Ask A Scientist General Science Archive

Subject: NEWTON BBS  ---General 
name         Shannon
status       student
grade        9-12
location     IN

Question -   What is an example of a physical equilibrium containing 
two or more phases?
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Ice water at zero degrees Celsius.  Both solid and liquid phases are
present and in equilibrium.

Christopher Perkins
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Ice at 0 C. involves three phases -- solid, liquid, vapor. Water at 25 C.
involves two phases -- liquid and vapor. "Dry ice" frozen CO2 involves two
phases -- solid and vapor. A saturated solution of salt, ice, water, and
vapor involves two components and four phases -- solid NaCl, ice, water,
and vapor.

Vince Calder
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Crushed ice in water, in a perfect "Thermos" bottle an hour after mixing,
is a physical equilibrium containing two phases.
One phase is liquid water, the other is solid water.
Neither phase is moving, neither is changing into the other, and
both phases are at the same temperature, 0 degrees C.

The water was originally warmer (20 C, and the ice was originally cooler 
(5 C).
The water warmed the ice from -5 C to 0 C, then continued applying heat, 
because the water was still +10 C.
Then the ice absorbed heat by melting, but didn't increase in temperature 
any more.
As ice melted the percentage of the bottle's contents in the solid phase 
diminished some, and the liquid phase increased.
Eventually all the water was very close to 0 degrees C as well,
so everything was the same temperature, so heat stopped flowing.

No more heat flow, and no more transfer from solid phase to liquid phase 
or v/v.
That is an equilibrium condition.
If heat did not leak through the walls of the bottle, it would stay ice 
and water forever.
The percentages would stay the same too.

If the bottle was evacuated if air and sealed shut,
some water and ice would evaporate, and water vapor would be the only gas 
present.
This water vapor would be a 3rd phase, a gas phase.
While the water was warmer than 0 C, vapor would be evaporating faster 
from the water than from the ice,
and in fact it would be re-condensing on the colder places, the tops of 
floating ice.
This can be a significant path for heat flow.   It is what happens in 
"heat-pipes".
But eventually it, too, stops.
Then you would have 3 phases: ice and water and about 1/100 atmosphere of 
"steam",
together in equilibrium, unchanging forever.

Oil and water mixed together and sealed in a jar might be considered two 
phases in equilibrium, too.
A very small percentage of the oil wants to dissolve in the water,
and a different very small percentage of the water wants to dissolve in 
the oil.
Give the jar 10 seconds of good shaking, and all this dissolving is done.
The liquids settle into separated top and bottom layers,
and after that they are pretty much in equilibrium.
(Well, in hundreds or millions of years perhaps there would be a chemical 
reaction
between them, but we will neglect that for now.)

If you put this jar in the sun and it starts growing algae or mold inside,
that is a chemical non-equilibrium situation.
There is an on-going chemical reaction changing the contents.
It is important be aware of any injected energy (such as sunlight),
if you think you are trying to achieve or make use of an equilibrium.
But then, you specified "physical" equilibrium, which limits our thinking 
to thermal and mechanical forces.

If the jar is sterilized, and the water is made dark with food-coloring, 
and the oil is almost as heavy as water,
Then it is possible that this jar in the sunlight would act like a 
"Lava-Lamp".
More of sunlight's energy would go into the water than the oil,
and the warmer water might expand and float upwards in globs, then cool 
and fall, repeatedly.
The continuing physical action would be a visible sign
that there is a continuing non-equilibrium condition inside the jar,
in this case a temperature difference between the oil and water.

Half-melted solder in a thermally insulated bottle can be a three-phase 
equilibrium, too.
The liquid is all the same composition, a 3:1 mix of atoms, so it is only 
one phase.
but solid solder is actually a mix of two phases: tiny crystals of "mostly 
tin" and "mostly lead".
At the eutectic ("lowest-melting-mix") temperature all three phases can 
coexist stably.

Jim Swenson
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