Sciencemadness Discussion Board - On Solubility and What it Means

On Solubility and What it Means

aga - 12-1-2016 at 14:00

Having used solubility curves to determine how many grammes of X will dissolve in water (or whatever solvent) it recently became apparent that i do not really know what 'dissolve' or 'solvate' mean.

What is it ?

In the case of Water as a solvent, it appears to auto-dissasociate into an equilibrium of :-

2 H₂O <=> H₃O+ + OH-

Now 'dissolve' table salt, NaCl in that water.

It is said to disassociate to Na+ + Cl-

So now we have a mix of species :-
H₂O + H₃O+ + OH- + Na+ + Cl-

It gets more complex : do the atoms/molecules (of the solvent and the dissolved substance) re-arrange themselves in a 'dissolved' state into a different form than they would have in an undissolved state ?

In the case of NaCl, yes, it stops being a solid, so some reaction happens (it reacts to water by not being a solid anymore)

Taking this a bit further, i did a few experiments to see what happens to the Volume of two substances when mixed.

Hypothesis: If they react to each other, then possibly their combined volume will change as they become different species.

The question is : are they simply mixing, or are they re-arranging themselves (i.e. reacting) by being mixed ?

In each of the following cases the materials were measured in a 25ml cylinder then mixed in a small beaker, then re-measured in the same cylinder.

1. Water + NaCl
20ml distilled water + approx 5ml (5.87g) NaCl
Result : 24ml

2. acetone + 36w% H₂SO₄
10ml acetone + 10ml sulphuric acid 36w%
Result: 19.5ml

3. OTC turpentine + acetone
10ml + 10ml
Result: 21ml

4. OTC turpentine + distilled water
10ml + 10ml
Result: 19.5ml

5. acetone + distilled water
10ml + 10ml
Result: 19.5ml

The results are inconclusive, however there does appear to be a volume change in all cases, apart from the NaCl experiment, where the actual volume of NaCl could not be accurately determined.

This is an experiment that can easily be done by Anyone.

Measure the volume of two chemicals as accurately as you can, mix them thoroughly, then measure the volume of the mixture.

Perhaps by comparing and tabulating results we can know more about what 'dissolve' actually means.

P.S. Before any smartass says 'you measured them wrong you agahole' it should be noted that i can measure liquid volumes to 0.1 ml without difficulty (so long as they're not viscous oils that stick to the glass for longer than my attention span).

[Edited on 12-1-2016 by aga]

DraconicAcid - 12-1-2016 at 15:40

I fail to see how measuring the changes of volume upon dissolution is going to give you any mental clarity about the process of dissolution. Volumes are not additive when you make a solution.

blogfast25 - 12-1-2016 at 15:58

The question isn't very well posed to begin with. More about that later...

A few quickies.

Auto-dissociation of water, remember what I wrote in the other thread:

$$K_W=[\mathrm{H_3O^+}][\mathrm{OH^-}]=10^{-14}$$

That's a very small number. At pH = 7, in very pure water that also means:

$$[\mathrm{H_3O^+}]=[\mathrm{OH^-}]=10^{-7}\:\mathrm{mol/L}$$

$$pH = -\log[\mathrm{H_3O^+}]=7.00$$

Re. NaCl solution, estimate the contraction by means of the density of solid NaCl = 2.165 g/cm3.

*********************

On a much broader note, "Solvation" is such a massive subject it's only possible to meaningfully approach it by type of solution. To mention a few:

1. Solvation of ionic compounds in polar solvents.(A decent page on NaCl/water solvation.)
2. Solvation of acids and bases. (A decent page on that.)
3. Solvation of covalent compounds.

Quote:

Perhaps by comparing and tabulating results we can know more about what 'dissolve' actually means.

Errrmm... no. That body of theory is so well developed no one here can add a single new iota to it!

[Edited on 13-1-2016 by blogfast25]

Etaoin Shrdlu - 12-1-2016 at 18:48

Quote: Originally posted by aga

Hypothesis: If they react to each other, then possibly their combined volume will change as they become different species.

Most substances dissolved in each other result in a volume different from the added volumes without regard to whether they form new species or not.

blogfast25 - 12-1-2016 at 19:38

Quote: Originally posted by Etaoin Shrdlu

Most substances dissolved in each other result in a volume different from the added volumes without regard to whether they form new species or not.

What he's trying to get at (I think) is that there's some interaction between solvent and solute. That is of course true but the nature of these interactions between solvent and solute species varies rather wildly from one type of solution to the next.

It's probably also useful to distinguish between 'solubility', 'dissolution', 'solvation', terms that are easily conflated because they are related, yet are not the same.

[Edited on 13-1-2016 by blogfast25]

aga - 13-1-2016 at 00:16

Quote: Originally posted by DraconicAcid

I fail to see how measuring the changes of volume upon dissolution is going to give you any mental clarity about the process of dissolution.

Depends on how a particular mind works and how clear it is in the first place

Quote: Originally posted by blogfast25

The question isn't very well posed to begin with.

Granted.

Quote: Originally posted by blogfast25

On a much broader note, "Solvation" is such a massive subject it's only possible to meaningfully approach it by type of solution. To mention a few:

1. Solvation of ionic compounds in polar solvents.(A decent page on NaCl/water solvation.)
2. Solvation of acids and bases. (A decent page on that.)
3. Solvation of covalent compounds.
...

It's probably also useful to distinguish between 'solubility', 'dissolution', 'solvation', terms that are easily conflated because they are related, yet are not the same

Thanks for the links.

It's embarassing to think that i've used those words for a long time and had no idea what they really mean.

Time to Learn.

blogfast25 - 13-1-2016 at 06:53

1. Solubility (limit): the maximum amount of solute that can be dissolved in a certain amount of solvent, at a specified temperature and pressure, usually expressed as g solute per 100 g solvent.

2. Dissolution: the process of dissolving a solute in a solvent. This is accompanied by a negative change in Gibbs Free Energy ΔG < 0. It's the sum of all Gibbs Free Energy changes resulting from solute/solvent interactions.

3. State of solvation: the ensemble of interactions between solute and solvent which involve (simply put):

* van der Waals forces
* Hydration/complexation
* Dissociation/deprotonation
* Hydrogen bonding

[Edited on 13-1-2016 by blogfast25]

Praxichys - 13-1-2016 at 07:57

Here is a great page on K_sp and the common ion effect, and how to mathematically predict the limits of solubility.

The page blogfast recommends on acids/bases is good, but don't forget that polyprotic acids like sulfuric (diprotic) and phosphoric (triprotic) form equillibria of their polysubstituted ions.

For instance, dissolving sodium bisulfate in water creates an equillibrium of Na+, H+, HSO4- and SO4- rather than just the Na+ and HSO4- that might be expected. In many cases, the concentration of these species is small enough to ignore, but in other cases it can cause problems with precipitation of intermediates.