When does dissolving occur?


by elementerystu
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elementerystu
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Dec18-07, 06:24 PM
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When does dissolving occur? it would help if you put in a diagram to s I'd understand more.
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DaveC426913
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Dec18-07, 09:58 PM
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At 80C, with a good whisk.

Perhaps you could be more specific.
ShawnD
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Dec19-07, 12:05 AM
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Quote Quote by DaveC426913 View Post
Perhaps you could be more specific.
Context would be nice. Is this a thermodynamics question?

Wikipedia puts it nicely: (I've removed some words so it's easier to understand)

"The solvation process will only be thermodynamically favored if the overall Gibbs energy of the solution is decreased [....].. This means that the change in enthalpy minus the change in entropy [....] is a negative value"


What the terms mean, sort of:
Gibbs Free Energy - Determines whether a reaction is spontaneous or not. Negative free energy means it will happen, positive means it will not happen.
Entropy - Randomness. More randomness means higher entropy. Solids have low entropy because the atoms are rigidly held in place. Solutes have higher entropy because they can freely move around in solution.
Enthalpy - You can probably think of enthalpy as being a kind of stored energy. When you burn something like gasoline and it releases heat, you would say the enthalpy is negative because the molecules are losing that heat energy. Boiling water would be positive enthalpy since energy is being added to the water; the water is storing that energy.


When you're talking about dissolving solids in water, most solids will lower the temperature of water, which means the enthalpy change is positive (positive enthalpy favors the reaction not happening). Given that enthalpy is usually positive, the only thing driving the reaction is the entropy change, which is usually (always?) positive (positive entropy favors the reaction happening). Dissolution happens when the entropy (randomness) change is greater than the enthalpy (stored energy) change. Given that entropy is temperature dependent, solubility is also temperature dependent.

mysqlpress
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#4
Dec24-07, 04:00 AM
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When does dissolving occur?


Gibbs free energy is the energetically dominant factor

Generally, enthalpy change of solution is okay preferable to decide whether the dissolution is favorable or not.
pheidippides
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#5
Dec28-07, 07:30 PM
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guessing by the name elementerystu, he/she is in elementary school, so maybe a simpler answer is desired.

well, anyway, lets take the example of salt and water. Salt and water are both polar substances (let's just say that means they are charged.) Because they are both the same (polar), this means that the salt can be dissolved by the water.

the same is true with a nonpolar and another nonpolar substance. A good example of this is that of acetone. Acetone is what is used as nail polish remover. If you put, say, plastic in the acetone, it will dissolve because both are nonpolar.

If you put plastic in water, obviously it would not dissolve, and this would be called a suspension, instead of a solution. another example of a suspension is mud or even dusty air.



Dissolving will always occur whenever the solute (thing getting dissolved) is in the presence of the solvent (the thing doing the dissolving.) However, this only happens to a point. If you dump too much salt in the water, the water will not be able to dissolve all the salt. This is called saturation. You can, however, put more salt in the water by raising the temperature of the water. As soon as the water cools, the extra salt will then fall out of solution and rest on the bottom of whatever container you used.

You can also dissolve a gas in a liquid (carbon dioxide in water, what you would call soda) or a solid in a solid, etc etc.

So to put this all in a diagram:

Solvent:....Solute:......Result:
Polar.........Nonpolar....suspension
nonpolar....nonpolar....solution
polar.........polar.........solution
water........oil............suspension
acetone....plastic.......solution
acetone....styrofoam...solution
acetone....salt...........suspension
water.......dirt............suspension

i could keep going, but i think thats what you were looking for. If i'm completely off...well then maybe you might want to make a post being more specific. if you're looking for some kinda graphic then just do a google search you should find something.
Gokul43201
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Dec28-07, 07:44 PM
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Quote Quote by pheidippides View Post
Solvent:....Solute:......Result:
Polar.........Nonpolar....suspension
nonpolar....nonpolar....solution
polar.........polar.........solution
water........oil............suspension
acetone....plastic.......solution
acetone....styrofoam...solution
acetone....salt...........suspension
water.......dirt............suspension

i could keep going, but i think thats what you were looking for.
Let's try to go a little further, shall we?

acetone......dirt........?
methanol.....salt........?
CaCO3.......water.......?
CaSO4.......water.......?


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