Understanding Phase Change Hysteresis: A Comprehensive Guide

[turin]

Does phase change have hysteresis?

 

[Janitor]

Do you have a specific example you are thinking of? Are you talking about the quantum regime and wavefunction phases? Or the macroscopic regime and some other kind of wave?

 

[turin]

Not phase of a wave. I'm talking phases of matter (solid, liquid, gas).

They learn at the freshman level of physics that the latent heat of vaporization is the amount of heat that you have to give the system to change it from liquid to gas or the amount of heat that you have to take away from the system to change it from gas to liquid. Phase change is basically declared a reversible process. What I'm wondering is whether there is any hysteresis WRT temperature, pressure, or time. As an example, if I have some solid H₂O at 0^oC and 1 atm, and I add 80 cal of heat to it, then I should get 1 g of liquid H₂O. The temperature and pressure are supposed to stay at 0^oC and 1 atm, but frankly, I'm not so sure I understand what that means. The process seems rather dynamic, and I only really think I understand pressure and temperature as it pertains to a static system (and even then not very well).

I would be really surprised if there were not hysteresis WRT time. I'm not so sure what to think about the properties of temperature and pressure.

 

[Janitor]

Sorry--I totally misinterpreted your question.

I was thinking of "phase shift."

 

[clint]

Taking H2O from solid to liquid at a fixed pressure takes heat, its Latent heat. Say you have a block of ice at -10C and at 1 atmosphere of pressure. You then hook up a heater to the block, and keeping the pressure constant, then you turn on the heater at a moderate but constant heating rate. The temperature of the ice will rise up to around 0C (depending on how pure the water is), and the temperature will stay at 0C for a while, then only when all is turned to liquid will the temperature increase again.

During the time the temperature is constant, the heater is supplying the increase in entropy needed in going from the ordered solid (with low entropy), to the disordered liquid (with higher entropy). If you turn off the heater too soon, the phase transition is incomplete, and there is hystersis (meaning that the state of the matter depends on its history, e.g. how long the heater was on for etc.). If the heater setting is too high, there isn't enough time for the entropy change to occur at the transition, and although the temperature of the ice/liquid is over 0C, the transition is not complete and there is hysteresis (in this case one has a "metastable" state of matter which eventually decay and revert to normal states of matter, eg all liquid).

In any case if one keeps the heater on long enough, the transition is complete and one has a liquid with no memory. One can reverse the cycle and take away heat, get back the solid, then heat again etc, and both the transition temperature and Latent heat be the same at each crossing.

One can then repeat the above experiment at a different constant pressure. If one keeps the pressure at say 5 atms the whole time, the ice to liquid transition temperature will be lower than 0C, and the Latent heat will be different, but the same arguments above apply.

 

[turin]

That was almost exactly what I was looking for, clint. Thanks. I still have one little bit of confusion, though:

You say to keep the pressure constant. I'm not sure I understand what pressure you're talking about in a solid. At least, how would I practically determine the pressure? I guess I can kind of think of the solid in terms of layers that are all pushing on each other, but I can't see any reason to think that this would be uniform throughout the material. I guess I also have a similar confusion concerning the liquid state. Is the pressure defined on the boundary of the material?

 

[clint]

Turin, hope this helps , clint

Say one chooses to do an experiment across a liquid/solid phase transition, with constant pressure consitions. One starts with the material in its high temperature liquid state which is sealed inside a closed high pressure cylinder with a piston at one end (or both ends, doesn't matter). An external force pushes the piston in. The pressure of the liquid is uniform.

Then as one cools the liquid, one eventually encounters a phase transition into the solid, which is a first order phase transition, with a volume change. For constant pressure conditions, one either has to push in the piston (as for most liquids), or retract it (like water) in order to keep the pressure constant (and not the volume).

One complication that you were leading to, is that once the solid has formed, if one then wants to increase the pressure of the solid, this will lead to nonuniform pressure inside the solid. The experimental output data will then reflect this, and be distributed; this is not desirable.

 

[turin]

... this will lead to nonuniform pressure inside the solid. The experimental output data will then reflect this, ...

How will the data reflect this?