Dissolving stuff

[patchbunny]

If you read the rest of this thread, let me ask you - why do you think it doesn't?

I figured the ionic state kept the sodium ion from somehow stripping hydrogen from the water molecules, but I hadn't a clue how (chemistry, as I said, was not my best subject). Evilgiraffe's post reveals more steps than I had thought present.

 

[Evilgiraffe]

I figured the ionic state kept the sodium ion from somehow stripping hydrogen from the water molecules, but I hadn't a clue how (chemistry, as I said, was not my best subject). Evilgiraffe's post reveals more steps than I had thought present.

Evilgiraffe has realized that he has screwed up the charges somewhere between step 2 and step 3. But... the point remains that elemental Sodium is capable of losing an electron to water, leading to the generation of Hydrogen and all the explosive potential that that entails.

I must add that this lies firmly in the realm of solution electrochemistry and is far outside my "expertise" as a solid state inorganic chemist

 

[SezMe]

Excelent explanation for the layman. Thanks.

What he said. I don't think I got much out of my freshman 8 am Saturday chemistry lab.

 

[Olowkow]

What I used to like to demonstrate to undergrads (until I ran out of free gold wire remnants) was showing how gold dissolves in lead, when you try to solder to it.

 

[Complexity]

All hail Evilgiraffe.

 

[This Guy]

Yeah, there really is.... and all it says about evolution is that it would have been impossible if the sun didn't exist (and the earth had no molten core).

Shocking, huh?

I was just trying for a laugh. Though I hadn't read about, or considered the affect (effect? I always screw those up) the molten core would play. Thanks for that!

It is odd though, that with the various science mags and WWW pages I visit to read about scientific information, it seems the only time I run across comments about the 2nd law of thermodynamics, is when it's being used by creationist in a vain attempt to discredit evolution.

Thanks Evilgiraffe, for the great explanation! Gotta love this forum! I've learned SO much here.

 

[fuelair]

Evilgiraffe has realized that he has screwed up the charges somewhere between step 2 and step 3. But... the point remains that elemental Sodium is capable of losing an electron to water, leading to the generation of Hydrogen and all the explosive potential that that entails.

I must add that this lies firmly in the realm of solution electrochemistry and is far outside my "expertise" as a solid state inorganic chemist

Well posted and welcome in!! I am teaching Chemistry and wish my students (H.S.) would ask questions like the OP. Most ask none.

 

[sol invictus]

It is odd though, that with the various science mags and WWW pages I visit to read about scientific information, it seems the only time I run across comments about the 2nd law of thermodynamics, is when it's being used by creationist in a vain attempt to discredit evolution.

There's a debate about the "arrow of time" that's gotten some press lately.

Here's the question: the fundamental laws of physics are time-symmetric. In other words, anything which can happen going forwards in time can also happen going backwards. Make a movie of something happening, say a car crash, play it in reverse, and you're watching an allowed (if unusual) physical process.

The only law which isn't symmetric is the 2nd law (it says entropy increases with time, which means entropy decreases going back in time - so that's not symmetric - and it's why those reversed movies look so strange). What is the origin of that asymmetry, considering that the second law is derived from time-symmetric fundamental laws? (I should emphasize that the second law is a law of thermodynamics, and thermodynamics is an approximate description of lots of smaller degrees of freedom, like H2O, Na, and Cl molecules in the example here. The interactions of those molecules are governed by time-symmetric equations.)

We can make an analogy with the salt: if we add a crystal of salt to some water, without any interference from us it dissolves in a few minutes and mixes. Once it's dissolved, we could sit there for (much, much) longer than the age of the universe before the crystal spontaneously re-assembles itself (let's assume the water can't evaporate). That's because the dissolved salt has more entropy than the un-dissolved salt. So time points in the direction of dissolving, and the asymmetry is because the initial state was a crystal of salt in the water - a very special, low-entropy state.

So the answer to why the 2nd law breaks time-reversal invariance, ultimately, must be that the universe began in a very special and low entropy state. We live during its return to equilibrium, the phase where the entropy is increasing (the salt is in the process of dissolving). But why did the universe start in such a special state? What, or who, put the salt crystal into the water?

 

[dogjones]

The "what happens" of dissolution is fairly simple. Ions at the surface of a crystal are less strongly bound than those in the bulk crystal (they have fewer neighbours).

Thermal vibration of the surface ions means that, at any given time, a few of them have broken away from their neighbours and have moved away from the surface, leaving a hole.

Consider the crystal in a vacuum (or air, same diff to a first approximation)

Let's say a sodium atom breaks free of the surface. It's positively charged, the hole it left behind is lined with chloride ions and so is negative. Electrostatics dictates that the most likely outcome is that the sodium ion drops back into its hole as though nothing happened.

Now consider the crystal surrounded by water. When the sodium atom breaks free, leaving a hole. The positively charged sodium ion will quickly be surrounded by the partially negative oxygen ends of water molecules. The sodium ion is now solvated, it doesn't fit back into the hole it left behind and floats off into solution.

Repeat ad nauseam to dissolve the entire crystal.

A similar argument work if a chloride ion breaks free of the surface, but this time it is the partially positive hydrogen atoms of water that coordinate to the ion to form the solvation sphere.

The "why it happens" of solvation depends pretty much on the second law of thermodynamics.

Many thanks EvilG! Great explanation.

 

[quarky]

why does cold water hold more salt in solution than hot water?

can ethanol be salted to remove water, as oppossed to distilling?

could a swimming pool be designed so that the water was completely full of solvate and couldn't get dirty?

 

[Terry]

why does cold water hold more salt in solution than hot water?

It doesn't. Solubility of solids generally increases with temperature. Solubility of gasses decreases though.

 

[Madalch]

can ethanol be salted to remove water, as oppossed to distilling?

Yes, but you have to start with fairly concentrated ethanol to start with, which means distilling it first.

could a swimming pool be designed so that the water was completely full of solvate and couldn't get dirty?

No, because two different solutes (to a first approximation) do not affect the solubilities of each other.

That's a really lousy first approximation, though. If you saturated the water with, say, salt, the swimming pool water would be a much poorer solvent for organic materials, and a much better solvent for some inorganic materials (although some things would precipitate out- various chlorides, for example). But it would still get dirty, since dirt would still get suspended in the water.

If you saturated the water with something organic, such as butanol (I won't suggest ethanol, since you can't saturate water with it), the water would be a poorer solvent for ionic compounds, but a much better solvent for organics.

Either way, I'd hate to immerse myself in that swimming pool.

 

[quarky]

Yes, but you have to start with fairly concentrated ethanol to start with, which means distilling it first.


No, because two different solutes (to a first approximation) do not affect the solubilities of each other.

That's a really lousy first approximation, though. If you saturated the water with, say, salt, the swimming pool water would be a much poorer solvent for organic materials, and a much better solvent for some inorganic materials (although some things would precipitate out- various chlorides, for example). But it would still get dirty, since dirt would still get suspended in the water.

If you saturated the water with something organic, such as butanol (I won't suggest ethanol, since you can't saturate water with it), the water would be a poorer solvent for ionic compounds, but a much better solvent for organics.

Either way, I'd hate to immerse myself in that swimming pool.

What if the water was supersaturated with long chain polymers and benign additives,and that all the filth we added, when we took a swim, would float to the top, where there was a thin film of non-polar solvent, and that was what we had to clean, instead of a huge quantity of water.?
Btw, said layer would necessarily inhibit mosquitoes and the like.

as per cold water's affinity to dissolve NaCl, vs hot water, Methinks there is an exception here.

 

[Madalch]

What if the water was supersaturated with long chain polymers and benign additives,and that all the filth we added, when we took a swim, would float to the top, where there was a thin film of non-polar solvent, and that was what we had to clean, instead of a huge quantity of water.?
Btw, said layer would necessarily inhibit mosquitoes and the like.

It would also inhibit swimmers. Do you want to swim in a pool with a layer of slime on top?

as per cold water's affinity to dissolve NaCl, vs hot water, Methinks there is an exception here.

Sodium chloride solubility at 0oC = 30 g/100mL; at 100oC, it's 40 g/100mL.

Some solids do dissolve better in cold water than hot water- it's because the dissolution of these compounds is exothermic rather than endothermic (which is more common).

 

[Elvis666]

The "what happens" of dissolution is fairly simple. Ions at the surface of a crystal are less strongly bound than those in the bulk crystal (they have fewer neighbours). . . .

From where I'm standing, 38 years from my last chemistry class, that was a masterful explanation. This kind of thing is one of the reasons I hand around here. Thank you.

 

[quarky]

It would also inhibit swimmers. Do you want to swim in a pool with a layer of slime on top?


Sodium chloride solubility at 0oC = 30 g/100mL; at 100oC, it's 40 g/100mL.

Some solids do dissolve better in cold water than hot water- it's because the dissolution of these compounds is exothermic rather than endothermic (which is more common).

I may be mistaken, but isn't there a point on that soulbility chart wherein colder water holds more salt than warmer water?

 

[quarky]

(I'm not entirely oppossed to the idea of swimming with a slime layer on top...if it was a clean slime layer. actually, the slime layer on top would be black, ideally; to prevent algae growth below)

It wouldn't have to be tolouene...more like hair conditioner.

 

[Bob Blaylock]

Could someone explain simply what happens when salt is dissolved in water?

Yes; in fact, there are several people who could explain this.

 

[EyeOn]

An excellent description, but note that the water will be attracted to the sodium ions while they are still part of the crystal; the cations won't "break free", they will be pulled off by the water molecules.

Isn't that because the electronegativity of water is stronger, thus able to pull the sodium chloride apart?

 

[Madalch]

Isn't that because the electronegativity of water is stronger, thus able to pull the sodium chloride apart?

Compounds don't have electronegativity; atoms have electronegativity.

The water molecule is polar because of the difference between the electronegativity of the oxygen and that of the hydrogen. So the negative end of the molecule is attracted to the positive sodium cations, and the positive side of the molecule is attracted to the chloride anions.