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#1
Old 04-02-2015, 08:11 PM
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How does water dissolve salt and why does it stop?

What is the mechanism of saturation? What is happening at the molecular level that causes the water to lose the ability to pull any more salt apart into its ions?

My wild ass guess was that the ions are binding to individual water molecules and when every molecule has an ion attached, it's game over, but that can't be true because raising the temperature allows the water to dissolve more salt even though you haven't added more water molecules.... so how does that work?
#2
Old 04-02-2015, 08:34 PM
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When added to water, soluble ionic compounds like sodium chloride tend to, well, dissolve. The random motion of water molecules causes them to collide with the sodium and chlorine ions in the salt, knocking them free.

At the same time, sodium and chlorine ions in the water are finding each other and bonding. With low amounts of Na+ and Cl- present in the water, they'll break apart again before you ever notice they're present (but they are there).

As more and more salt is added, eventually the water gets so crowded that there's just not enough water molecules to efficiently knock sodium and chlorine apart, and it'll precipitate noticeably to the bottom of the container. If you add energy to the water, though, the molecules can move faster and can disrupt more bonds per unit of time. (As you note, raising the temperature increases solubility.)

These reactions can be described by an equilibrium constant, a value (dependent on temperature) which is derived from the concentrations of each species and the chemical reaction equation. For sodium chloride: NaCl(aq) -> Na+(aq) + Cl-(aq)
(sodium chloride separates into positive sodium ions and negative chlorine ions). The solubility constant is given by the concentrations of the aqueous ions (ignoring solids or liquids) so it's just Ksp = [Na+] * [Cl-] (where the square brackets are a notation indicating the concentration of the substance in question -- and it's a very high number (~36) because room temperature water can hold about 6 moles of NaCl per liter.

Once the system has reached equilibrium with the concentrations described above, it's in "dynamic equilibrium." The salt is still constantly dissolving and precipitating, but in equal amounts to maintain similar proportions.

Last edited by Baffle; 04-02-2015 at 08:38 PM. Reason: oops... NaCl is a solid.
#3
Old 04-02-2015, 09:44 PM
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To expand on your answer, what physically is happening is that due to the nature of water, which is shaped like a V, the molecule has a slight negative charge towards the oxygen (the point of the V) and a slight positive charge opposite of that - (sort of in the empty space in the V). The oxygen is actually tugging on the electrons a little stronger than the hydrogens, and that creates a permanent charge imbalance in the molecule, which is called a dipole. You can think of this working on the same principle as a magnet - the negative end of water is attracted to sodium ions, and surround them, while the positive end of water is attracted to chloride ions, and surround them, forming a kind of weak bond.
#4
Old 04-02-2015, 11:32 PM
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I knew temperature was part of the deal, I just couldn't see the relationship.

So the way the formula reads for me is that the ability of water to pull salt apart is determined by how many water molecules you have and how fast they're moving.

Sort of like the freeways here in So Cal. The faster you go, the more things break when you crash them together.

Thanks!
#5
Old 04-03-2015, 01:41 AM
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A sodium ion can be surrounded by 6 water molecules in a layer, or shell. Another layer can form after that. When NaCl is introduced into water it is ripped apart and the Na+ surrounded by the water molecules a few layers (at least one) deep. The Cl- sits outside this cluster.

As you increase the temperature (speed of collisions) you give the water more of an opportunity to pull apart the Na from the Cl.

Once you have less than 6 water molecules per NaCl, the NaCl finds it easier (less energetic) just to hang out by itself.
#6
Old 04-03-2015, 01:43 AM
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Quote:
Originally Posted by antechinus View Post
A sodium ion can be surrounded by 6 water molecules in a layer, or shell. Another layer can form after that. When NaCl is introduced into water it is ripped apart and the Na+ surrounded by the water molecules a few layers (at least one) deep. The Cl- sits outside this cluster.

As you increase the temperature (speed of collisions) you give the water more of an opportunity to pull apart the Na from the Cl.

Once you have less than 6 water molecules per NaCl, the NaCl finds it easier (less energetic) just to hang out by itself.
It should be more than 6. Water is 55 mol/L, and each liter can dissolve 6 mol of salt; so by that ratio it needs closer to 10 water molecules per sodium ion.
#7
Old 04-03-2015, 02:11 AM
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Per sodium or per NaCl?
#8
Old 04-03-2015, 08:54 AM
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http://en.m.wikipedia.org/wiki/Dissolution_(chemistry)

When anything dissolves in any other thing, we have to consider the favorable interactions in the undissolved substance, the favorable interactions among the molecules of solvent, the interactions between the solvent and the solute, and the entropy of all the different states in question.
#9
Old 04-05-2015, 08:36 AM
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Another cause of saturation, or rather another way to look at it, is that the salt is precipitating at the same rate that is is dissolving...

This idea lets you understand supersaturation, where its precipitating FASTER than its dissolving..
or would if it could..

Last edited by Isilder; 04-05-2015 at 08:36 AM.
#10
Old 04-05-2015, 10:37 PM
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Whether or not it matters, NaCl solubility in water doesn't increase much as the temperature goes up. In grams/dL, salt's solubility is 35.7 g/dL at 0 C and 38.99 g/dL at 100 C. Sucrose, for comparison, is 181.9 g/dL at 0 C and 476.0 g/dL at 100 C.

Tangential question I'm too lazy to look up- how does dissolution rate of NaCl fare as temperature of the water goes up? Will the 38.99 g dissolve a lot faster in 100 C water than the 35.7 g in the 0 C water?
#11
Old 04-06-2015, 01:31 AM
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How about those salts that have decreasing solubility as the temperature is increased?

Also, bear in mind that the equilibrium when a salt dissolves is that not all of it is dissociated into positive and negative ions. Depending on the atomic charge, ionic strength and other factors some of the dissolving salt will remain as undissociated molecules. Theis is the basis for the Debye-Huckel limiting law and the definition (IIRC) of the activity coefficient. In fact, some salts, like lead acetate, exist as mostly undissociated molecules in solution.

The point is that dissolving and diassociation are not necessarily the same.

Last edited by Waterman; 04-06-2015 at 01:32 AM. Reason: typo
#12
Old 04-06-2015, 08:07 AM
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Quote:
Originally Posted by california jobcase View Post
Whether or not it matters, NaCl solubility in water doesn't increase much as the temperature goes up. In grams/dL, salt's solubility is 35.7 g/dL at 0 C and 38.99 g/dL at 100 C. Sucrose, for comparison, is 181.9 g/dL at 0 C and 476.0 g/dL at 100 C.

Tangential question I'm too lazy to look up- how does dissolution rate of NaCl fare as temperature of the water goes up? Will the 38.99 g dissolve a lot faster in 100 C water than the 35.7 g in the 0 C water?
The wiki link I posted earlier addressees this. It will dissolve faster.

Quote:
Originally Posted by Waterman View Post
How about those salts that have decreasing solubility as the temperature is increased?
You typically see this when the dissolution or hydration is highly exothermic, e.g. calcium sulfate. Le Châtelier's principle is a good first order approximation.
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