What exactly happens during a phase change?

[Iwanttolearnphysics]

TL;DR Summary

I want to be able to clear all my misunderstandings about phase changes.

• What will happen if say, I have both ice and water inside the refrigerator and I set the fridge to exactly 0 degrees Celsius?
• Since both melting and freezing happen at 0 degrees, what will happen to my water? what about to my ice? Will they stay the same?
• I know this is an ignorant question, but I can't for the life of me, understand how these two things can happen at the same time.
• I've seen videos of the triple point of water but I don't understand what's happening at the molecular level.
• In the triple point of water, where ice, water vapor and water all coexist, what happens in terms of PE? KE should not change right? because temperature is the same during a phase change.
• I will probably have more questions later on.

Thank you.

 

[Borek]

Think in terms of a dynamic equilibrium - if you have a mixture of ice and water at 0°C, all of the time both freezing and melting take place: some of the ice melts but simultaneously exactly the same amount of water freezes. So the composition of the mixture (amount of ice/amount of water) is constant.

Same for triple point, just three phases and six simultaneous processes.

 

[PeroK]

Thermodynamics is fundamentally statistical. If you say the air has a temperature of 0°C, then that determines the average kinetic energy of the air molecules. The interaction (collisions) between the air and a block of ice may be such that microscopic melting and re-freezing is happening with approximately equal frequency. In theory, therefore, an equilibrium could be achieved, where the ice remains on the verge of melting indefinitely. Likewise, the water may remain on the verge of freezing indefinitely. Or, some intermediate mixture of half-ice, half-water may remain in that equilibrium.

In any case, ice melting must become an indefinitely slower process as the air temperature reduces to 0°C. Whatever happens will happen very slowly.

 

[Iwanttolearnphysics]

Think in terms of a dynamic equilibrium - if you have a mixture of ice and water at 0°C, all of the time both freezing and melting take place: some of the ice melts but simultaneously exactly the same amount of water freezes. So the composition of the mixture (amount of ice/amount of water) is constant.

Same for triple point, just three phases and six simultaneous processes.

Okay, so I can think of it like how chemical equations have the little reversible arrows? That clears it up for me. Thanks!

 

[Iwanttolearnphysics]

Thermodynamics is fundamentally statistical. If you say the air has a temperature of 0°C, then that determines the average kinetic energy of the air molecules. The interaction (collisions) between the air and a block of ice may be such that microscopic melting and re-freezing is happening with approximately equal frequency. In theory, therefore, an equilibrium could be achieved, where the ice remains on the verge of melting indefinitely. Likewise, the water may remain on the verge of freezing indefinitely. Or, some intermediate mixture of half-ice, half-water may remain in that equilibrium.

In any case, ice melting must become an indefinitely slower process as the air temperature reduces to 0°C. Whatever happens will happen very slowly.

Okay. Thank you! I don't think I have any more questions then. Someone mentioned I should read more about latent heat and I think that cleared up a lot of my misconceptions about "heat" in general.

 

[DaveE]

Okay. Thank you! I don't think I have any more questions then. Someone mentioned I should read more about latent heat and I think that cleared up a lot of my misconceptions about "heat" in general.

That's funny. I deleted my latent heat comment because I thought it wasn't that relevant to your original question. It explains why only some of the ice melts, in an average, thermodynamic sense. There may not be enough energy to melt all of the ice. But doesn't really address what happens at each molecule.

 

[Borek]

Okay, so I can think of it like how chemical equations have the little reversible arrows?

Yes, plus remember it nicely combines with the latent heat part - heat produced by freezing is consumed by melting, so the net energy change is zero.

 

[russ_watters]

That's funny. I deleted my latent heat comment because I thought it wasn't that relevant to your original question. It explains why only some of the ice melts, in an average, thermodynamic sense. There may not be enough energy to melt all of the ice. But doesn't really address what happens at each molecule.

In my opinion, energy (latent heat) is entirely the issue here. Melting requires adding heat, freezing requires taking it away. If the ice, water and freezer are all at exactly 0C there is no heat transfer and nothing happens. That's it.

IMO talking about the statistics of simultaneous melting and freezing at the molecular level is not helpful for this scenario. (Noting that the title question and OP content don't really match and there are extra concepts tossed in).

 

[Iwanttolearnphysics]

Yes, plus remember it nicely combines with the latent heat part - heat produced by freezing is consumed by melting, so the net energy change is zero.

Okay. Since posting this I've been reading about thermodynamics and I think I kind of get it now. Of course, I don't completely get everything since I've just started studying physics. Thank you very much!

 

[Iwanttolearnphysics]

That's funny. I deleted my latent heat comment because I thought it wasn't that relevant to your original question. It explains why only some of the ice melts, in an average, thermodynamic sense. There may not be enough energy to melt all of the ice. But doesn't really address what happens at each molecule.

No, it helped me understand what's happening better. Thank you for pointing me to the right direction.

 

[Iwanttolearnphysics]

In my opinion, energy (latent heat) is entirely the issue here. Melting requires adding heat, freezing requires taking it away. If the ice, water and freezer are all at exactly 0C there is no heat transfer and nothing happens. That's it.

IMO talking about the statistics of simultaneous melting and freezing at the molecular level is not helpful for this scenario. (Noting that the title question and OP content don't really match and there are extra concepts tossed in).

Yeah, I wasn't really sure how to ask my question because there's so many gaps in my knowledge, I didn't know how to ask what I wanted to find out. Thank you everyone for giving me things to read. I was busy doing just that after reading your replies.