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Tom Mazanec
2015-Mar-09, 01:46 PM
My WAG about how matter changes its state at different temperatures would be that these would be gradual. Solids would soften to clay consistency, to tar consistency, to honey consistency, to water consistency.
Liquids would just evaporate faster at higher temperatures.
You would not have ice at minus one Celsius and water at plus one, or water at 99 and steam vapor at 101.
What am I missing?

Swift
2015-Mar-09, 02:05 PM
My WAG about how matter changes its state at different temperatures would be that these would be gradual. Solids would soften to clay consistency, to tar consistency, to honey consistency, to water consistency.
Liquids would just evaporate faster at higher temperatures.
You would not have ice at minus one Celsius and water at plus one, or water at 99 and steam vapor at 101.
What am I missing?
Not all phase transitions are as abrupt as you describe. From the wikipedia article (http://en.wikipedia.org/wiki/Phase_transition)


First-order phase transitions are those that involve a latent heat. During such a transition, a system either absorbs or releases a fixed (and typically large) amount of energy per volume. During this process, the temperature of the system will stay constant as heat is added: the system is in a "mixed-phase regime" in which some parts of the system have completed the transition and others have not. Familiar examples are the melting of ice or the boiling of water (the water does not instantly turn into vapor, but forms a turbulent mixture of liquid water and vapor bubbles). Imry and Wortis showed that quenched disorder can broaden a first-order transition in that the transformation is completed over a finite range of temperatures, but phenomena like supercooling and superheating survive and hysteresis is observed on thermal cycling.[3][4][5]

Second-order phase transitions are also called continuous phase transitions. They are characterized by a divergent susceptibility, an infinite correlation length, and a power-law decay of correlations near criticality. Examples of second-order phase transitions are the ferromagnetic transition, superconducting transition (for a Type-I superconductor the phase transition is second-order at zero external field and for a Type-II superconductor the phase transition is second-order for both normal state-mixed state and mixed state-superconducting state transitions) and the superfluid transition.

JohnD
2015-Mar-09, 07:54 PM
The explanation for boiling is that the molecules have to be moving fast enough (=are hot enough) to over come surface tension.
That is a fixed amount - well amost! Water's goes from 72 dynes at room to about 60 at boiling, but it's still a certain number.

Is that adequate?
JOhn

JohnD
2015-Mar-10, 02:18 PM
It must be! No one has contradicted me!

JOhn

malaidas
2015-Mar-10, 02:41 PM
I don't know if you are right or wrong, asserting you are because no-one has contradicted you is non sequitur though.

grant hutchison
2015-Mar-10, 07:50 PM
It must be! No one has contradicted me!If your explanation was correct, boiling point would be independent of ambient pressure.

Grant Hutchison

Squink
2015-Mar-10, 09:04 PM
When lyophilizing, the hurried grad student will often pump down his watery sample and boil it til it freezes.

profloater
2015-Mar-10, 10:29 PM
what a good question! it seems to depend on nucleation of a change. Many systems go supercritical. It seems purity encourages a "late" change of state and tiny inpurities which are normallt present, allow the sudden change we expect. Boiling water is normally limited by energy transfer rate but when there is plenty of energy, it becomes explosive. However very pure water will stay liquid above BP and then flash over when nucleated. Same with freezing. The normal state of affairs is to have impurities, you have to be abnormally careful to get purity and then the system breaks the "rules"

John Mendenhall
2015-Mar-10, 10:47 PM
Various phase changes? Those above, and supercritical fluids. And supercooled fluids. And eutectic mixes. And glasses. And brines. What about oils and greases and sll the rest of the organic zoo? Some fast and sharp, some not, a pointed out above.

JohnD
2015-Mar-12, 09:50 AM
Grant offers a cogent riposte!
But if I rephrase and say "intermolecular force", of which surface tension is but an expression, then is that an adequate reply?
John

PS Malaidas, irony is my downfall

grant hutchison
2015-Mar-12, 09:10 PM
Grant offers a cogent riposte!
But if I rephrase and say "intermolecular force", of which surface tension is but an expression, then is that an adequate reply?I don't think it moves you much further forward. The intermolecular forces determine the energy required for molecules to leave the liquid phase. But then they form a vapour phase, which is in equilibrium with the liquid phase when the rate of exit from the liquid matches the rate of reentry to the liquid. That's the saturated vapour pressure (SVP). As the liquid gets hotter, the rate of exit from the liquid phase gets higher, so the SVP needs to be correspondingly higher to push molecules back into the liquid at the same rate they're leaving. When SVP exceeds ambient pressure in an open vessel, it becomes impossible to maintain enough vapour pressure over the liquid to maintain the equilibrium situation. That's the boiling point, and it depends on the ambient pressure over the liquid.
So we see that what looks like an "abrupt phase change" (boiling) is actually just a point on a continuous, smooth process of phase equilibrium. The abrupt change involves the crossing of a threshold pressure in an open vessel, but we wouldn't see that in a closed vessel, when the pressure would continue to rise to maintain liquid/vapour equilibrium.

Grant Hutchison

profloater
2015-Mar-12, 10:16 PM
But the point is phase change involves an energy transfer and if that transfer is restricted the change is slowed down exactly to the available energy rate.

grant hutchison
2015-Mar-12, 11:37 PM
But the point is phase change involves an energy transfer and if that transfer is restricted the change is slowed down exactly to the available energy rate.But the original question isn't about "suddenness" in terms of time, but in terms of temperature transitions. Tom wants to know why we have "water at 99 and steam vapor at 101". Whereas actually we have both at both temperatures, and all the way to the critical point, provided we contain the mixture in a closed vessel.

Grant Hutchison

ShinAce
2015-Mar-13, 12:36 AM
One explanation for the suddenness is 'Boltzmann suppression'.

Consider the energy difference between liquid water at 100C and water vapor at 100C. The Boltzmann factor guarantees that the transition between the two will be quick (over a small tempretature range). This is due to it being an exponential.

The same thing happens in ionization. The vast majority of hydrogen at the sun's surface is not ionized. The smallness of the fraction of ionized hydrogen is also due to Boltzmann suppression. Double the sun's surface temp from 5000K to 10000K and you'll have on the order of 7 million times more ionized hydrogen. You can consider the ionization of hydrogen like the boiling of water. In this case, the ionization 'boils' at 157 000K.

A factor of 7 million 'more' for doubling the temp is amazing! If it were water, that means the rate at which a glass of water sitting around will evaporate away is strongly dependent on temperature. You can try this at home if you have a cold room. A glass of water will last longer in a cold room than a warm room by a large margin.

With boiling water, once the water has the energy it gets from being at 100C, the boltzmann suppression essentially disappears and the water is suddenly(or quickly) able to boil.

profloater
2015-Mar-13, 09:50 AM
But a vessel of water at 100 never spontaneously turns into steam, whereas small droplets of water on a very hot surface do flash to steam. The so called total heat of steam at 100C is much greater than the total heat of water at 100C and the difference is the latent heat value, which as we know is much greater than all the energy involved in bringing water from 0 to 100. A vessel of water cannot receive that latent heat quickly enough but a droplet on a very hot surface can. Wind across the surface of a lake carries away molecules with just enough energy to escape and by carrying away the latent heat the whole lake is cooled. The cooling is not conduction into the air it is the loss of latent heat from the surface. The water example is useful because the energy of transition of phase is so high.

JohnD
2015-Mar-14, 09:40 PM
Grant,
further thought (?) returns me to my previous argument.
As you point out, boiling point depends on ambient pressure, and on Partial Pressure.
The molecules of liquid water will have a range of energies. Even at room temperature, some will have enough to make the phase transition into vapour.
As the water heats, more molecules will have that energy, to break the intermolecular force, and will do so, leaving their cooler, sluggardly siblings behind, and removing their higher energy selves from the liquid.
THAT's the phase transition, the explanation of the Latent Heat of Vapourisation, and the school room experiment that water boils at a constant temperture.

Ditto Latent Heat of Crystallisation. The IMFs ina crystal are stronger than those in liquid.
Can I extend this to sublimation?
Not with a few more glasses.

John