# Is it possible to evaporate water from mixture withot raising the temperature of the

other components?

Hello,
Lets say I have a glass of water with sugar or salt.
Is it possible to somehow turn the water into a vapor without inputing energy into the other materials in the mixture?

A microwave affects only water molecules, but when you raise their temperature- then obviously the heat treansfers into the the rest of the food.
I thought, that maybe if I somehow raise the tempeature rapidly the water will vaporize before any heat transferrence could occur... or is it just hogwash, since the energy to break molecuar bonds between the water and the material it is released from is constant and need to affect both the water and the other molecules?

Thank you very much.

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Drakkith
Staff Emeritus

There is no way to heat ONLY the water and avoid heating the other materials for the reason you said already.

Spinnor
Gold Member

Take a bottle of sugar water at room temp. and cap it and then reduce the pressure. Reduce the pressure enough and the water will boil away leaving the sugar behind.

DaveC426913
Gold Member

Or don't raise the temp at all. Blow air across it, increasing evaporation rate.

Drakkith
Staff Emeritus

Oh, I thought the OP already knew about that and was asking for another way lol. I think I got caught up on the heating part.

Thank you,

Oh, I thought the OP already knew about that and was asking for another way lol. I think I got caught up on the heating part
Lol, actualy I did know that though kinda forgot about it...
And it also did help me, though indeed I too was caught up with the heating in my original question...

Or don't raise the temp at all. Blow air across it, increasing evaporation rate
But that would affect not only the water molecules? Wouldn't it?

DaveC426913
Gold Member

But that would affect not only the water molecules? Wouldn't it?
Depends on what else would evaporate. Salt would not. Sugar would not.

If you leave a plate of these solutions out on a counter, the water will evaporate, leaving the precipitates behind.

Hmm...
(Im a bit ashamed for asking such rudimentary questions...:P)
But isn't evaporation is about transferring energy from the air to the substance?
Thus heating salt as well?...

Hmm...
(Im a bit ashamed for asking such rudimentary questions...:P)
But isn't evaporation is about transferring energy from the air to the substance?

No the energy transfer is not from the air to iquid but between individual molecules of the liquid. Molecules keep colliding with each other. Sometimes at the surface, some molecule collisions give a lot of energy to some particular particles which, if moving in the right direction, can escape the forces that are holding the liquid together. So the total energy of the liquid decreases as a result of evaporation. This is why sweat helps to cool your body.

Sorry, should have explained myself better.

I meant that the mixture as a whole is pretty homogenous, so all of the water molecules are equaly binded with the molecules from the other material.
Thus processes that affect the water also will affect the other material just as much.
So the particles on the surface with enough energy to break bonds will be a) of both kinds, b)tied to the water molecules which escape.

1) I'm not sure that salt per se is actually tied to water, but rather is engulfed by its molecules... but I do know salt water is used as an air dessicant, so evaporation must differ than in regular water...

2) Sure it's the high energized particles that escapes, but isn't the initial energy from the air transfers to the entire fluid (surface), even if only one type of particles utilizes it for freedom?

3) Umm... Is breakage of molecular bonds possible when only one side has the energy required? Shouldn't some enrgy transsfer the other molecules?..

thank you.

DaveC426913
Gold Member

Sorry, should have explained myself better.

I meant that the mixture as a whole is pretty homogenous, so all of the water molecules are equaly binded with the molecules from the other material.
Thus processes that affect the water also will affect the other material just as much.
So the particles on the surface with enough energy to break bonds will be a) of both kinds, b)tied to the water molecules which escape.

1) I'm not sure that salt per se is actually tied to water, but rather is engulfed by its molecules... but I do know salt water is used as an air dessicant, so evaporation must differ than in regular water...

2) Sure it's the high energized particles that escapes, but isn't the initial energy from the air transfers to the entire fluid (surface), even if only one type of particles utilizes it for freedom?

3) Umm... Is breakage of molecular bonds possible when only one side has the energy required? Shouldn't some enrgy transsfer the other molecules?..

thank you.
They don't bind to water in that sense. When in water, salt and sugar are in solution.
http://en.wikipedia.org/wiki/Solution#Solid

Regardless, they will not evaporate; they do not normally have a gaseous state.

russ_watters
Mentor

Since the dissolved salt is already at room temperature, the only heat transfer from it is a small, but transient heat transfer out of it at the surface caused by the evaporation of the water around it. But once it reaches an equilibrium temp, that heat transfer stops while the evaporation continues.

This
all of the water molecules are equaly binded with the molecules from the other material.
I meant that the mixture as a whole is pretty homogenous...
Homogeneity is condition where the concentration of solute is the same everywhere in the solvent.

Also molecules aren't equally bonded with each other. For a solute to dissolve in a solvent, the adhesive forces must be more than the cohesive forces i.e. the solute-solvent attraction should be greater than solute-solute or solvent-solvent attraction.

Thus processes that affect the water also will affect the other material just as much.
So the particles on the surface with enough energy to break bonds will be a) of both kinds, b)tied to the water molecules which escape.
Yes right, but the ones that escape are only the solvent molecules and not the solute because they are not held by the forces of attraction from other solvent molecules. Although the solute apples and attractive force, solvent is in much larger quantity and hence net attraction on solvent molecules is less.

2) Sure it's the high energized particles that escapes, but isn't the initial energy from the air transfers to the entire fluid (surface), even if only one type of particles utilizes it for freedom?
As I already said, the energy transfer is between the molecules in the solution itself and not air.

3) Umm... Is breakage of molecular bonds possible when only one side has the energy required? Shouldn't some enrgy transsfer the other molecules?..
Sorry couldn't understand what you said.