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To melt or not to melt ?


Hal.

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

Change of state of matter is always a reversible process.

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

No, it could be a decomposition reaction or something.

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

as long as you prove the liquid material is actually the same substance it was before it was melted then you don't have to show that itis reversible because if it works one way then it will work the other.

 

if it isn't the same substance when it is in the liquid state then it isn't melting.

 

phase changes are always reversible

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

 

Change of state of matter is always a reversible process.

 

Every time I go to the shops I always come back , so it looks like a reversible process . There is no need to show that it is possible to return or that I actually have returned from the shop , in order to show that I went to the shop .

 

If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

 

No, it could be a decomposition reaction or something.

 

 

Maybe the wording could be changed to the following John as it is not the intention to deduce a conclusion of every instance .

 

If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , is it possible this show's melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

 

If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

 

 

 

as long as you prove the liquid material is actually the same substance it was before it was melted then you don't have to show that itis reversible because if it works one way then it will work the other.

 

if it isn't the same substance when it is in the liquid state then it isn't melting.

 

phase changes are always reversible

 

InsaneAlien , do you then think that it is possible to define melting independent of reversibility ? The logical question also follows of , do you also think it is possible to define fusion independent of reversibility ?

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InsaneAlien , do you then think that it is possible to define melting independent of reversibility ? The logical question also follows of , do you also think it is possible to define fusion independent of reversibility ?

 

I think you are getting the definition a bit backwards because of discussions that went on in the last thread.

 

reversibility is a logical conclusion of the fact that the material doesn't change at a molecular level (only intermolecular bonds are disrupted) so by taking energy out of the system, it should go back to its initial phase.

 

phase changes are defined independent of reversibility but reversibility is an inherent property of a phase change.

 

For instance, water can be defined independently of wetness but wetness is an inherent property of water (at least in its liquid state.)

 

in the last thread the only reason reversibility came into it was to show that the 'molten' wood was not actually wood but something else entirely which meant that it was a decomposition reaction rather than a phase change. I think this is where the confusion is coming from.

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I think you are getting the definition a bit backwards because of discussions that went on in the last thread.

 

 

I am not trying to define conventional melting , so the definition I am talking of is not the definition of which you think I am getting backwards . It is a new definition , a definition which is independent of reversibility , whether it is new or not is also open to debate . I had to make a new thread as discussions in the other did not , as far as I know , contain enough Chemists who are natural melters and freezers of substances .

 

reversibility is a logical conclusion of the fact that the material doesn't change at a molecular level (only intermolecular bonds are disrupted) so by taking energy out of the system, it should go back to its initial phase.

 

phase changes are defined independent of reversibility but reversibility is an inherent property of a phase change.

 

For instance, water can be defined independently of wetness but wetness is an inherent property of water (at least in its liquid state.)

 

 

All agreeable to me !

 

in the last thread the only reason reversibility came into it was to show that the 'molten' wood was not actually wood but something else entirely which meant that it was a decomposition reaction rather than a phase change. I think this is where the confusion is coming from.

 

 

I agree that this instance of a decomposition reaction is not a phase change , I never did say any decomposition reaction was a phase change . What I talk of is quite simple . An example may clear the minds of any confusion to make it clear what it is I talk of .

 

If a person has solid ice which undergoes a phase change to liquid water I contend that the ice has been melted without any need to show that this is reversible , thus making the definition of melting independent of reversibility . I also contend that liquid water which undergoes a phase change to solid ice has been frozen without any need to show that this is reversible , thus making the definition of freezing independent of reversibility .

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If a person has solid ice which undergoes a phase change to liquid water I contend that the ice has been melted without any need to show that this is reversible , thus making the definition of melting independent of reversibility . I also contend that liquid water which undergoes a phase change to solid ice has been frozen without any need to show that this is reversible , thus making the definition of freezing independent of reversibility .

 

I'm not sure what the issue is then, you don't need to show reversibility, you need to show the liquid is the same substance as the solid phase.

 

now, of course you don't need to do this every time, its commonly known that ice and water are the same substance just in different pahses, but for a new material you might have to.

 

also, freezing the material again to show that the change is reversible is just one of the ways of proving it is a phase change rather than a decomposition. it is not a necessary component. its just that if it is a phase change then the change can happen both ways as a result of thermodynamics. if it can't happen then the either the laws of thermodynamics are ridiculously wrong or it wasn't a phase change. and you know how well tested the laws of thermodynamics are.

 

The key point is that the substance remains the same at a molecular level. reversibility is just a result of that.

 

I think i'm putting the point across poorly.

 

Take gravity and orbits. If there is gravitational attraction then orbits are possible. you don't have to show there are possible orbits to show there is gravity, but this in noway affects the possibility of orbits.

 

like this, you don't have to prove reversibility in a phase change but its still going to be there as a result of the nature of phase changes.

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insane_alien ,

 

from what I read I would conclude that you would be of the opinion that reversibility is not a precondition for melting , but because reversibility is used conventionally as one means of confirming a phase change has originally taken place , without it another means of confirming the same material is in a different phase is required . The idea of melting , independent of any need to prove reversibility and it's opposite , namely fusion , independent of any need to prove reversibility , then becomes merely a problem of identification of the material before and after the phase change .

 

I do not suggest that a phase change ( melting ) can take place without the opposite phase change ( freezing ) being possible afterwards . That would be a completely different argument to my argument that melting can take place independent of the need to prove reversibility .

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This is going to degenerate into a discussion of the definition of melting.

There are basically two definitions.

The general one where any sort of change takes place (their eyes met across a crowded room and her heart melted) which is fine for bad literature,, but short on rigour.

The scientific one where a material changes phase without changing it's composition (and even that one can be less clear than it looks).

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John Cuthber ,

 

The definition of melting is the topic in the thread , the thread began with the discussion of the definition of melting in mind , that was it's desired theme , it can't therefore degenerate .

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That would be a completely different argument to my argument that melting can take place independent of the need to prove reversibility .

 

Oh, I didn't think you were really trying to argue that.

 

It seemed a bit too self evident if we start from the safe assumption 'melting exists'.

 

Of course melting can take place without you having to prove anything. You wouldn't refreeze some icecubes everytime to prove they melted.

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as long as you prove the liquid material is actually the same substance it was before it was melted then you don't have to show that itis reversible because if it works one way then it will work the other.

 

if it isn't the same substance when it is in the liquid state then it isn't melting.

 

phase changes are always reversible

 

Does that include nuclear fission? If that doesn't fall in the classification of phase change, then is this circuitous logic?

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Does that include nuclear fission? If that doesn't fall in the classification of phase change, then is this circuitous logic?

 

nuclear fission is a nuclear reaction. you are changing the substance. That cannot come under phase change (although the daughter products may be in a different phase from the parent) as the substance after is different from the substance before.

 

I don't see how this makes the logic circuitous

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I do not suggest that a phase change ( melting ) can take place without the opposite phase change ( freezing ) being possible afterwards . That would be a completely different argument to my argument that melting can take place independent of the need to prove reversibility .

So, you actually thought that all this time (in this thread, and the other one about melting wood) we all suggested that you must always (in every individual instance) prove that you can freeze the molten material?

 

We all know that it's pretty difficult to "prove" that an ice cube has molten when it was in your drink, and is now in your stomach.

 

So, no, if you talk about science, you do not have to prove every word you say there and then on the spot. It's alright if someone else has done the work for you, and you are confident that it has been proven sufficiently. Like in the case of water, we can all agree that it can melt and freeze, and that this is reversible. So, if the ice cubes in your drink melt, you can say they melt, without having to show for these individual ice cubes that they will freeze again. But if needed, there exists literature which can backup your remark. And with that literature, you can show that it's a reversible process, and not a decomposition reaction.

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nuclear fission is a nuclear reaction. you are changing the substance. That cannot come under phase change (although the daughter products may be in a different phase from the parent) as the substance after is different from the substance before.

 

I don't see how this makes the logic circuitous

 

Now that you point that out I understand. Simple, in retrospect.

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So, you actually thought that all this time (in this thread, and the other one about melting wood) we all suggested that you must always (in every individual instance) prove that you can freeze the molten material?

 

 

I did not think it was practical to prove all instances of melting by proving the resultant melted material could be refrozen . But , people do rely on a previous instance of refreezing with the same material to ascert something has been melted . I however did think that any definition of melting which is how I think of melting is independent of any ascertion refreezing could or actually did take place . I think likewise of fusion .

 

Plain and simply , if your definition of melting is dependent on reversibility it differs from my definition of melting which doesn't . If your definition of fusion depends on reversibility it differs from my definition of fusion which doesn't . I would not say , " it melted because it was refrozen to give the original material " , melting there being dependent on reversibility , I would say , " it melted because the material was the same material after the phase change " , melting there being independent of reversibility .

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Plain and simply , if your definition of melting is dependent on reversibility it differs from my definition of melting which doesn't . If your definition of fusion depends on reversibility it differs from my definition of fusion which doesn't . I would not say , " it melted because it was refrozen to give the original material " , melting there being dependent on reversibility , I would say , " it melted because the material was the same material after the phase change " , melting there being independent of reversibility .

So, could melting, according to you, be irreversible? That means that the opposite (freezing) does not require the exact opposite of enthalpy change?

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So, could melting, according to you, be irreversible? That means that the opposite (freezing) does not require the exact opposite of enthalpy change?

 

 

If a guess would be appropriate , yes , no , yes , no , yes , no , yes , no , yes , no , yes , no .......................................................

 

Maybe melting could be irreversible because of the limitations of technology , irreversible because of undiscovered means , irreversible because it can't humanly or naturally be done is something that hasn't been shown yet , has it ?

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If a guess would be appropriate , yes , no , yes , no , yes , no , yes , no , yes , no , yes , no .......................................................

 

Maybe melting could be irreversible because of the limitations of technology , irreversible because of undiscovered means , irreversible because it can't humanly or naturally be done is something that hasn't been shown yet , has it ?

Well... technology has enabled us to discover that it is in fact reversible. That is also why the enthalpy of melting is the same as the enthalpy of freezing (but with an opposite sign). And if there is an irreversible aspect to it, we don't call it melting.

 

That's why I asked you the question.

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CaptainPanic , any definition of melting independent of reversibility can't use observed irreversible aspects to show melting hasn't taken place , if it didn't . To do so would mean that the definition of melting independent of reversibility isn't independent of reversibility in practice . Other way's would have to be found .

 

 

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If a material undergoes a phase change from solid to liquid ( for example from ice to water ) , does this show melting has taken place , without having to show that the reverse phase change ( from liquid to ice ) is attainable ?

If the liquid has the same composite makeup as the solid then it would be a true phase change and it could be considered melting. For example, solid H2O is still H2O when it becomes a liquid. OTOH, if the resulting liquid has a new composite makeup then it would be a reaction, not melting.

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CaptainPanic , any definition of melting independent of reversibility can't use observed irreversible aspects to show melting hasn't taken place , if it didn't . To do so would mean that the definition of melting independent of reversibility isn't independent of reversibility in practice . Other way's would have to be found .

 

Yes, but we know from endless experiments that it is reversible. There is no irreversible aspect. If there is, please show me.

 

You just can't seem to give me a decent explanation why you refuse to have it in your definition of melting/freezing that the two are reversible, when they always are.

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CaptainPanic , I suppose that the way people have tried to teach me Science in the past is a way where , if I didn't understand the phenomenon in discussion through the example being given , then the example being given would be changed to hopefully give a better understanding of the phenomenon in discussion . I , CaptainPanic , think you need no further description of my view as you know exactly what I am talking about . Reversibility is not needed to show that melting has taken place .

 

I also think it is natural to think of melting without reversibility . When the ice cream on a cone melts and falls to the ground a kid doesn't stop to ponder the merits of reversibility when declaring , " my ice cream has melted " . Take a tennis ball and throw it from you to another person standing 20 metres away with one bounce as it goes , do you need to show the second parabola has happened after the first , when it is only the intention to show the first has happened , without taking into consideration that it is always taken as proven that all instances of the first parabola will be accompanied by another instance of a second parabola ?

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I had already agreed that (1) in language the concept of melting is quite stretched (and wrong), and (2) you do not have to always prove that something melts in order to be allowed to make the statement.

 

Anyway... I am curious. Tell me how you would determine whether this component melts or decomposes (I just picked something off the net, and I also do not know).

Let's assume you have just been put in charge to do the research to figure out what happens to this component if you heat it up... and our first observation is that it turns to a liquid.

Poly(2-Vinylpyridine)

Material Safety Data Sheet

 

Physical Appearance: White Powder

Boiling Point: Not Available

Critical Temperature: Not Available

Vapor Pressure: Not Available

Volatility: Not Available

 

Flammability of Product: May be combustible at high temperatures.

Auto-Ignition Temp.: Not Available

Flash Points: Not Available

Flammable Limits: Not Available

Products of Combustion: Not Available

Fire & Explosion Hazards in Presence of Various Substances: Not Available

 

Synonyms: PVP; Polyvinylpyridine

Formula: C4H9(C6H7N)nH

Reactivity: 0

Personal Protection: B

 

Odor: Odorless

Melting Point: Not Available

Specific Gravity: Not Available

Vapor Density: Not Available

Solubility: Soluble in Water

 

Section 3 (Fire & Explosion Hazards Data)

[...]

(I didn't copy paste all the other "Not Available".)

 

Point is, that this is a science forum, and we also deal with these things. Not only with the verbal communication between a child and its mother regarding an ice cream. Our scientific definition, which we discuss on this science forum will also have to be used for problematic things like this, when it's not child's play to make a correct statement.

Edited by CaptainPanic
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