## Definition of "melting point"

chornedsnorkack - 8-3-2024 at 10:41

What is "melting point"?
The temperature where disordered fluid comes to equilibrium with ordered crystal?
Or the temperature where the viscosity equals 10 000 centipoises?

Sulaiman - 8-3-2024 at 11:45

https://en.wikipedia.org/wiki/Melting_point
chornedsnorkack - 9-3-2024 at 01:15

https://glassproperties.com/viscosity/Glass-Viscosity-Fixpoi...

Your link discusses only one definition. Mine, only the other. Does anyone have a discussion of both definitions together?

bnull - 9-3-2024 at 05:15

The definition of melting point seems to depend on the substance you're talking about.

Glasses and some substances don't have a "sharp" melting point, a temperature below which they are solid and above which they are liquid; they soften as the temperature increases. That's why they use the viscosity as a parameter.

For substances that don't go through glass transitions, melting point is the temperature in which solid and liquid coexist in equilibrium: the liquid and solid phases are always exchanging molecules (or atoms or whatever the basic unit is). Some substances have a range of temperatures as the melting point, maybe because of experimental conditions, maybe because its their property. Even so, the solid and liquid phases are distinct.

Both definitions are valid within their respective contexts. Yours for the glasses, the majority of polymers and some mixtures, and Sulaiman's for all the rest.

chornedsnorkack - 9-3-2024 at 07:17

It seems to me that some substances have melting points by both definitions. And some have more than one by one or other definition.
Water is a classical example of substance that has only one melting point and that by first definition. Equilibrates with solid at 0 degrees by definition, viscosity 1,8 centipoises. Viscosity 1 centipoise at 20 Celsius (which is why centipoise is such a good unit), 0,28 centipoises at boiling point 100 Celsius.
But silicon dioxide is a substance for which, at 1 bar, existence of "liquid" phase is questionable... and therefore the existence of "melting point" by first definition?
Silicon dioxide does have a reasonably reachable point where an ordered phase - cristobalite - comes to an equilibrium with a disordered, soft phase. This is at 1713 Celsius.
But the problem here is that the viscosity of the said soft phase at the equilibrium temperature is quoted as around 2*109 centipoises. Which is far below "melting point" by the second definition. So is the equilibrium even a melting point?
As the soft phase of silicon dioxide is heated, it comes to an equilibrium with its vapours at 1 bar at about 2900 degrees. At which point its viscosity seems to still be above 10 000 centipoises. Which means that although silicon dioxide can be heated to the working point of 1 000 000 centipoises - somewhere around 2300...2400 degrees - it does not have a melting point.
Since silicon dioxide at 2900 Celsius has not reached "melting point" and does not qualify as "liquid", what is it doing? Boiling? Sublimating?

Texium - 9-3-2024 at 08:50

What you are describing for silicon dioxide is glass transition, not melting. We colloquially use the word “melt” to refer to the softening of glasses and polymers, but by the one true definition of melting point, this is a different phenomenon entirely.
chornedsnorkack - 9-3-2024 at 09:17

 Quote: Originally posted by Texium What you are describing for silicon dioxide is glass transition, not melting. We colloquially use the word “melt” to refer to the softening of glasses and polymers, but by the one true definition of melting point, this is a different phenomenon entirely.

From the description of "glass transition" in your link, it appears to be the transition between glass and "viscous liquid", around 1015 centipoise annealing point.
Since the "melting point" of glass is defined at 10 000 centipoises, it apparently is the "transition" between liquid and "viscous liquid".
If transition between crystal and liquid is "melting" and transition between "viscous liquid" and liquid is also "melting", does it mean that transition between crystal and "viscous liquid" is not "melting"?

bnull - 9-3-2024 at 09:50

You're applying the definition of melting point as given for glass to silicon dioxide (silica). Although glass is mostly silica, glass and silica are not the same substance. Your link above is about glass.

And silica has a dozen or more forms that vary with temperature and pressure. While the transition temperatures from one form to another--and their respective melting points--vary with the structure, the boiling point is apparently unique and well defined (see attachment). Silica melts at about 1700°C and boils at about 2950°C. You can easily verify that by using an acetylene/oxygen torch (>3000°C) to melt and possibly boil a fragment of quartz.

Attachment: sio2 - mp, bp.pdf (148kB)
This file has been downloaded 58 times

chornedsnorkack - 9-3-2024 at 16:03

 Quote: Originally posted by bnull You're applying the definition of melting point as given for glass to silicon dioxide (silica). Although glass is mostly silica, glass and silica are not the same substance. Your link above is about glass.

Is "glass" a "substance", different from silica, or a "state of matter", or for the matter several states of matter, which can be reached by many substances, including and not limited to silica?

bnull - 11-3-2024 at 05:41

Melting point is the temperature where there is a sharp distinction between the physical states of the substance above (liquid) and below (solid) said temperature.

Consider water. Lower the temperature of water slowly. There will be a value below which the water is solid, and above which it is a liquid, never rubbery or deformable.

Now take glass. Is there a temperature that marks the limit between solid and liquid? No. What we have is a continuum from solid, hard glass, to fluid, liquid glass, as we raise the temperature. The use of the expression "melting point" for glass is incorrect from the scientific point of view. It is not a true melting point. Heat a glass rodNote 1 in the hot flame of a bunsen burner and watch what happens: it goes from rigid to deformable to liquid (if you heat it long enough). Can you say where it goes from solid directly to liquid? No. You have first a hard(ish) material, then it becomes softer and deformable, and finally liquid. In the case of the glass rod, it is rigid, then begins to sag, then starts dropping molten glass.

Let's say that melting point is "the temperature where the viscosity equals 10 000 centipoises". Is it a good physico-chemical definition?

Of course, not. Viscosity of water at 0 °C is very close to 1.8 mPa*s (1.8 cP), almost one five-hundredth of your figure. If your definition were valid for water, then Roald Amundsen would have described the sea during his 1911 expedition to the South Pole as if being made of molasses. Also, it is arbitrary and implies that all substances reach the same value viscosity before solidifying. What stops me from saying that the viscosity must be equal to 12 Pa*s (12000 cP) so we can say it is a melting pointNote 2?

I can offer you some options to investigate the matterNote 3. They're not mutually exclusive so you can try them all. (1) Measure the viscosity of many substances at and near the (let's call it) "IUPAC melting point" (from the latest CRC Handbook if possible). Monitor the temperature and viscosity very carefully. That's quite good a research for an amateur scientist (if you have the time and resources), and you can even share the experimental data with the community if you want to. (I could give you a link with the experimental data for thousands and thousands of substances, but this post belongs to Beginnings and I don't feel like it nowNote 4.) (2) Send an e-mail to the guys at IUPAC and repeat your question. IUPAC is like the Holy See of chemistry. If they can't answer it, we're all doomed. (3) Read the literature, specially the ISO/ASTM standards or whatever it is that is acceptable. And the theoretical papers, if you're so inclined. It is a good addition to your knowledge base.

Note 1: An old CD polycarbonate case may do, as long as the flame is not too hot. That's how I made a micro-funnel.

Note 2: ASTM International, of course.

Note 3: Because I'm an old fart and am giving up. I don't want to write a PhD thesis "On the Inadequacy and Misuse of Language in Physical Sciences" in a forum thread.

Note 4: Who am I kidding? Here's the link: https://www.cheric.org/research/kdb/.

DraconicAcid - 11-3-2024 at 07:06

Quote: Originally posted by chornedsnorkack
 Quote: Originally posted by bnull You're applying the definition of melting point as given for glass to silicon dioxide (silica). Although glass is mostly silica, glass and silica are not the same substance. Your link above is about glass.

Is "glass" a "substance", different from silica, or a "state of matter", or for the matter several states of matter, which can be reached by many substances, including and not limited to silica?

Depending on context, "glass" can refer to a specific group of polysilicates, or to a kind of non-crystalline solid. In this context, the latter.

chornedsnorkack - 11-3-2024 at 08:21

 Quote: Originally posted by bnull Now take glass. Is there a temperature that marks the limit between solid and liquid? No. What we have is a continuum from solid, hard glass, to fluid, liquid glass, as we raise the temperature.

Yes, in case of soda-lime glass.
A point I note about silicon dioxide is that like soda-lime glass, it has the glass transition from hard glass to viscous liquid... but unlike soda-lime glass, does not have a transition from viscous liquid to fluid liquid, because instead it has a direct transition from viscous liquid to gas.
 Quote: Originally posted by bnull Also, it is arbitrary and implies that all substances reach the same value viscosity before solidifying. What stops me from saying that the viscosity must be equal to 12 Pa*s (12000 cP) so we can say it is a melting pointNote 2?

Indeed. The viscosity at the point where crystal is at equilibrium with liquid varies over a huge range. It is 1,8 centipoises for water (at 0 degrees) and 2 000 000 000 centipoises for silica (at 1720 degrees). Which makes important differences.
 Quote: Originally posted by bnull I can offer you some options to investigate the matterNote 3. They're not mutually exclusive so you can try them all. (1) Measure the viscosity of many substances at and near the (let's call it) "IUPAC melting point" (from the latest CRC Handbook if possible). Monitor the temperature and viscosity very carefully. That's quite good a research for an amateur scientist (if you have the time and resources), and you can even share the experimental data with the community if you want to. (I could give you a link with the experimental data for thousands and thousands of substances, but this post belongs to Beginnings and I don't feel like it nowNote 4.) (3) Read the literature, And the theoretical papers, if you're so inclined. It is a good addition to your knowledge base.

Yes.
An example which I thought worth sharing:
A simple substance (and therefore resistant to composition changes over thermal history) is Se.
Which turns out to have a glass transition temperature... conveniently slightly above room temperature:
https://onlinelibrary.wiley.com/doi/abs/10.1002/pol.1963.110...

Much easier experimentally than soda-lime glass (glass transition around 550 degrees) or silica (glass transition about 1200 degrees).

SplendidAcylation - 25-3-2024 at 14:27

Off the top of my head, it is the temperature where the Gibbs free energy for the phase transition is zero... Am I close?
chornedsnorkack - 26-3-2024 at 01:28

 Quote: Originally posted by SplendidAcylation Off the top of my head, it is the temperature where the Gibbs free energy for the phase transition is zero... Am I close?

It is one way to express the first definition, yes.