted well by fluid dynamics, has constant volume and zero shear strength. No real liquid is truly newtonian but the substances we think of as liquids (water, ethanol, etc.) are close enough.

Pitch may still sneak in as a near-newtonian liquid since it does deform under an infinitessimal shear stress, although very slowly, i.e. it flows. Glass however does not. Despite the red herring of antique window glass being thicker at the bottom, there are counterexamples of that. There is also other antique glasswork as further evidence: vases etc., optical pieces (lenses and mirrors) that would show even tiny flow effects, and volcanic glass blades that are still razor sharp after 50,000 years or so. From the point of view of flow, cold glass is a solid even if it is structurally amorphous.

OTOH, if you want to define a liquid in terms of structure, or solids and liquids in terms of a melting/freezing phase transition, then you can argue that glass is a liquid. Non-flowing, cold glass is structurally identical to yellow-hot, flowing glass and there isn’t a sharp at point at which you can say one becomes the other.

Personally, I think if it doesn’t drip, it isn’t a liquid, but really this is more about words - descriptions and definitions - than about properties.

By the OP’s definition, wood is not a solid. Neither is meat.

Shaving cream is a mixture of a gas and a liquid; more specifically, it is a liquid foam, which is a dispersion of gas in liquid. If the gas globules are of colloidal size, the foam is a colloidal foam.

I heard this in school too, but later heard that it was estimated that it would take longer than the universe has existed for glass to measurably flow at room temperature.

The reason why cathedral glass is thicker at the bottom is that variable glass thickness was an unintended consequence of the method of manufacture at the time, and the glass installers tended to put the thicker edge on the bottom.

P.S. Not all solids are crystalline solids.

[quote=“Tim_R.Mortiss, post:4, topic:512181”]

My Ceramic Engineering professor …QUOTE]

Tim, Are you a Ceramic Engineer? If so, from which institute of higher education? Just curious…there aren’t many of us.

Rutgers University, BTW

I heard that professors somewhere, Cambridge I think, would do a sort of experiment that emphasized the long time they could expect to spend occupying their office. They would arrange a glass rod acting as a beam, supported at the ends and carrying a load slung on a thread at the center. This would be set up someplace out of the way, perhaps up high on the corner bookshelf. The glass would flow enough to take on a bend. In this situation the forces on the glass were a significant fraction of the breaking force, and only a little flow is needed to make a visible bend.

Gotta say, though, I can’t remember where I heard this, and think it was a long time ago so even if what I heard was true I might have gotten something wrong. No cite whatsoever.

The big problem, here, is that local schemes of classification are not absolute enough to be extended into a big outline of all matter. The solid/liquid/gas distinction tests whether there is flow and whether volume is conserved. However, it breaks down at temperatures and pressures beyond the critical point, and I think it would be true that most of the mass of the universe, or at least most of the visible mass, is supercritical and therefore neither liquid nor gas but more like a compressible liquid. The crystalline/amorphous distinction tests whether there is order (and by the way polymers can be amorphous or crystalline or contain a mix, and the details of this are argued extensively by the experts). The gas/plasma distinction tests the permanence of the association between electrons and nucleii. Beyond all of these are the various forms of degeneracy, the Bose-Einstein condensates, the colloids and emulsions and aerosols and hydrasols and foams and gels and polyphase mixtures of any sort, and all kinds of things we have not yet discovered. All these distinctions are useful in specific problems, but none of them can claim to be universal or to fit into an outline that defines all matter.

I would agree with Napier, unless you pose pretty strict conditions, phase is ambigous. I haven’t checked but I do suppose that soft glass will creep under on earth desert temperatures, and “cold molding” is certainly used to create high quality metal parts (Rafale landing units) using very high pressure at room temperature.
It may even be a poor way of considering matter, stunting our ability to conceive materials by diminishing our vision of their state.

What about toothpaste?

Toothpaste is a colloid, which is a mixture in which one substance is dispersed evenly throughout another (as opposed to a solution, in which a substance is completely dissolved in another). In the case of toothpaste, the dispersed phase is silica, the abrasive that cleans your teeth.

With a colloid, the dispersed substance consists of a larger particle size than in a solution, so the dispersed substance is said to be suspended. The cutoff between the two is when the particles are large enough to scatter light, making the resulting mixture opaque. Milk is another example of a colloid.

According to wikipedia, toothpaste is an example of a viscoelastic colloidal gel, which flows like a liquid under shear, but maintains its shape when shear is removed. This is why toothpaste can be squeezed out of the tube, but remains on the brush without flowing off.

I thought about this after posting. There are many exceptions to (what I would call) the classical rules attempting to distinguish a solid from a liquid.

Above, it was said that shaving cream is a mix of a gas and liquid. Actually, part of daugther’s lesson involved leaving shaving cream to sit overnight. Although hard to describe, I would say most (or all) the liquid component evaporated and a fine, light but somewhat cohesive airy powdery substance was left behind.

I would say both shaving cream and toothpaste are slurries except the shaving cream is propelled by a gas from the can. Yet, is a slurry just a fancy word for a suspension? Perhaps a thick suspension? …As for a colloid, Wiki attempts to define a colloid which seems even harder to define from other categories.

  • Jinx

Pitch flows under steady pressure but shatters if you hit it with a hammer. Pitch is weird. Or what Napier and cplif said:Our categories are neat and simple but the universe disagrees.

Lots of things will shatter if deformed more rapidly and flow if deformed more slowly. Silly Putty is a nice example because its behaviors are distinct under conditions that are easy to create with your hands. Glass is (as I understand) a much slower and stiffer example. Supposedly water is a much faster one (I have heard of very rapid shattering of droplets into shards), and in its solid state a pretty slow one (e.g. glaciers).

The science of rheology opens up a world of this sort of thing. If you crumple a plastic bag and then drop it on the table and sit and watch it change, and consider how differently it does this at different temperatures or after being held crumpled for different times, you can learn worlds about it.

I think anything whose molecules are reluctant to move relative to one another but have no permanent associations falls into this category. Pitch has many friends.

This and the other contenders for the longest running experiment are the coolest things I’ve read today.