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What is a solid.

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What's the difference between a solid and a liquid, according to time?

 

A body can behave as a visco-elastic plastic, so it's neither a solid or a liquid, only one or the other, depending on time?

 

Rocks flow over time, at an instant they are a solid, but over million year time-scales they behave as liquids.

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Think of it this way: a "solid" is a liquid with a very high viscosity. It is flowing but at a very slow rate in comparison to say water or corn syrup. A solid would be something that "pushes back" at an exerted force with an equal (or almost equal) amount of force, while a liquid would not. Thus it flows. Hope that helped.

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In a gas, molecules are free to change distance and to change position.

In a liquid, molecules are free to change position but not distance.

In a solid, molecules are neither free to change distance nor position. They vibrate in place.

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In a gas, molecules are free to change distance and to change position.

In a liquid, molecules are free to change position but not distance.

In a solid, molecules are neither free to change distance nor position. They vibrate in place.

 

And here I always thought it had to do with adding (or taking away) heat without raising the temperature, latent heat of vaporization and fusion (see wikipedia) or some such. Guess ya learn something new everyday.:eek:

Edited by npts2020

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Geo, your idea is correct.. in a larger time scale then we consider regular, you speak of rocks flowing.. they behave as liquids... no they behave as liquids in their time range they indded are liquids and particles in the time range they are particles... sounds familiar? hmm , if this analogy would hold we would have particles so minute compared to protons as a rock to a liquid behaviour over your 'geological' time. However, your argument holds perfectly fine.

 

answers were somewhat unclear in this post tbh :)

 

(including this of course, don't stray ofF course!

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I disagree that time has anything to do with it. If you click the link in my previous post or read about phase change, you will be able to see that there are well defined temperatures (at a given pressure, usually atmospheric) that determine when a given substance is gas, liquid or solid. The only time it is more than one is while undergoing phase change when latent heat of vaporization or fusion is being added (or taken away). Plasma is very unusual and an entirely different matter...:D

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this has been going of for far too long for what it is.

 

yes some solids can deform in a manner similar to fluid flow given enough time. and that is the nub of it, similar. as in reminicent of but not actually the same at all.

 

lets say you take your solid that flows nice and slowly, a nice chunk of rock and a liquid, water(for simplicity.

 

and we stick some nice controlled conditions on it, lets put them in a container. now, in the interests of getting a few million years out the way, you liquify the rock and let it settle into the container then resolidify it again.

 

if we then tag certain bits of the rock sample with a marker we can follow and do the same with the water and leave it for a few trillion years then we'd notice some very big differences.

 

the markers in the liquid are jiggling about all over the place (and given some trillions of years the markers in water should have covered every cubic angstrom of the container at least twice.) but the markers in the solid have stayed put.

 

see solids will deform and 'flow' when under stress and out of hydrostatic equilibrium, put it in hydrostatic equilibrium or remove stress and you get bugger all.

 

in a liquid the molecules are going to be jiggling about all over the place regardless. and then there is the fact that not all solids exhibit a flow regardless of how much time you give it.

 

in any case this 'flow' is well known and the type of materials have a whole materials classification for themselves. solids which behave in such a manner are called 'plastics'* and even these materials have a temperature regime in which they can be described as plastic. outside these regimes they are either completely solid or liquid.

 

*this is not reffering to just what are normally thought of as plastics(long chain organic polymers) but to any and all materials that exhibit plasticity which basically means all of them. plastic behaviour is when stress and strain have a non linear relationship (if its linear then it is elastic behaviour) this results in a permanent deformation when the material is exposed to sufficient stress (although more precisely a stress-time regime.)

 

does this answer everyones questions?

 

yes its not as cut and dry as it was in your highschool physics class but shocker they kept the the more subtle and complex things away from you when you hadn't a hope in hell of understanding them but its still not too difficult.

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this has been going of for far too long for what it is.

 

yes some solids can deform in a manner similar to fluid flow given enough time. and that is the nub of it, similar. as in reminicent of but not actually the same at all.

 

lets say you take your solid that flows nice and slowly, a nice chunk of rock and a liquid, water(for simplicity.

 

and we stick some nice controlled conditions on it, lets put them in a container. now, in the interests of getting a few million years out the way, you liquify the rock and let it settle into the container then resolidify it again.

 

if we then tag certain bits of the rock sample with a marker we can follow and do the same with the water and leave it for a few trillion years then we'd notice some very big differences.

 

the markers in the liquid are jiggling about all over the place (and given some trillions of years the markers in water should have covered every cubic angstrom of the container at least twice.) but the markers in the solid have stayed put.

 

see solids will deform and 'flow' when under stress and out of hydrostatic equilibrium, put it in hydrostatic equilibrium or remove stress and you get bugger all.

 

in a liquid the molecules are going to be jiggling about all over the place regardless. and then there is the fact that not all solids exhibit a flow regardless of how much time you give it.

 

in any case this 'flow' is well known and the type of materials have a whole materials classification for themselves. solids which behave in such a manner are called 'plastics'* and even these materials have a temperature regime in which they can be described as plastic. outside these regimes they are either completely solid or liquid.

 

*this is not reffering to just what are normally thought of as plastics(long chain organic polymers) but to any and all materials that exhibit plasticity which basically means all of them. plastic behaviour is when stress and strain have a non linear relationship (if its linear then it is elastic behaviour) this results in a permanent deformation when the material is exposed to sufficient stress (although more precisely a stress-time regime.)

 

does this answer everyones questions?

 

yes its not as cut and dry as it was in your highschool physics class but shocker they kept the the more subtle and complex things away from you when you hadn't a hope in hell of understanding them but its still not too difficult.

 

This does not really contradict what I have been trying to point out about latent heat and phase change in regards to defining the state of a substance. Plasmas and bose-einstein condensates are the only substances I can think of off hand that do not fit those definitions. I could have written about it in my own words but wikipedia already had good explanations IMO.

 

One small point, however, "melting" a substance (especially a rock) and resolidifying it is not necessarily the same as leaving it alone for a trillion years. If you heat a diamond up at atmospheric pressure beyond its melting point and resolidify it at the same pressure will it still be a diamond (hint diamonds can't form except under very high pressures)? Whereas, a diamond if left alone at normal earth surface temperatures and pressures will be a diamond forever AFAIK.

Edited by npts2020

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If you heat a diamond up at atmospheric pressure beyond its melting point and resolidify it at the same pressure will it still be a diamond? (hint diamonds can't form except under very high pressures)

 

you are missing the point somewhat.

 

the point of melting it to fit the container allows the substance to settle into the container and get to the lowest energy like the water can do.

 

also, diamonds don't melt at STP, they sublime. i also wasn't talking about diamonds either.

 

and no, it doesn't contradict phase change latent enthalpies because it wasn't supposed to. although phase change enthalpies are more of an indicatr that phase has been changed, it does not define the current state of matter. and you do get phase change enthalpies for bose-einstein condensates (IIRC) and definitely for plasmas. you also get them for solid-solid and liquid-liquid phase changes.

 

either way, the OP had a query about the properties of particular states of matter, not the transition between them

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You may be interested in doing a little reading about the Deborah number used in rheology of polymers: http://en.wikipedia.org/wiki/Deborah_number

 

In short, it is a number that quantifies how "solid" a fluid can act like.

 

Every liquid can act "solid-like" depending on the time scale, and I think that it is equally fair to say that every solid can act "liquid-like" again depending on the scale.

 

I good example from Mythbusters that they did a while back was to see how well bullets penetrate water and remain deadly. They found that the slower-velocity bullets, like from hand guns, penetrated the water well and could easily harm or kill someone several meters underwater. But, the high-velocity guns, like a sniper rifle, the bullet would actually fracture and fall apart very quickly after hitting the water. In this case, the bullet was moving so fast that the water was "solid-like" from the perspective of the bullet. The bullets disintegrated because they were in effect hitting a solid. In the time scales of the high-velocity bullet, the water didn't have enough time to be pushed out of the way like with the slower bullets.

 

A glacier is another good example. It is seemingly very solid. But, the experiment has been done where they placed beacons in the ice on a glacier that was between two mountains. They came back several years later, and the beacons had shifted in the flow direction into a perfect parabolic profile, just like the Navier-Stokes equations predict. On the time scale of years, the glacier acts like a fluid.

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I realize it has been a long time (over 30 years) since I had to learn about this sort of thing but it was always my understanding that the reason that the point of latent heat was used for determining the phase of most materials was precisely because of the ambiguity of many states of matter, especially when time is involved. Am I wrong about this?

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npts, I think you're answering a different question. You're right that phase changes are defined thermodynamically, and I don't think anyone is disagreeing with you. However the question seemed to be about the behaviour of solids, and ways that they are similar to liquids.

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npts, I think you're answering a different question. You're right that phase changes are defined thermodynamically, and I don't think anyone is disagreeing with you. However the question seemed to be about the behaviour of solids, and ways that they are similar to liquids.

 

Thanks for the clarification. Apparently, I misunderstood the original question and took it to be asking when a solid becomes a liquid instead of what properties distinguish one from the other.


Merged post follows:

Consecutive posts merged

IMO it is just easier and more well defined to describe phases of matter thermodynamically then through physical properties of a substance since a phase of one type of material can act in the same manner as a different phase of some other material.

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If I stand on the branch of a tree near the runk and walk outwards the branch will break.

Not many people would describe that as the timber "flowing".

 

Similarly, over long periods of time the movement of bits of the earth's crust caused by gravity and the upwelling of magma produce huge forces that break rocks. The broken rocks can get shoved aside then cold-welded back together.

This isn't the behaviour of a liquid- it's the behaviour of a solid under sufficient stress that it breaks.

Rocks are not liquid on any timescale but they will deform if subject to sufficiently great forces.

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If I stand on the branch of a tree near the runk and walk outwards the branch will break.

Not many people would describe that as the timber "flowing".

 

Similarly, over long periods of time the movement of bits of the earth's crust caused by gravity and the upwelling of magma produce huge forces that break rocks. The broken rocks can get shoved aside then cold-welded back together.

This isn't the behaviour of a liquid- it's the behaviour of a solid under sufficient stress that it breaks.

Rocks are not liquid on any timescale but they will deform if subject to sufficiently great forces.

 

 

I still think that you can think of it as a continuum of time scale behavior. Some very solid solids, like rocks or wood, probably would need time scales longer than the life of the universe to act "fluid-like", but there is still a theoretical time scale. The molecules of a rock are very, very, very slowly moving about and in a time scale of billions, trillions, quadrillions of years, will eventually rearrange themselves into the absolute lowest minimum of energy. Just because such behavior is essentially unobservable (by the tools we have today) doesn't mean that it isn't happening. Just happening very slowly.

 

And, the flip side is that there are going to be materials that act fluid-like in all but the quickest timescales. Most gases are going to be fluid-like in all but the shortest time-scales.

 

A better description of how a material acts is what is the cause of the stress in the material. In a solid, it is displacement, in a fluid is the rate of shear. And, in polymers that have both liquid and solid-like properties, it is some combination of both.

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I still think that you can think of it as a continuum of time scale behavior. Some very solid solids, like rocks or wood, probably would need time scales longer than the life of the universe to act "fluid-like", but there is still a theoretical time scale. The molecules of a rock are very, very, very slowly moving about and in a time scale of billions, trillions, quadrillions of years, will eventually rearrange themselves into the absolute lowest minimum of energy. Just because such behavior is essentially unobservable (by the tools we have today) doesn't mean that it isn't happening. Just happening very slowly.

 

And, the flip side is that there are going to be materials that act fluid-like in all but the quickest timescales. Most gases are going to be fluid-like in all but the shortest time-scales.

 

A better description of how a material acts is what is the cause of the stress in the material. In a solid, it is displacement, in a fluid is the rate of shear. And, in polymers that have both liquid and solid-like properties, it is some combination of both.

 

So why even bother defining different phases of any material? It seems to me that there is so much overlap of properties that the only consistent way of doing it is thermodynamically by latent heat of vaporization and fusion.

 

IMO the OP is saying there is no distinguishable difference between phases (of at least some substances) given a long enough time scale but in fact thermodynamics says otherwise.

Edited by npts2020

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So why even bother defining different phases of any material? It seems to me that there is so much overlap of properties that the only consistent way of doing it is thermodynamically by latent heat of vaporization and fusion.

 

IMO the OP is saying there is no distinguishable difference between phases (of at least some substances) given a long enough time scale but in fact thermodynamics says otherwise.

 

How does the latent heat definition explain how a glacier can act like a fluid over a time scale of years? How does the latent heat definition explain how glass can flow again over a large number of years?

 

The latent heat definition has its flaws, too.

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How does the latent heat definition explain how a glacier can act like a fluid over a time scale of years? How does the latent heat definition explain how glass can flow again over a large number of years?

 

The latent heat definition has its flaws, too.

 

I agree, but at least you can for the most part definitively describe the phase of a material that way, even if the characteristics of a given phase are ambiguous.

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If the difference is tied to the strength of atomic/molecular bonds, wouldn't solids flow due to external factors -- like how much pressure's involved?

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