So what are the definitions of gas, liquid, solid?

[swampwiz]

Obviously, we know intuitively what they mean, but it seems that physicists have developed an objective definition for all of these.

If I were to guess, I'd say that:

- a gas is vastly less compressible than a liquid or solid (i.e., which are considered in thermodynamics as basically imcompressible).

- a solid has an elastic force (at some strength) of deformation - such that the material bounces back (to some extent) when the deformation force is removed.

- a liquid is anything that is not a gas or a solid, and has some shear force that is generated when there is a flow of particles with a velocity field that has a differential in a normal direction from the flow.

I've heard of a term for a material as being viscoelastic. I would guess that it is still a solid, but with a strong amount of the liquid shear force.

 

[snorkack]

Both gas and liquid generate shear forces by viscous friction when moved. What distinguishes them from solids is that the shear force of fluids goes to zero rapidly when flow stops. What precisely is the definition of gel point?
The distinction between liquid and gas is not so much compressibility. It is rather extendibility. Gas fills the volume uniformly. Liquid separates into bubbles. This is what happens at critical point.

 

[DrClaude]

- a gas is vastly less compressible than a liquid or solid (i.e., which are considered in thermodynamics as basically imcompressible).

I guess this is not what you meant.

 

[kuruman]

Where do glasses fit in your scheme of things?

 

[Baluncore]

I would say that in the short term, solids hold their shape, liquids flow, and gasses expand to fill the available volume. But all three have viscosity, so given time, even solids flow.

Why do you need a trichotomous classification of multidimensional matter?
An arbitrary classification will be useless if it does not index the dimensions you require. For example, time and viscosity, plasma, glass, and quasicrystals will test the boundaries of classifications based on chemical bonding.

 

[swampwiz]

I guess this is not what you meant.

Yes, obviously I had a brain fart, and I had meant the opposite. :)

 

[swampwiz]

Both gas and liquid generate shear forces by viscous friction when moved. What distinguishes them from solids is that the shear force of fluids goes to zero rapidly when flow stops. What precisely is the definition of gel point?
The distinction between liquid and gas is not so much compressibility. It is rather extendibility. Gas fills the volume uniformly. Liquid separates into bubbles. This is what happens at critical point.

Yes, gas is vastly more compressible than liquid; engineering-wise liquids are considered incompressible. As for the bubble-ability, that is a manifestation of the phase change from liquid to gas, and I don't think that is a proper comprehensive definition.

 

[swampwiz]

I would say that in the short term, solids hold their shape, liquids flow, and gasses expand to fill the available volume. But all three have viscosity, so given time, even solids flow.

Why do you need a trichotomous classification of multidimensional matter?
An arbitrary classification will be useless if it does not index the dimensions you require. For example, time and viscosity, plasma, glass, and quasicrystals will test the boundaries of classifications based on chemical bonding.

Solids hold their shape == a deformation has some elastic snap-back. Of course, the effect of hysteresis means that solids don't snap-back all the way. In the OP, I had proposed that a liquid has no snap-back.

As for wanting the trichotomous classification definition, I just figured that physicists like to have good definitions for cases that are a bit nebulous - e.g., the definition of physical touching being that the forces of repulsion > forces of attraction, etc. This definition should resolve all such tests that you say.

 

[swampwiz]

Where do glasses fit in your scheme of things?

Does glass have any snap-back? Then under *my* definition, I would call it a solid. But the whole point of my thread is to get the definition that material scientists use.

 

[kuruman]

Does glass have any snap-back? Then under *my* definition, I would call it a solid. But the whole point of my thread is to get the definition that material scientists use.

Maybe the distinction between liquid and solid isn't as sharp as you think. You may find this article informative and the conclusion section might guide your thinking.

 

[swampwiz]

Maybe the distinction between liquid and solid isn't as sharp as you think. You may find this article informative and the conclusion section might guide your thinking.

Yes, it is bit nebulous, but I was hoping for some definition like the one for objects touching. I guess you're saying that a solid has non-molecular bonding (IOW, crystallization) that does not break, while for liquids such bonds break - and it is the breaking of the bonds from which the phase change heat of fusion/melting derives?

 

[kuruman]

Something like that. I am content with the idea that the distinction is semantic in the sense that one can argue both ways.

 

[Baluncore]

But the whole point of my thread is to get the definition that material scientists use.

They will redefine their definition as and when they need to.
Why must there be only one definition?

 

[Vanadium 50]

I think this is futile. You want a nice clean three-way division that fits into one-third of a forum post when in reality you have sols and gels and liquid crystals and clusters and sand piles and glasses and and and...

 

[snorkack]

Yes, gas is vastly more compressible than liquid; engineering-wise liquids are considered incompressible. As for the bubble-ability, that is a manifestation of the phase change from liquid to gas, and I don't think that is a proper comprehensive definition.

The thing is that liquid and gas have discontinuity at boiling. For example, water has critical point at 373,95 Celsius. If you examine water and steam at, say, 373,8 C, their densities and compressibilities would be actually close to one another - but they would be one a liquid and the other a gas because the transition is discontinuous unless you warm them by 0,15 C more to get around the critical point.
Liquid and glass don´t have discontinuous transition at such a specific point.

 

[nasu]

As for the bubble-ability, that is a manifestation of the phase change from liquid to gas, and I don't think that is a proper comprehensive definition.

This is the point. There is no "proper" comprehensive definition. There are just fuzzy, approximate ones because we are trying to describe nature and not make a mathematical construction. Division into three phases is just the first approximation. There are many other phases there. Your division based on macroscopic properties is good enough (once you get it right). There are also microscopic properties that can be used, like long range and short range order, translation symmetry, binding strength. But still, no "rigid" boundaries.

 

[Arjan82]

So the 'official' definition for a fluid (gas or liquid) that I've been thought is that a fluid deforms continuously under the application of an arbitrarily small shear force. A solid does not.

Then you've got the elasto-plastic materials which can hold a small shear force and deform back 'a bit' after shear.

Solids stop deforming a short time after you apply a shear force (as long as it doesn't break or deform plastically etc.)

The difference between a gas and a liquid, I think I remember, is that for a liquid intermolecular attractive forces become important (van-der-Waals forces? but maybe there are more? That's really out of my league...).

[Edit]
Looking into Batchelor's "An Introduction To Fluid Dynamics", he mentions for the difference between gases an liquids, besides what I already mentioned:
- The vast difference in compressibility between the two
- The fact that the thermal movement of a liquid is in the order of the diameter of the molecules, and least an order of magnitude larger for a gas.
[/edit]

 

[swampwiz]

So the 'official' definition for a fluid (gas or liquid) that I've been thought is that a fluid deforms continuously under the application of an arbitrarily small shear force. A solid does not.

Then you've got the elasto-plastic materials which can hold a small shear force and deform back 'a bit' after shear.

Solids stop deforming a short time after you apply a shear force (as long as it doesn't break or deform plastically etc.)

The difference between a gas and a liquid, I think I remember, is that for a liquid intermolecular attractive forces become important (van-der-Waals forces? but maybe there are more? That's really out of my league...).

[Edit]
Looking into Batchelor's "An Introduction To Fluid Dynamics", he mentions for the difference between gases an liquids, besides what I already mentioned:
- The vast difference in compressibility between the two
- The fact that the thermal movement of a liquid is in the order of the diameter of the molecules, and least an order of magnitude larger for a gas.
[/edit]

Yes, this is why I proposed that a good definition for a solid is that it have deformation-based resistance - i.e., elastic (even though this could be very non-linear) stiffness.

 

[sophiecentaur]

Obviously, we know intuitively what they mean,

Actually, we only think we know and the old classification problem rears its ugly head. No medium is linear so no material can be classified into one of your three over all conditions.

You would have the same problem trying to classify PF posts. There are some which we could call totally brilliant and accurate and some which we could call total rubbish. But every one has some of each quality about it.

 

[snorkack]

The difference between a gas and a liquid, I think I remember, is that for a liquid intermolecular attractive forces become important (van-der-Waals forces? but maybe there are more? That's really out of my league...).

[Edit]
Looking into Batchelor's "An Introduction To Fluid Dynamics", he mentions for the difference between gases an liquids, besides what I already mentioned:
- The vast difference in compressibility between the two
- The fact that the thermal movement of a liquid is in the order of the diameter of the molecules, and least an order of magnitude larger for a gas.
[/edit]

But note that when you approach the critical point, the difference in terms of intermolecular attractive forces, compressibility and free path is no longer vast - yet still discontinuous.

 

[Arjan82]

But note that when you approach the critical point, the difference in terms of intermolecular attractive forces, compressibility and free path is no longer vast - yet still discontinuous.

You're absolutely right. I should have emphasized that the distinction I mention is only clear for 'regular' or everyday liquids (water at room temperature), gases (air at atmospheric conditions) and solids (a bit of steel or whatever). When you approach more exotic states (triple point) or more exotic materials (non-Newtonian fluids) things become a lot less cut and dried.